US 5,498,544 AGrant
Method and an Acetyl CoA Carboxylase Gene for Conferring Herbicide Tolerance
Issue Date:1996-03-12
•6 Claims
•9 Drawing Sheets
Abstract
The present invention provides an expression cassette encoding a plant acetyl CoA carboxylase gene and methods for conferring herbicide tolerance and/or altering the oil content of plants by introducing and expressing a plant acetyl CoA carboxylase gene in plant cells. An expression cassette of the invention can comprise a gene coding a plant acetyl CoA carboxylase or a functional mutant thereof or an antisense DNA sequence complementary to the sequence for a plant acetyl CoA carboxylase gene combined with a promoter functional in a plant cell. The method of imparting herbicide tolerance to a plant includes the steps of introducing the expression cassette into the cells of a plant tissue and expressing the plant acetyl CoA carboxylase gene in an amount effective to render the acetyl CoA carboxylase and/or plant cell substantially tolerant to the herbicides. The method of altering the oil content in a plant includes the steps of introducing an expression cassette into plant cells and expressing the acetyl CoA carboxylase gene in an amount effective to alter the oil content of the cells. The expression cassette can also be introduced into a prokaryotic cell to increase yield of a plant acetyl CoA carboxylase so that crystallized enzyme can be used to screen and identify other herbicides that bind to and inhibit the enzyme.
Metadata
Assignee
- Regents of the University of Minnesota
Inventors
- Burle G. Gengenbach
- David A. Somers
- Donald L. Wyse
- John W. Gronwald
- Margaret A. Egli
- Sheila M. Lutz
Application Information
Application Number:US 014326&
Filing Date:1993-02-05
Priority Date:1988-11-10
Art Unit:183
Classifications
IPC:
C12N 1582
Field of Search:
435800536320.1;172.3205;DIG. 52;5623.6;23.2
Patent Drawings (9 sheets)
Description
BACKGROUND OF THE INVENTION
Acetyl CoA carboxylase (ACCase) is an enzyme involved in many important metabolic pathways in plant, animal and bacterial cells. The enzyme is especially important in fatty acid synthesis in plants and is sensitive to inhibition by some types of herbicides. Structurally, ACCases are biotinylated and are quite large enzymes consisting of one or more subunits. For example, most ACCases of animals, higher plants, and yeast are dimers of 420 to 700 kD native MW and contain subunits of 200 to 280 kD. Diatom and algal ACCases are 700 to 740 kD tetramers of 160 to 180 kD subunits. Bacterial ACCase consists of three dissociable proteins; biotin carboxylase (51 kD), biotin carboxyl carrier protein (22.5 kD), and biotin transcarboxylase (130 kD).
Acetyl CoA Carboxylase (ACCase) catalyzes the formation of malonyl-CoA from acetyl-CoA and bicarbonate in animal, plant, and bacterial cells. Malonyl-CoA is an essential substrate for (i) de novo fatty acid (FA) synthesis, (ii) fatty acid elongation, (iii) synthesis of secondary metabolites such as flavonoids and anthocyanins, and (iv) malonylation of some amino acids and secondary metabolites. Synthesis of malonyl-CoA is the first committed step of flavonoid and fatty acid synthesis and current evidence suggests that ACCase catalyzes the rate-limiting step of fatty acid synthesis. Formation of malonyl-CoA by ACCase occurs via two partial reactions and requires a biotin prosthetic group:
(i) E-biotin+ATP+HCO.sub.3 .fwdarw.E-biotin-CO.sub.2 +ADP+Pi
(ii) E-biotin-CO.sub.2 +Acetyl-CoA.fwdarw.E-biotin+malonyl-CoA
(NET) Acetyl-CoA+ATP+HCO.sub.3 .fwdarw.malonyl-CoA+ADP+Pi
In E. coli, these reactions are catalyzed by three distinct components; biotin carboxylase, biotin transcarboxylase, and biotin carboxyl carrier protein, which can be separated and yet retain partial activity. Plant and animal ACCases contain all three activities on a single polypeptide.
In plants, most ACCase activity is located in plastids of green and non-green plant tissues including leaves and oil seeds. Leaf ACCase activity is primarily located in mesophyll cells, but lesser amounts have been found in C-4 bundle sheath cells and in epidermal cells. The subcellular location of ACCase activity in epidermal cells is unknown, but since synthesis of very long-chain fatty acids (VLCFA) for formation of waxes, cutin, and suberin occurs on the endoplasmic reticulum (ER), malonyl-CoA might also be derived from a cytosolic ACCase. In contrast, rat ACCase is primarily cytosolic or associated with the outer mitochondrial membrane.
De novo fatty acid synthesis in chloroplasts involves successive 2-carbon additions to acetate, using malonate as the 2-C donor. All intermediates are attached to acyl carrier protein (ACP). Synthesis in plastids resembles that in E. coli in that the fatty acid synthesis complex can be dissociated into separate enzymes: .beta.-ketoacyl-ACP synthase (KAS), .beta.-ketoacyl-ACP reductase, .beta.-hydroxyl-ACP dehydratase, and enoyl-ACP reductase, acetyl-CoA:ACP transacylase, and malonyl-CoA:ACP transacylase. A highly active KASIII isozyme catalyzes the condensation of acetyl-CoA and malonyl-ACP. Successive additions of malonyl-CoA to acyl-ACPs catalyzed by KAS I form C16 acyl-ACP, some of which is converted to C18 acyl-ACP by KAS II and then to C18:1-ACP. Fatty acid metabolism then diverges; de-esterification allows movement to the cytoplasm (eukaryotic path) where fatty acids may be further unsaturated and/or elongated by additions of malonyl-CoA in the ER. Alternatively, fatty acids are linked to glycerol-3-phosphate (prokaryotic path), further unsaturated, and used for synthesis of chloroplast lipids. A portion of cytoplasmic lipids returns to the chloroplast. The relative contributions of these two paths are species-specific but appear to be relatively flexible in mutants blocked in either path. In oil-storing organs such as cotyledons and monocot embryos, the triacylglycerides are stored in cytoplasmic oil bodies surrounded by a single unit membrane.
Condensation of malonyl-CoA with phenylpropionyl-CoAs or acetyl-CoA leads to synthesis of flavonoids, anthocyanins, or to polyacetates. Condensation is increased by light, elicitors, or pathogens and may be the rate-limiting step in synthesis of some phytoalexins. In addition to the secondary metabolites derived by de novo synthesis, malonyl conjugates of flavonoid glycosides, formed by malonyl-CoA:flavonoid glycoside malonyltransferase, D-amino acids and 1-amino-carboxyl-cyclopropane (ethylene precursor) are found in plants. Malonylated compounds accumulate in vacuoles, probably after synthesis in the cytoplasm.
An important property of ACCase is the central role it plays in fatty acid synthesis and accumulation in plants and seeds. Available evidence supports the idea that ACCase activity is the rate-limiting step for de novo fatty acid synthesis in plants. High rates of ACCase activity in vitro parallel or slightly precede high rates of lipid deposition or [.sup.14 C]acetate incorporation into lipids in developing leaves and oil seeds. Significant changes in plant ACCase activity occur during chloroplast development and increase in ACCase activity correlates with lipid deposition in developing oil seeds. Turnham et al., Biochem. J., 212:223 (1883); and Beittenmiller et al., Plant Physiol., 100:923 (1992).
Among other properties, ACCase in most monocots is also inhibited by several herbicides. [.sup.14 C]acetate incorporation into maize lipids is strongly inhibited by fluazifop and sethoxydim due to inhibition of plastid ACCase. In barley, however, fluazifop had little effect on [.sup.14 C]acetate incorporation into very long-chain fatty acids. Since synthesis of very long-chain fatty acids occurs in the cytosol on the ER, and de novo fatty acid synthesis occurs in the plastids, cytosolic malonyl-CoA might be supplied by a herbicide insensitive ACCase isozyme.
There are three general mechanisms by which plants may be resistant to, or tolerant of, herbicides. These mechanisms include insensitivity at the site of action of the herbicide (usually an enzyme), rapid metabolism (conjugation or degradation) of the herbicide, or poor uptake and translocation of the herbicide. Altering the herbicide site of action from a sensitive to an insensitive form is the preferred method of conferring tolerance on a sensitive plant species. This is because tolerance of this nature is likely to be a dominant trait encoded by a single gene, and is likely to encompass whole families of compounds that share a single site of action, not just individual chemicals. Therefore, detailed information concerning the biochemical site and mechanism of herbicide action is of great importance and can be applied in two ways. First, the information can be used to develop cell selection strategies for the efficient identification and isolation of appropriate herbicide-tolerant variants. Second, it can be used to characterize the variant cell lines and regenerated plants that result from the selections.
Tissue culture methods have been used to select for resistance (or tolerance) using a variety of herbicides and plant species (see review by Meredith and Carlson, 1982, in Herbicide Resistance in Plants, eds. Lebaron and Gressel, pp. 275-291, John Wiley and Sons, NY). For example, P. C. Anderson et al., in U.S. Pat. No. 4,761,373, disclose the use of tissue culture methods to produce maize plants resistant to herbicidal imidazolidones and sulfonamides. The resistance is due to the presence of altered acetohydroxy acid synthase which is resistant to deactivation by these herbicides.
Certain 1,3-cyclohexanediones exhibit general and selective herbicidal activity against plants. One such cyclohexanedione is sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1 -one}. Sethoxydim is commercially available from BASF (Parsippany, N.J.) under the designation POAST.TM..
Other herbicidal cyclohexanediones include clethodim, (E,E)-(.+-.)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio )propyl]-3-hydroxy-2-cyclohexen-1-one; available as SELECT.TM. from Chevron Chemical (Valent) (Fresno, Calif.); cloproxydim, (E,E)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]butyl]-5-[2-(ethylthio)propyl] -3-hydroxy-2-cyclohexen-1-one; available as SELECTONE.TM. from Chevron Chemical (Valent) (Fresno, Calif.); and tralkoxydim, 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-mesitylcyclohex-2-enone, available as GRASP.TM. from Dow Chemical USA (Midland, Mich.).
For purposes of reference in the present specification, the herbicides described in the two preceding paragraphs and other structurally related herbicidal compounds, are collectively referred to as the cyclohexanedione family of herbicides.
Certain aryloxyphenoxypropanoic acids exhibit general and selective herbicidal activity against plants. In these compounds, the aryloxy group may be phenoxy, pyridinyloxy or quinoxalinyl. One such herbicidal aryloxyphenoxypropanoic acid is haloxyfop, {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid}, which is available as VERDICT.TM. from Dow Chemical USA (Midland, Mich.). Another is diclofop, {(.+-.)-2-[4-(2,4-dichlorophenoxy)-phenoxy]propanoic acid}, available as HOELON.TM. from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.).
Other members of this family of herbicides include fenoxyaprop, (.+-.)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoic acid; available as WHIP.TM. from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.); fluazifop, (.+-.)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE.TM. from ICI Americas (Wilmington, Del.); fluazifop-P, (R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE 2000.TM. from ICI Americas (Wilmington, Del.); and quizalofop, (.+-.)-2-[4[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propanoic acid; available as ASSURE.TM. from E. I. DuPont de Nemours (Wilmington, Del.).
For purposes of reference in the present specification, the herbicides referred to in the two preceding paragraphs and other structurally related herbicidal compounds, are collectively referred to as herbicidal aryloxyphenoxypropanoic acids.
Thus, there is a need for methods to develop plants that are resistant or tolerant to herbicides. There is also a need to increase the oil and/or fatty acid content of the plants and seeds, as well as for methods to increase their resistance to herbicides. There is a need to identify and clone genes important in conferring herbicide tolerance and in increasing the oil content of plants.
SUMMARY OF THE INVENTION
The present invention provides an expression cassette encoding a plant acetyl CoA carboxylase gene and methods for conferring herbicide tolerance and/or altering the oil content of plants by introducing and expressing a plant acetyl CoA carboxylase gene in the plant cells. An expression cassette according to the invention comprises a gene coding for plant acetyl CoA carboxylase or a functional mutant thereof operably linked to a promoter functional in a plant cell. The gene coding for a plant acetyl CoA carboxylase can encode an unaltered plant acetyl CoA carboxylase or an altered plant acetyl CoA carboxylase substantially tolerant to inhibition by cyclohexanedione or aryloxyphenoxypropanoic acid herbicides as well as encoding an antisense DNA sequence that is substantially complementary to a plant acetyl CoA carboxylase gene or to a portion thereof. The expression cassette is operably linked to a promoter functional in the plant cell. The promoter can be an inducible or tissue specific promoter or provide for overexpression of at least about a 2-fold amount of a plant acetyl CoA carboxylase. An expression cassette of the invention can also optionally further comprise a chloroplast transit peptide sequence operably linked between the promoter and the plant acetyl CoA carboxylase gene, as well as 3' regulatory DNA sequences required for expression of the expression cassette in a plant cell and/or a plasmid.
The method of imparting cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance to a plant includes the steps of introducing an expression cassette comprising a gene coding for a plant acetyl CoA carboxylase or a functional mutant thereof operably linked to a promoter functional in a plant cell into the cells of plant tissue and expressing the gene in an amount effective to render the acetyl CoA carboxylase and/or the plant tissue substantially tolerant to the herbicides. Herbicide tolerance can be achieved in the plants by at least two methods, including increasing the level of gene expression of a native or unaltered acetyl CoA carboxylase, or by introducing an altered gene coding for an acetyl CoA carboxylase that is less sensitive to herbicide inhibition. The level of gene expression can be increased by either combining a plant acetyl CoA carboxylase gene with a promoter that provides for a high level of gene expression, such as a 35S cauliflower mosaic virus promoter (CaMV), or by introducing multiple copies of the gene into the cell so that the multiple copies of the gene are integrated into the genome of transformed plant cells. The preferred plant cells into which to introduce the expression cassette of the invention, to achieve herbicide tolerance, are monocot plant cells. Once transformed cells exhibiting herbicide tolerance are obtained, transgenic plants and seeds can then be regenerated therefrom, and evaluated for stability of the inheritance of the herbicide tolerance trait.
The invention also provides a method for altering, preferably raising, the oil content in a plant. The method includes the steps of introducing an expression cassette comprising a gene coding for a plant acetyl CoA carboxylase or a functional mutant thereof operably linked to a promoter functional in a plant cell into the cells of plant tissue and expressing the gene in an amount effective to alter the oil content of the plant cell. An alteration in oil content can include a change in total oil content over that normally present in that type of plant cell or a change in the type of oil present in the cell. An alteration in oil content in the plant cell, according to the method of the invention, can be achieved by at least two methods including:
(1) an increase or decrease in expression of an altered plant acetyl CoA carboxylase gene; or
(2) by introducing an altered or functional mutant plant acetyl CoA carboxylase gene.
The level of gene expression of an unaltered plant acetyl CoA carboxylase gene can be increased by either combining an unaltered plant acetyl CoA carboxylase with a promoter that provides for a high level of gene expression, or by introducing multiple copies of an expression cassette into cells so that multiple copies of the gene are integrated into the genome. When an altered or functional mutant plant acetyl CoA carboxylase gene codes for an enzyme that exhibits an increase in specific activity, it can lead to an increase in total oil content of the plant cell. When an altered or functional mutant acetyl CoA carboxylase gene codes for an enzyme having a decrease in specific activity, it may lead to a decrease in the total oil content of the plant cell. Preferably, the expression cassette is introduced into dicot plants such as soybeans, canola, and sunflower. In an especially preferred version, transformed cells exhibiting about a 1.2- to 5-fold increase in total oil content and/or expression or specific activity of acetyl CoA carboxylase are selected for and used to generate transgenic plants and seeds exhibiting a substantial increase in oil content. A substantial increase in oil content depends on the oil content normally present in the plant or seed and can range from about a 1.2 to a 20-fold increase.
The invention also provides for a method of producing plant acetyl CoA carboxylase in a host cell. The method includes the steps of introducing an expression cassette comprising a gene encoding a plant acetyl CoA carboxylase or functional mutant thereof into a host cell and expressing the gene in an amount sufficient to permit crystallization of the plant acetyl CoA carboxylase. An expression cassette can include a promoter that is functional in either a eukaryotic or a prokaryotic cell. Preferably, the expression cassette is introduced into a prokaryotic cell, such as E. coli, that is routinely used for production of recombinantly produced proteins. Recombinantly produced and crystallized plant acetyl CoA carboxylase can then be used to identify other herbicides and that bind to and inhibit acetyl CoA carboxylase in plants. In addition, the availability of large amounts of purified enzyme can permit the screening of the efficacy of such herbicides in terms of their ability to bind to, or otherwise inhibit, the activity of the enzyme.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic depiction of the fatty acid biosynthesis pathway in plants.
FIG. 2 is a graph depicting the effect of sethoxydim on the growth of mutant maize callus.
FIG. 3 is a graph depicting the shoot length growth of maize seedlings seven days after treatment with sethoxydim.
FIG. 4 is a graph depicting the shoot length growth of maize seedlings fourteen days after treatment with sethoxydim.
FIG. 5: Total soluble and biotinylated polypeptides in ACCase purification fractions from seedling leaves of maize inbred A619. Proteins were separated by SDS-PAGE in 7.5% gels and then silver-stained (Panel A). An identical gel was Western-blotted and a longitudinal section of each lane was probed with avidin (Panel B). Lanes were 1: crude (10 .mu.g); 2: (NH.sub.4).sub.2 SO.sub.4 (10 .mu.g); 3: S-300 (5 .mu.g); 4: Blue Sepharose (2 .mu.g); 5: Mono-Q ACCase II (5 .mu.g); and 6: Mono-Q ACCase I (5 .mu.g). Diagonal lines between lanes indicate position of molecular weight markers shown on the left.
FIG. 6: Immunoprecipitation of ACCase activity from B73 leaf, embryo, endosperm, and BMS suspension cultured cells. Equal activities (0.58 nmol min.sup.-1) were incubated with 16 .mu.L serum (immune plus preimmune), immune complexes were precipitated with Protein A-agarose, and ACCase activity remaining in the resulting supernatant fraction was measured relative to the preimmune serum control.
FIG. 7: Herbicide inhibition of acetyl-(AcCoA) or propionyl-CoA (Prop-CoA)-dependent H.sup.14 CO.sub.3 -incorporation into acid-stable product by ACCase I and II Mono-Q fractions. Activities in the presence of haloxyfop (1 .mu.M) are expressed relative to the minus herbicide control.
FIG. 8: Comparison of the peptide sequence of maize cDNA clones #15-14 and #18-5 with chicken ACCase. The approximate locations of the biotin carboxylase, biotin binding site, and biotin transcarboxylase functional domains are indicated for the chicken sequence. The percentages of amino acid identity are indicated by cross-hatched boxes for the maize coding sequence. Regions of genomic DNA Type I and Type II clone sequences that align with cDNA #18-5 are indicated by solid heavy lines. The approximate locations of subclone #28 and #16 from genomic Type I and subclone #34 from genomic Type II clones are indicated.
FIG. 9: Northern blot of total RNA from leaf, immature embryo and endosperm tissue (16 days after pollination), and Black Mexican Sweetcorn (BMS) cells. Lanes contain 10 .mu.g total RNA and were probed with the 2 kb EcoRI fragment of lambda clone #15-14.
FIG. 10: DNA sequence (SEQ ID No:1) of a 2 kb EcoRI fragment of lambda clone #15-14 including a portion of a maize ACCase gene located at bases 2883 to 883 from the 3' stop codon. A) DNA sequence of bases 2883 to 1779. B) DNA sequences of bases 1778 to 883.
FIG. 11: Graph of ACCase activity during seed development in two high oil soybean cell lines and one low oil soybean cell line.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an expression cassette encoding a plant acetyl CoA carboxylase gene and methods for conferring herbicide tolerance and/or altering the oil content of plants by introducing and expressing a plant acetyl CoA carboxylase gene in the plant cells using said cassette. In plants, acetyl CoA carboxylase plays a central role in regulating fatty acid synthesis and in the sensitivity of monocots to cyclohexanedione or aryloxyphenoxypropanoic acid herbicides.
In accord with the present invention, a plant acetyl CoA carboxylase gene is identified, isolated and combined with a promoter functional in a plant cell to provide a recombinant expression cassette. A plant acetyl CoA carboxylase gene can be introduced and expressed in a plant cell. Depending on the type of plant cell, the level of gene expression, and the activity of the enzyme encoded by the gene, introduction of a plant acetyl CoA carboxylase gene into the plant cell can confer herbicide tolerance and/or alteration of the oil of the plant cell.
In monocots, an exogenously introduced plant acetyl CoA carboxylase gene can be expressed at a level effective to render the cells of the plant tissue substantially tolerant to cyclohexanedione or aryloxyphenoxypropanoic acid herbicide levels which normally inhibit a native or endogenous acetyl CoA carboxylase. A native acetyl CoA carboxylase is an enzyme that is normally encoded and expressed in the plant cell prior to transformation. An exogenously introduced plant acetyl CoA carboxylase gene is a gene which has been isolated and amplified from either the same or different type of cell. Exogenous introduction and expression of a plant acetyl CoA carboxylase gene in both monocots and dicots can result in alteration of the oil content and quality of plant tissue and seeds. Exogenous introduction and expression in a host cell, such as a bacteria, can provide for sufficient amounts of plant acetyl CoA carboxylase to allow for crystallization and isolation of the enzyme. Crystallized plant acetyl CoA carboxylase is useful to identify other herbicides that bind to and can inhibit plant acetyl CoA carboxylases. The enzyme could also be used to screen potential herbicidal compounds for efficacy.
A. Formation of an Expression Cassette
An expression cassette of the invention can comprise a gene encoding a plant acetyl CoA carboxylase or functional mutant thereof operably linked to a promoter functional in a plant cell. The gene can code for a plant acetyl CoA carboxylase that is substantially tolerant to herbicides, preferably cyclohexanedione and/or aryloxyphenoxypropanoic acid herbicides. An expression cassette of the invention can also include an antisense DNA sequence that is substantially complementary to an acetyl CoA carboxylase gene or a portion thereof operably linked to a promoter functional in a plant cell.
Isolation and Identification of a Gene Coding for a Plant Acetyl CoA Carboxylase
A gene encoding a plant acetyl CoA carboxylase can be identified and isolated by standard methods, as described by Sambrook et al., Guide to Molecular Cloning: A Laboratory Manual, Cold Spring Harbor, N.Y. (1989). The gene can be obtained either from monocot or dicot plant cells. When the gene encoding a plant acetyl CoA carboxylase is obtained from a dicot plant, the enzyme encoded by the gene exhibits tolerance to cyclohexanedione or aryloxyphenoxypropanoic acid herbicides. The gene can also be obtained from herbicide-tolerant maize cell lines, prepared as described in U.S. Pat. No. 5,162,602, which is hereby incorporated by reference.
A gene encoding a plant acetyl CoA carboxylase can be identified by screening of a DNA or cDNA library generated from plant cells. Screening for DNA fragments that encode all or a portion of the gene encoding a plant acetyl CoA carboxylase can be accomplished by complementation of an auxotrophic mutant of acetyl CoA carboxylase in E. coli (fabE) (Bachman, Microbiological Reviews, 47:180 (1983)) or yeast (accl) (Michionada, Eur. J. Biochem., 111:79 (1980)) or by screening of plaques for binding to antibodies that specifically recognize a plant acetyl CoA carboxylase. DNA fragments that can restore ACCase activity in E. coli or yeast and/or plaques carrying DNA fragments that are immunoreactive with antibodies to a plant ACCase can be subcloned into a vector and sequenced and/or used as probes to identify other cDNA or genomic sequences encoding all or a portion of a plant acetyl CoA carboxylase gene.
Specific examples of cDNA sequences encoding a portion of a plant acetyl CoA carboxylase gene include DNA fragments that include a DNA sequence that substantially corresponds to the coding sequence for the transcarboxylase active site of a plant acetyl CoA carboxylase, DNA fragments that include a DNA sequence that substantially corresponds to a coding sequence for the biotin binding site of a plant acetyl CoA carboxylase, a DNA fragment encoding the 5' transcriptional start sequence of a plant acetyl CoA carboxylase gene, and a DNA fragment encoding the 3' transcriptional stop sequence for the acetyl CoA carboxylase gene. Substantially corresponding DNA sequences share about 90% to about 100% DNA sequence homology. Especially preferred cDNA probes can be obtained from lambda clone #18-5 which include DNA sequences corresponding to the transcarboxylase active site domain and the biotin binding site domain. Lambda clone #18-5 includes EcoRI subclones of 3.9 kb, 1.2 kb, or 0.23 kb. Lambda subclone #18-5I is an 3.9 kb EcoRI subclone. The lambda subclone #18-5I has been deposited with the American Type Culture Collection, Rockville, Md., and given Accession No. 69236.
In a preferred version, a plant acetyl CoA carboxylase gene is identified and isolated from an herbicide tolerant maize cell line prepared as described in Example II. A cDNA library can be prepared by oligo DT priming. Plaques containing DNA fragments can be screened with antibodies specific for maize acetyl CoA carboxylase. DNA fragments encoding a portion of an acetyl CoA carboxylase gene can be subcloned and sequenced and used as probes to identify a genomic acetyl CoA carboxylase gene. DNA fragments encoding a portion of a maize acetyl CoA carboxylase can be verified by determining sequence homology with other known acetyl CoA carboxylases, such as chicken or yeast acetyl CoA carboxylase, or by hybridization to acetyl CoA carboxylase specific messenger RNA. Once DNA fragments encoding portions of the 5', middle and 3' ends as well as the transcarboxylase active site or biotin binding site of a plant acetyl CoA carboxylase are obtained, they can be used to identify and clone a complete genomic copy of a maize acetyl CoA carboxylase gene.
To isolate a complete copy of a maize acetyl CoA carboxylase gene, a maize genomic library can then be probed with cDNA probes prepared as described above. Portions of the genomic copy or copies of a plant acetyl CoA carboxylase gene can be sequenced and the 5' end of the gene are identified by standard methods including either DNA sequence homology to other acetyl CoA carboxylase genes or by RNAase protection analysis, as described by Fritch et al. (1989). Once portions of the 5' end of the gene are identified, complete copies of a plant acetyl CoA carboxylase gene can be obtained by standard methods, including by cloning or by polymerase chain reaction (PCR) synthesis using oligonucleotide primers complementary to the DNA sequence at the 5' end of the gene. The presence of an isolated full-length copy of a plant acetyl CoA carboxylase gene can be verified by hybridization, partial sequence analysis, or by expression of a plant acetyl CoA carboxylase enzyme. The maize acetyl CoA carboxylase gene cloned and expressed from a maize herbicide tolerant cell line can be assessed for tolerance to cyclohexanedione or aryloxyphenoxypropanoic acid herbicides by standard methods, as described in Example I.
An expression cassette of the invention can also contain an antisense DNA sequence. A antisense DNA sequence is a sequence that is substantially complementary to all or a portion of a coding sequence of a plant acetyl CoA carboxylase gene. A substantially complementary sequence has about 90% to about 100% DNA sequence homology with that of the coding sequence of all or a portion of a plant acetyl CoA carboxylase. The antisense DNA sequence when expressed can act to inhibit the synthesis and expression of a native plant acetyl CoA carboxylase. Antisense sequences are preferably about 200 to 1000 nucleotides long in order to provide sufficient inhibition of synthesis and/or expression of a native acetyl CoA carboxylase. The inhibition of acetyl CoA carboxylase synthesis and gene expression by antisense DNA sequences can be confirmed in a transformed plant cell by standard methods for measuring the presence and/or activity of the enzyme such as described in Examples I and V.
An expression cassette of the invention can also include a functional mutant of a plant acetyl CoA carboxylase gene. Mutants of a plant acetyl CoA carboxylase gene are substantially homologous to a plant acetyl CoA carboxylase gene and are functional if the acetyl CoA carboxylase expressed retains significant enzyme activity. A mutant substantially homologous to a plant acetyl CoA carboxylase can share about 90% to 99.99% DNA sequence with that gene. For example, a mutant acetyl CoA carboxylase gene can code for a herbicide tolerant acetyl CoA carboxylase, or for an acetyl CoA carboxylase with altered substrate specificity so that the total amount of oil content in the plants or seeds is increased, or for an enzyme with an altered substrate specificity so that synthesis of secondary metabolites such as flavonoids or anthocyanins is decreased. A preferred mutant is a gene coding for an acetyl CoA carboxylase that is substantially tolerant to cyclohexanedione or aryloxyphenoxypropanoic acid herbicides.
Functional mutants of a gene coding for a plant acetyl CoA carboxylase can be obtained by several methods. The alteration or mutation of the ACCase gene can be accomplished by a variety of means including, but not limited to, the following methods.
1. spontaneous variation and direct mutant selection in cultures;
2. direct or indirect mutagenesis procedures on tissue culture of all cell types, seeds or plants; and
3. mutation of the cloned acetyl CoA carboxylase gene by methods such as site specific mutagenesis (Sambrook et al. cited supra), transposon mediated mutagenesis (Berg et al., Biotechnology, 1:417 (1983)) and deletion mutagenesis (Mitra et al., Molec. Gen. Genetic., 215:294 (1989)).
Mutants can be identified by a change in a functional activity of the enzyme encoded by the gene or by detecting a change in the DNA sequence using restriction enzyme mapping or partial sequence analysis.
In a preferred version, a functional mutant gene encoding for a plant acetyl CoA carboxylase tolerant to cyclohexanedione and/or aryloxyphenoxypropanoic acid herbicides is isolated from a maize herbicide tolerant cell line. The maize herbicide tolerant cell line was obtained as described in U.S. Pat. No. 5,162,602, issued Nov. 10, 1992, the disclosure of which is incorporated by reference herein, and in Examples I-III. Briefly, partially differentiated cell cultures are grown and subcultured with continuous exposures to low herbicide levels. Herbicide concentrations are then gradually increased over several subculture intervals. Maize cells or tissues growing in the presence of normally toxic herbicide levels are repeatedly subcultured in the presence of the herbicide and characterized. Stability of the herbicide tolerance trait of the cultured cells may be evaluated by growing the selected cell lines in the absence of herbicides for various periods of time and then analyzing growth after exposing the tissue to herbicide.
Maize cell lines which are tolerant by virtue of having an altered acetyl CoA carboxylase enzyme can be selected by identifying cell lines having enzyme activity in the presence of normally toxic levels of sethoxydim or haloxyfop. The tolerant maize cells can be further evaluated for whether acetyl CoA carboxylase is altered to a less sensitive form or increased in its level of expression.
Maize cell lines with a acetyl CoA carboxylase less sensitive to herbicide inhibition can be used to isolate a functional mutant gene of a plant acetyl CoA carboxylase. A DNA library from a maize cell line tolerant to herbicides can be generated and DNA fragments encoding all or a portion of an acetyl CoA carboxylase gene can be identified by hybridization to a cDNA probe encoding a portion of the maize ACCase gene. A complete copy of the altered gene can be obtained either by cloning and ligation or by PCR synthesis using appropriate primers. The isolation of the altered gene coding for acetyl CoA carboxylase can be confirmed in transformed plant cells by determining whether the acetyl CoA carboxylase being expressed retains enzyme activity when exposed to normally toxic levels of herbicides.
Promoters
Once a plant acetyl CoA carboxylase gene or functional mutant thereof or an antisense DNA sequence is obtained and amplified, it is combined with a promoter functional in a plant cell to form an expression cassette.
Most genes have regions of DNA sequence that are known as promoters and which regulate gene expression. Promoter regions are typically found in the flanking DNA sequence upstream from the coding sequence in both procaryotic and eukaryotic cells. A promoter sequence provides for regulation of transcription of the downstream gene sequence and typically includes from about 50 to about 2,000 nucleotide base pairs. Promoter sequences also contain regulatory sequences such as enhancer sequences that can influence the level of gene expression. Some isolated promoter sequences can provide for gene expression of heterologous genes, that is a gene different from the native or homologous gene. Promoter sequences are also known to be strong or weak or inducible. A strong promoter provides for a high level of gene expression, whereas a weak promoter provides for a very low level of gene expression. An inducible promoter is a promoter that provides for turning on and off of gene expression in response to an exogenously added agent or to an environmental or developmental stimulus. Promoters can also provide for tissue specific or developmental regulation. An isolated promoter sequence that is a strong promoter for heterologous genes is advantageous because it provides for a sufficient level of gene expression to allow for easy detection and selection of transformed cells and provides for a high level of gene expression when desired.
The promoter in an expression cassette of the invention can provide for overexpression of acetyl CoA of a plant acetyl CoA carboxylase gene or functional mutant thereof. Overexpression of the gene is that amount of gene expression that results in an increase in tolerance of the plant cells to an herbicide or that results in an increase in the total oil content of the cells. Overexpression of an acetyl CoA carboxylase gene is preferably about a 2- to 20-fold increase in expression of an acetyl CoA carboxylase over the expression level of the native acetyl CoA carboxylase. The promoter can also be inducible so that gene expression can be turned on or off by an exogenously added agent. For example, a bacterial promoter such as the P.sub.tac promoter can be induced to varying levels of gene expression depending on the level of isothiopropylgalactoside added to the transformed bacterial cells. It may also be preferable to combine the gene with a promoter that provides tissue specific expression or developmentally regulated gene expression in plants.
Specific promoters functional in plant cells include the 35S cauliflower mosaic virus promoter, nopaline synthase (NOS) promoter and the like. Currently, a preferred promoter for expression in monocots is the 35S cauliflower mosaic virus promoter.
An acetyl CoA carboxylase gene can be combined with the promoter by standard methods as described in Sambrook cited supra. Briefly, a plasmid containing a promoter such as the 35S cauliflower mosaic virus promoter can be constructed as described in Jefferson, Plant Molecular Biology Reporter 5:387 (1987) or obtained from Clontech Lab in Palo Alto, Calif. (e.g. pBI121 or pBI221). Typically these plasmids are constructed to provide for multiple cloning sites having specificity for different restriction enzymes downstream from the promoter. A gene for plant acetyl CoA carboxylase can be subcloned downstream from the promoter using restriction enzymes to ensure that the gene is inserted in proper orientation with respect to the promoter so that the gene can be expressed. In a preferred version, a maize acetyl CoA carboxylase is operably linked to a 35 S CaMV promoter in a plasmid such as pBI121 or pBI221. Once a plant acetyl CoA carboxylase gene is operably linked to a promoter and the plasmid, the expression cassette so formed can be subcloned into other plasmids or vectors.
Optional Sequences in the Expression Cassette
The expression cassette can also optionally contain other DNA sequences. The expression cassette can further be comprised of a chloroplast transit peptide sequence operably linked between a promoter and a plant acetyl CoA carboxylase gene. If the expression cassette is to be introduced into a plant cell, the expression cassette can also contain plant transcriptional termination and polyadenylation signals and translational signals linked to the 3' terminus of a plant acetyl CoA carboxylase gene. The expression cassette can also optionally be further comprised of a plasmid.
Because one site of action for biosynthetic pathways involving plant acetyl CoA carboxylase is the chloroplast, an expression cassette of the invention can be combined with a DNA sequence coding for a chloroplast transit peptide, if necessary. A chloroplast transit peptide is typically 40 to 70 amino acids in length and functions post-translationally to direct the protein to the chloroplast. The transit peptide is cleaved either during or just after import into the chloroplast to yield the mature protein. The complete copy of a gene encoding a plant acetyl CoA carboxylase may contain a chloroplast transit peptide sequence. In that case, it may not be necessary to combine an exogenously obtained chloroplast transit peptide sequence into the expression cassette.
Optionally, the DNA fragment encoding a transit peptide can be obtained from a variety of plant nuclear genes, so long as the products of the genes are expressed as preproteins comprising an amino terminal transit peptide and transported into chloroplast. Examples of plant gene products known to include such transit peptide sequences are the small subunit of ribulose bisphosphate carboxylase, ferredoxin, chlorophyll a/b binding protein, chloroplast ribosomal proteins encoded by nuclear genes, certain heatshock proteins, amino acid biosynthetic enzymes such as acetohydroxy acid synthase, 3-enolpyruvylphosphoshikimate synthase, dihydrodipicolinate synthase, and the like. Alternatively, the DNA fragment coding for the transit peptide may be chemically synthesized either wholly or in part from the known sequences of transit peptides such as those listed above.
Regardless of the source of the DNA fragment coding for the transit peptide, it should include a translation initiation codon and an amino acid sequence that is recognized by and will function properly in chloroplasts of the host plant. Attention should also be given to the amino acid sequence at the junction between the transit peptide and the plant acetyl CoA carboxylase enzyme where it is cleaved to yield the mature enzyme. Certain conserved amino acid sequences have been identified and may serve as a guideline. Precise fusion of the transit peptide coding sequence with the acetyl CoA carboxylase coding sequence may require manipulation of one or both DNA sequences to introduce, for example, a convenient restriction site. This may be accomplished by methods including site directed mutagenesis, insertion of chemically synthesized oligonucleotide linkers and the like.
Once obtained, the chloroplast transit peptide sequence can be appropriately linked to the promoter and a plant acetyl CoA carboxylase gene in an expression cassette using standard methods. Briefly, a plasmid containing a promoter functional in plant cells and having multiple cloning sites downstream can be constructed as described in Jefferson cited supra. The chloroplast transit peptide sequence can be inserted downstream from the promoter using restriction enzymes. A plant acetyl CoA carboxylase gene can then be inserted immediately downstream from and in frame with the 3' terminus of the chloroplast transit peptide sequence so that the chloroplast transit peptide is linked to the amino terminus of the plant acetyl CoA carboxylase. Once formed, the expression cassette can be subcloned into other plasmids or vectors.
When the expression cassette is to be introduced into a plant cell, the expression cassette can also optionally include 3' nontranslated plant regulatory DNA sequences. The 3' nontranslated regulatory DNA sequence preferably includes from about 300 to 1,000 nucleotide base pairs and contains plant transcriptional and translational termination sequence. Specific examples of 3' nontranslated regulatory DNA sequences functional in plant cells include about 500 base pairs of the 3' flanking DNA sequence of the pea ribulose biphosphate carboxylase small subunit E9 gene, the 3' flanking DNA sequence of the octopine synthase gene, and the 3' flanking DNA sequence of the nopaline synthase gene. These 3' nontranslated regulatory sequences can be obtained as described in An, Methods in Enzymology, 153:292 (1987) or are already present in plasmids available from commercial sources such as Clontech, Palo Alto, Calif. The 3' nontranslated regulatory sequences can be operably linked to the 3' terminus of a plant acetyl CoA carboxylase gene by standard methods.
An expression cassette of the invention can also be further comprised of a plasmid. Plasmid vectors included additional DNA sequences that provide for easy selection, amplification and transformation of the expression cassette in procaryotic and eukaryotic cells. The additional DNA sequences include origins of replication to provide for autonomous replication of the vector, selectable marker genes, preferably encoding antibiotic resistance, unique multiple cloning sites providing for multiple sites to insert DNA sequences or genes encoded in the expression cassette, and sequences that enhance transformation of prokaryotic and eukaryotic cells. The preferred vectors of the invention are plasmid vectors. The especially preferred vector is the pBI121 or pBI221 vector formed as described by Jefferson cited supra.
Another vector that is useful for expression in both plant and procaryotic cells is the binary Ti vector PGA582. This binary Ti vector has been previously characterized by An, cited supra., and is available from Dr. An. This binary Ti vector can be replicated in procaryotic bacteria such as E. coli and Agrobacterium. The Agrobacterium plasmid vectors can be used to transfer the expression cassette to plant cells. The binary Ti vectors preferably include the nopaline T DNA right and left borders to provide for efficient plant cell transformation, a selectable marker gene, unique multiple cloning sites in the T border regions, the colE1 replication of origin and a wide host range replicon. The binary Ti vectors carrying an expression cassette of the invention can be used to transform both prokaryotic and eukaryotic cells, but is preferably used to transform plant cells.
B. Method for Screening for Expression and/or Overexpression of a Plant Acetyl CoA Carboxylase Gene
A method for screening for expression or overexpression of a plant acetyl CoA carboxylase gene is also provided by the invention. Once formed, an expression cassette comprising an acetyl CoA carboxylase gene can be subcloned into a known expression vector. The screening method in the invention includes the steps of introducing an expression vector into a host cell and detecting and/or quantitating expression of a plant acetyl CoA carboxylase gene. This method of screening is useful to identify expression cassettes providing for an overexpression of a plant acetyl CoA carboxylase gene, antisense molecules that effectively inhibit acetyl CoA carboxylase synthesis, and expression of an acetyl CoA carboxylase in the chloroplast of a transformed plant cell.
Suitable known expression vectors include plasmids that autonomously replicate in prokaryotic and eukaryotic cells. Specific examples include plasmids such as the pBI121 or pBI221 plasmid constructed as described by Jefferson cited supra, a binary Ti vector such as PG582 as described by An cited supra, PUC119, or PBR322. The preferred expression system is a pBI121 or pBI221 plasmid.
An expression cassette of the invention can be subcloned into an expression vector by standard methods. The expression vector can then be introduced into prokaryotic or eukaryotic cells by standard methods including protoplast transformation, Agrobacterium mediated transformation, electroporation, microprojectiles and liposomes. The expression vector can be introduced into plant cells such as tobacco, Brassica, Black Mexican sweet corn, and Arabidopsis cells. The vector can also be introduced into procaryotic cells such as E. coli or Agrobacterium. Transformed cells can be selected typically using a selection marker encoded on the expression vector.
Transient expression of a plant acetyl CoA carboxylase gene can be detected and quantitated in the transformed cells. Gene expression can be quantitated by a quantitative Western blot using antibodies specific for the cloned acetyl CoA carboxylase or by detecting an increase in specific activity of the enzyme. The tissue and subcellular location of the cloned acetyl CoA carboxylase can be determined by immunochemical staining methods using antibodies specific for the cloned acetyl CoA carboxylase. Sensitivity of the cloned acetyl CoA carboxylase to herbicides can also be assessed. Expression cassettes providing for overexpression of a plant acetyl CoA carboxylase or acetyl CoA carboxylase tolerant to herbicides can then be used to transform monocot and/or dicot plant tissue cells and to regenerate transformed plants and seeds.
C. Method of Imparting Cyclohexanedione or Aryloxyphenoxypropanoic Acid Herbicide Tolerance to a Plant
The invention provides a method of conferring cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance to a plant. The method includes the steps of introducing an expression cassette comprising a gene coding for a plant acetyl CoA carboxylase or a functional mutant thereof operably linked to a promoter into the cells of plant tissue and expressing the gene in an amount effective to render the cells of the plant tissue substantially tolerant to herbicides. An effective amount of gene expression to render the cells of the plant tissue substantially tolerant to the herbicide depends on whether the gene codes for an unaltered acetyl CoA carboxylase gene or a mutant or altered form of the gene that is less sensitive to the herbicides. Expression of an unaltered plant acetyl CoA carboxylase gene in an effective amount is that amount that provides for a 2- to 50-fold increase in herbicide tolerance and preferably increases the amount of acetyl CoA carboxylase from at least about 2- to 20-fold over that amount of the native enzyme. An altered form of the enzyme can be expressed at levels comparable to that of the native enzyme or less if the altered form of the enzyme has higher specific activity. Acetyl CoA carboxylase substantially tolerant to herbicides is an enzyme that is tolerant of levels of herbicide which normally inhibit a native acetyl CoA carboxylase and preferably can function in concentrations of herbicide of about 2- to 20-fold greater than are toxic to the native enzyme.
Herbicide tolerance can be achieved by at least two methods including: 1) by increasing the level of gene expression of a native or unaltered acetyl CoA carboxylase gene; or 2) by introducing an altered gene coding for an acetyl CoA carboxylase that is less sensitive to herbicide inhibition. The level of gene expression can be increased by either combining a plant acetyl CoA carboxylase gene with a promoter that provides for a high level of gene expression such as the 35S CAMV promoter or by introducing the gene into the cells so that multiple copies of the gene are integrated into the transformed plant cells' genome. Formation of an expression cassette comprised of a plant acetyl CoA carboxylase gene operably linked to a promoter that can be expressed in an effective amount to confer herbicide tolerance has been described previously.
Most monocots, but not dicots, are sensitive to cyclohexanedione and/or aryloxyphenoxypropanoic acid herbicides. The preferred plant cells for introducing an expression cassette of the invention to achieve herbicide tolerance for the plant cells then are monocot plants. Monocot plants include corn, wheat, barley, sorghum, rice, and others. An expression cassette of the invention can be introduced by methods of transformation, especially effective for monocots including biolistic transformation of Type II embryogenic suspension cells as described by Gordon-Kamm et al. (1990), Fromm et al. (1990) and Walters et al. (1992) or by electroporation of type 1 embryogenic calluses described by D'Hafluin et al., The Plant Cell, 4:1495 (1992). Transformed cells can be selected for the presence of a selectable marker gene. Transient expression of a plant acetyl CoA carboxylase gene can be detected in the transgenic embryogenic calli using antibodies specific for the cloned plant acetyl CoA carboxylase. Transformed embryogenic calli can be used to generate transgenic plants that exhibit stable inheritance of either the altered acetyl CoA carboxylase gene or overexpression of the acetyl CoA carboxylase gene. Maize cell lines exhibiting satisfactory levels of tolerance to herbicide are put through a plant regeneration protocol to obtain mature maize plants and seeds expressing the tolerance traits such as described in D'Hafluin, cited supra., or An, cited supra. The plant regeneration protocol allows the development of somatic embryos and the subsequent growth of roots and shoots. To determine that the herbicide-tolerance trait is expressed in differentiated organs of the plant, and not solely in undifferentiated cell culture, regenerated plants are exposed to herbicide levels which will normally inhibit shoot and root formation and growth.
Mature maize plants are then obtained from maize cell lines that are known to express the trait. If possible, the regenerated plants are self-pollinated. Otherwise, pollen obtained from the regenerated plants is crossed to seed-grown plants of agronomically important inbred lines. Conversely, pollen from plants of these inbred lines is used to pollinate regenerated plants. The genetics of the trait are then characterized by evaluating the segregation of the trait in the first and later generation progeny. Stable inheritance of overexpression of a plant acetyl CoA carboxylase or a functional mutant of a plant acetyl CoA carboxylase conferring herbicide tolerance to the plant is achieved if the plants maintain herbicide tolerance for at least about three to six generations.
Seed from transformed monocot plants regenerated from transformed tissue cultures is grown in the field and self-pollinated to generate true breeding plants. Progenies from these plants become true breeding lines which are evaluated for herbicide tolerance in the field under a range of environmental conditions. Herbicide tolerance must be sufficient to protect the monocot plants at the maximum labeled delivery rate under field conditions which cause herbicides to be most active. Appropriate herbicide concentrations and methods of application are those which are known and have been developed for the cyclohexanedione and/or aryloxyphenoxypropanoic acid herbicides in question.
In a preferred version, an expression cassette comprised of a maize acetyl CoA carboxylase gene isolated from a maize cell line tolerant to sethoxydim and haloxyfop and linked to the 35S CaMV promoter is introduced into an herbicide sensitive monocot tissue using biolistic transformation. Transformed calli are selected and used to generate transgenic plants. Transformed calli and transgenic plants can be evaluated for tolerance to sethoxydim and haloxyfop and for stable inheritance of the tolerance trait.
D. Method for Altering the Oil Content in a Plant
The invention also provides a method of altering the oil content in a plant. The method include the steps of introducing an expression cassette comprising a gene coding for plant acetyl CoA carboxylase or functional mutant thereof operably linked to a promoter functional in a plant cell into the cells of plant tissue and expressing the gene in an amount effective to alter the oil content of the plant cell. An alteration in the oil content of a plant cell can include a change in the total oil content over that normally present in that type of plant cell, or a change in the type of oil from that normally present in the plant cell. Expression of the gene in an amount effective to alter the oil content of the gene depends on whether the gene codes for an unaltered acetyl CoA carboxylase or a mutant or altered form of the gene. Expression of an unaltered plant acetyl CoA carboxylase gene in an effective amount is that amount that may provide a change in the oil content of the cell from about 1.2- to 20-fold over that normally present in that plant cell, and preferably increases the amount of acetyl CoA carboxylase about 2- to 20-fold over that amount of the enzyme normally present in that plant cell. An altered form of the enzyme can be expressed at levels comparable to that of the native enzyme or less if the altered form of the enzyme has higher specific activity.
An alteration in oil content of the plant cells according to the method of the invention can be achieved in at least two ways including:
(1) an increase or decrease in expression of an unaltered plant acetyl CoA carboxylase gene; or
(2) by introducing an altered or functional mutant plant acetyl CoA carboxylase gene coding for an enzyme that exhibits a change in specific activity.
The level of gene expression of an unaltered plant acetyl CoA carboxylase gene can be increased by either combining an unaltered plant acetyl CoA carboxylase gene with a promoter that provides for a high level of gene expression, such as the 35S cauliflower mosaic virus or by introducing the expression cassette and/or selecting for plant cells having multiple copies of a plant acetyl CoA carboxylase gene integrated into the genome. A decrease in expression of an unaltered acetyl CoA carboxylase can be achieved by transformation with an ACCase antisense gene containing an expression cassette. When an altered or functional mutant plant acetyl CoA carboxylase gene codes for an enzyme that has an increase in specific activity, it may lead to an increase in total oil content of a plant cell even if the level of gene expression is comparable to that of the native enzyme. When an altered or functional mutant acetyl CoA carboxylase gene codes for an enzyme having a decrease in specific activity, it may lead to a decrease in the total oil content of the plant cell compared to that normally present.
An expression cassette as described above can be introduced into either monocots or dicots. Preferably, the expression cassette is introduced into dicot plants such as soybean, canola, and sunflower. An expression cassette can be introduced by standard methods including protoplast transformation, Agrobacterium-mediated transformation, microprojectiles, electroporation, and the like. Transformed cells or tissues can be selected for the presence of a selectable marker gene.
Transient expression of a plant acetyl CoA carboxylase gene can be detected in transformed cells or tissues by immunoreactivity with antibodies specific for the cloned acetyl CoA carboxylase. Overexpression of a plant acetyl CoA carboxylase can be detected by quantitative Western blots. A change in specific activity of the enzyme can be detected by measuring enzyme activity in the transformed cells. A change in total oil content can also be examined by standard methods, as described in Clark & Snyder, JACS, 66:1316 (1989).
Transgenic plants and seeds can be generated from transformed cells and tissues showing a change in oil content or in the amount or specific activity of a plant acetyl CoA carboxylase using standard methods. It is especially preferred that the oil content of the leaves, seeds, or fruits is increased.
In a preferred version, a maize acetyl CoA carboxylase gene is combined with a 35S cauliflower mosaic virus promoter in a vector such as pBI121 or pBI221 and introduced into soybean cells using the microprojectile method. Transformed soybean cells showing an increase in expression of acetyl CoA carboxylase of at least about 2-fold or at least a 1.2-fold increase in oil content are selected. Transformed soybean cells exhibiting overexpression of acetyl CoA carboxylase or showing an increase in total oil content are used to generate transgenic plants and seeds.
E. Method of Producing Plant Acetyl CoA Carboxylase
The invention also provides a method of producing plant acetyl CoA carboxylase in a host cell. The method includes the steps of introducing an expression cassette comprised of a gene encoding a plant acetyl CoA carboxylase or functional mutant thereof into a host cell and expressing the gene in an amount sufficient to allow for crystallization of the plant acetyl CoA carboxylase. An amount sufficient to allow for crystallization of a plant acetyl CoA carboxylase is about 20- to 100-fold increase over the amount of plant acetyl CoA carboxylase that can normally be purified from plant cells, preferably about 2 to 10 mg protein. Crystallized plant acetyl CoA carboxylase can be used to identify other herbicides that can bind to and inhibit acetyl CoA carboxylase function. In addition, the availability of large amounts of purified enzyme provides for screening of the efficacy of such herbicides.
An expression cassette can include a promoter that is functional in either a eukaryotic or prokaryotic cell. The expression cassette can be introduced into a prokaryotic cell such as E. coli, or a eukaryotic cell such as a plant or yeast. The preferred cell is a prokaryotic cell used routinely in producing recombinant proteins such as E. coli. The expression cassette can be introduced and transformed cells selected by standard methods.
The plant acetyl CoA carboxylase gene can be expressed in an prokaryotic cell until sufficient amount of the enzyme is produced so that it can be crystallized. Plant acetyl CoA carboxylase can be isolated from bacterial cells using standard methods, including those described in Example V. The purified acetyl CoA carboxylase can then be crystallized and characterized by standard methods.
EXAMPLE I
Identification of Herbicide Mechanism and Site of Action
The objective of this Example was to identify the mechanism whereby sethoxydim and/or haloxyfop inhibit fatty acid synthesis in maize. The results, reported in J. D. Burton et al., Biochem. Biophys. Res. Comm., 148, 1039 (Nov. 13, 1987), show that both sethoxydim and haloxyfop inhibit acetyl-coenzyme A carboxylase (ACCase) (EC 6.4.1.2) in maize chloroplasts.
A. Chemicals
Buffers and cofactors were purchased from Sigma Chemical Company (St. Louis, Mo.); [2-.sup.14 C]acetate was purchased from Research Products International; [2-.sup.14 C]pyruvate and [.sup.14 C]NaHCO.sub.3 were purchased from New England Nuclear; and [2-.sup.14 C]malonyl coenzyme A was purchased from Amersham. Sethoxydim was a gift from BASF (Parsippany, N.J.), and haloxyfop was provided by Dow Chemical USA (Midland, Mich.).
B. Plant Growth Conditions
Corn (Z. mays L., `B37.times.0h43`) seeds were germinated in darkness for 96 hr in vermiculite in an incubation chamber maintained at 30.degree. C., 80% RH. Seedlings were then transferred to a growth chamber with a 16 hr light (25.degree. C.) and an 8 hr dark (20.degree. C.) cycle, 90% relative humidity (RH). After greening 48 hr, seedlings were returned to the dark incubation chamber for 12 hr to deplete chloroplast starch reserves. Seedlings were harvested 6 days after planting. Pea (P. sativum L., `PI 9901-C`) seedlings were grown in vermiculite in a growth chamber with a 16 hr light (21.degree. C.) and 8 hr dark (16.degree. C.) cycle, 80% RH. Peas were harvested 10 to 13 days after planting. Black Mexican Sweet (BMS) corn suspension cultures were maintained in a supplemented Murashige-Skoog (MS) medium (C. E. Green, Hort. Sci., 12, 7-10 (1977 )), and subcultured weekly by 20-fold dilution of the suspension culture into fresh medium.
C. Chloroplast Isolation
Chloroplasts from corn and pea seedlings were isolated at 4.degree. C. (K. Cline et al., J. Biol. Chem., 260, 3691-3696 (1985)). Seedlings (50 g of shoots) were homogenized in 200 ml buffer A (50 mM HEPES-NaOH pH 7.5, 330 mM sorbitol, 0.1% w/v BSA, 1 mM MgCl.sub.2, 1 mM MnCl.sub.2, 2 mM EDTA, 5 mM isoascorbate, 1.3 mM glutathione) in an omnimixer (five, 3-sec bursts at full speed). The homogenate was filtered through six layers of cheesecloth and two layers of miracloth, and then centrifuged at 3000 g for 3 min with hand-braking. The pellet was gently resuspended in buffer A and layered onto a preformed linear Percoll gradient (50 mM HEPES-NaOH pH 7.5, 330 mM sorbitol, 1.9 mM isoascorbate, 1.08 mM glutathione, 0.1% w/v BSA, 50% Percoll) which was centrifuged at 3000 g for 20 min in a Sorvall HB-4 rotor. The lower band in the gradient, containing intact chloroplasts, was washed twice by gently resuspending it in 20 ml of buffer B (50 mM HEPES-NaOH, pH 7.5, and 330 mM sorbitol) followed by repelleting (3000 g, 5 min). The final pellet, consisting of intact chloroplasts, was resuspended in 2 to 3 ml of buffer B and stored on ice in the dark until use.
D. Fatty Acid Synthesis
[.sup.14 C]acetate and [.sup.14 C]pyruvate were used as precursors to measure fatty acid biosynthesis in isolated chloroplasts (B. Liedvogel et al., Planta, 169, 481-489 (1986)). [.sup.14 C]acetate incorporation was assayed in a 0.5 ml-volume containing: 50 mM HEPES-NaOH (pH 7.5), 330 mM sorbitol, 5 mM KH.sub.2 PO.sub.4, 10 mM NaHCO.sub.3, 1 mM MgCl.sub.2, 1 mM ATP, 0.1 mM CoA, 0.15 mM [.sup.14 C]acetate (3.33 mCi/mmol), and chloroplasts (20 to 50 .mu.g chlorophyll). [.sup.14 C]pyruvate incorporation into fatty acids was assayed in the same medium except that it included 2 mM TPP, 1 mM NAD.sup.+, 0.15 mM [.sup.14 C]-pyruvate (1.33 mCi/mmol), but no acetate. Assay suspensions were illuminated with 1400 .mu.E/m.sup.2.sec PAR at 25.degree. C. Assays were initiated by the addition of the labelled substrate and stopped by the addition of 0.5 ml of 40% KOH. To determine the incorporation of radiolabel into a non-polar (fatty acid) fraction, each treatment was saponified at 90.degree. C. for 30 min in capped vials (P. B. Hoj et al., Carlsberg Res. Commun., 47, 119-141 (1982)). The vials were acidified with 0.5 ml 40% H.sub.2 S0.sub.4, and carrier fatty acids (20 .mu.g each of C 14:0, C 16:0, and C 18:0) were added. The assay mixture was extracted twice with 4 ml hexane. The extracts were combined, dried under N.sub.2, and redissolved in 0.3 ml hexane. Aliquots (50 .mu.l) were counted for radioactivity by liquid scintillation spectrometry.
Incorporation of [.sup.14 C]malonyl-Coenzyme A into fatty acids (P. B. Hoj et al., supra; and J. B. Ohlrogge et al., Proc. Natl. Acad. Sci. USA, 76, 1194-1198 (1979)) was assayed using cell-free preparations from BMS tissue culture. Cells harvested during logarithmic growth phase were frozen in liquid nitrogen, ground with a mortar and pestle, and thawed in a medium containing: 0.1M HEPES-KOH, pH 7.5; 0.3M glycerol, and 5 mM DTT (buffer:tissue, 2:1, v/w). The homogenate was centrifuged at 12,000 g for 20 min. The supernatant was filtered through miracloth and centrifuged (125,000 g) for 60 min and then filtered through miracloth and assayed. Assays were conducted at 25.degree. C. in a 0.4 ml volume containing: 1.0 mM ATP, 0.32 mM NADPH, 0.38 mM NADH, 25 .mu.M CoA, 10 .mu.M acetyl-CoA, 25 .mu.g acyl-carrier protein, and 12 .mu.M malonyl-CoA (11.54 .mu.Ci/.mu.mol). Reactions were initiated by addition of [ .sup.14 C]malonyl CoA and stopped by addition of 0.4 ml 40% KOH. Label incorporation into fatty acids was determined as above. Chlorophyll (D. I. Arnon, Plant Physiol., 24, 1-15 (1949)) and protein (P. K. Smith et al., Anal. Biochem., 150, 76-85 (1985)) were determined as described therein.
E. Acetyl-Coenzyme A Carboxylase (ACCase) Activity
Maize chloroplasts, isolated as described above, were suspended in buffer C (0.1M Tricine-KOH, pH 8.0; 0.3M glycerol, and 1 mM DTT) and homogenized in a glass tissue homogenizer. The disrupted chloroplast fraction was centrifuged at 16,000 g for 15 min. The supernatant was desalted on a Sephadex G-25 column (1.5.times.5 cm equilibrated with 0.1M Tricine-KOH, pH 8.0; and 0.3M glycerol) and assayed directly. ACCase activity (B. J. Nikolau et al., Arch. Biochem. Biophys., 211, 605-612 (1981)) was assayed at 30.degree. C. in a 0.2 ml volume which contained 1 mM ATP, 3 mM acetyl coenzyme A, 2.5 mM MgCl.sub.2, 50 mM KCl, 0.5 mM DTT, and 15 mM [.sup.14 C]NaHCO.sub.3 (0.17 mCi/mmol). Reactions were initiated by addition of acetyl coenzyme A and stopped by addition of 25 .mu.l of 12N HCl. Product formation was determined by the radioactivity found in an acid stable fraction by liquid scintillation spectrometry. Enzyme activity was linear for 15 min.
F. Results
To probe for the site of herbicidal activity of sethoxydim and haloxyfop, labelled acetate, pyruvate, and malonyl-CoA were used individually as precursors for fatty acid synthesis. Isolated chloroplasts from corn seedlings incorporated [.sup.14 C]acetate and [.sup.14 C]pyruvate into a non-polar fraction (fatty acids). Acetate incorporation was linear for 30 min after a 5 min lag period, and dependent upon the addition of free acetyl coenzyme A. Addition of either 10 .mu.M sethoxydim or 1 .mu.M haloxyfop inhibited [.sup.14 C]acetate incorporation into fatty acids by 90% and 89%, respectively, as shown in Table I, below. Sethoxydim (10 .mu.M) and haloxyfop (1 .mu.M) also inhibited the incorporation of [.sup.14 C]pyruvate into fatty acids by 98% and 99%, respectively.
The effect of 10 .mu.M sethoxydim and 1 .mu.M haloxyfop on [.sup.14 C]malonyl-CoA incorporation into fatty acids was determined using cell-free extracts from corn suspension cultures. Neither sethoxydim (10 .mu.M) nor haloxyfop (1 .mu.M) inhibited fatty acid synthetase activity. Thus, both herbicides inhibited fatty acid synthesis in intact chloroplasts from corn seedlings with either acetate or pyruvate as a precursor, but did not inhibit incorporation of malonyl-CoA into fatty acids. This suggests that ACCase which catalyzes the formation of malonyl-CoA is the site of action of these herbicides.
EXAMPLE II
Selection and Characterization of Herbicide-tolerant Cell Lines
A selection protocol to identify and isolate herbicide-tolerant maize cells was developed to minimize the adverse effects of high herbicide concentrations on somatic embryo development and plant regeneration capacity. The procedure involved exposing tissue to gradually increasing concentrations of herbicide beginning with a sethoxydim concentration representing 1/20th of lethal dose and doubling the herbicide concentration at approximately two-week intervals until the lethal dose (10 .mu.M sethoxydim) was reached. In this way, the herbicide was allowed to take effect slowly with continuous selection pressure, thus permitting herbicide-tolerant cells to accumulate over time while not affecting the potential for plant regeneration.
A. Selection of a Sethoxydim-Tolerant Cell Line
Many selections were carried out utilizing the selection protocol described in the preceding paragraph. The selection of one such sethoxydim-tolerant cell line that was identified and characterized is described below in detail.
Approximately 100 grams of vigorously growing, regenerable, friable, embryogenic maize callus tissue established from an F.sub.1 immature embryo resulting from the cross A188.times.B73 were transferred to agar-solidified maintenance medium (Armstrong and Green, Planta, 164,207 (1985)) in petri plates containing 0.5 .mu.M sethoxydim (BASF) (Parsippany, N.J.). This callus line was designated 2167-9/2160-154. Forty plates were prepared and five clumps of callus tissue weighing about 0.5 grams each were placed on each plate. The 0.5 .mu.M sethoxydim concentration was chosen from growth inhibition studies to provide less than 10-20% growth inhibition during the first two weeks of herbicide exposure. After 14 days, 0.25-0.5 g pieces of tissue showing vigorous growth rate and retention of embryogenic morphology (i.e., presence of somatic embryos) were subcultured on fresh medium containing 1.0 .mu.M sethoxydim. Eighty plates containing five pieces of tissue per plate were prepared. For each subsequent transfer, all callus tissue showing growth and somatic embryo forming ability was placed on fresh media containing a two-fold increased sethoxydim concentration. Therefore, callus was transferred at two-week intervals to petri plates containing 0.5, 1.0, 2.0, 5.0 and 10.0 .mu.M sethoxydim. During the course of the selection process, the total number of lines decreased as the herbicide-mediated growth inhibition became more intense. Cell lines exhibiting growth on 10 .mu.M sethoxydim were designated as herbicide-tolerant and given an identification number. Two sethoxydim-tolerant lines were recovered that exhibited uninhibited growth at 10 .mu.M sethoxydim. These lines were designated 2167-9/2160-154 S-1 and 2167-9/2160-154 S-2.
B. Characterization of Herbicide-Tolerant Maize Cell Line 2167-9/2160-154 S-2
Tolerant cell line 2167-9/2160-154 S-2 ("S-2") was characterized to evaluate: (1) the magnitude of sethoxydim tolerance; (2) cross-tolerance of haloxyfop; and (3) the biochemical basis for the tolerance.
Callus tissue from S-2 that had been maintained on 10 .mu.M sethoxydim was transferred to media containing up to 100 .mu.M sethoxydim. One-half gram of S-2 tissue was plated on a 7 cm filter paper as a lawn overlaying 50 ml agar-solidified culture medium containing 0, 0.5, 1.0, 2.0, 5.0, 10.0, 50.0 and 100 .mu.M sethoxydim, and cultured for two weeks. Control cell line 2167-9/2160-154 was plated similarly on medium containing the same levels of sethoxydim. The results of this study are summarized in FIG. 2. The control cell line growth after two weeks was inhibited 50% at 1 .mu.M sethoxydim. Growth of S-2 was not inhibited at 100 .mu.M sethoxydim, indicating that S-2 was at least 100-fold more tolerant than the control callus line.
Growth of S-2 was inhibited with 0.65 .mu.M haloxyfop, whereas the control cell line was inhibited 50% with 0.02 .mu.M, indicating approximately a 30-fold increase in tolerance.
C. Acetyl-Coenzyme A Carboxylase (ACCase) Activity of Maize Cell Line S-2
Assays were conducted to determine if ACCase extracted from cell line S-2 was altered with respect to herbicide activity. ACCase activity of control tissue was 50% inhibited either by 1.5 .mu.M sethoxydim, or by 0.25 .mu.M haloxyfop. ACCase activity of S-2 tissue was inhibited 50% either by 70 .mu.M sethoxydim, or by 1.8 .mu.M haloxyfop, indicating at least 40-fold and 7-fold decreases in herbicide sensitivity on concentration basis, respectively.
EXAMPLE III
Plant Regeneration and Production of Herbicide-Tolerant Seed
A. Plant Regeneration Protocol
Sixteen ca. 150 mg clumps of S-2 callus were transferred per 25.times.100 mm petri plate containing agar-solidified N6 basal salts and 6% sucrose and incubated 7-14 days in low light (20 .mu.E m.sup.-2 s.sup.-1). Several plates containing callus on plant regeneration medium were prepared. Callus was transferred to agar-solidified Murashige-Skoog (MS) medium without hormones and incubated in high intensity light (200 .mu.E m-2 s.sup.-1) for shoot elongation. Developing plants (1-3 cm long) were isolated from the callus surface and transferred to magenta boxes containing agar-solidified MS salts, 2% sucrose with no hormones for two weeks of further growth. When plants reached the 2-3 leaf stage, they were transplanted to peat pots containing potting soil, and were incubated in the growth room until growing stably. Surviving plants were transferred to soil in 4" diameter plastic pots and grown in the greenhouse.
B. Expression of Herbicide Tolerance in Plants Regenerated from S-2 Callus Tissue
Groups of eight control (2167-9/2160-154 unselected) and eight S-2 plants were sprayed with either 0.0, 0.01, 0.05, 0.11, 0.22 or 0.44 kg/ha sethoxydim to determine whole plant sethoxydim-tolerance of greenhouse-grown plants. Control plants were killed by 0.05 kg/ha or more sethoxydim. Plants regenerated from the S-2 cell line survived the 0.44 kg/ha sethoxydim treatment, indicating that S-2 plants exhibit at least 20-fold more tolerance of sethoxydim than control. FIG. 3 shows the growth response of the regenerated plants seven days after treatment with 0.44 kg/ha sethoxydim. As shown in FIG. 4, shoot height of regenerated S-2 plants was only slightly reduced 14 days after treatment with 0.44 kg/ha sethoxydim.
C. Seed Production from S-2 Plants
Plants surviving sethoxydim treatments of up to 0.44 kg/ha were transplanted to the genetics plot on the University of Minnesota campus, St. Paul, Minn. Additional S-2 plants were transplanted to the field that had not been sprayed. Sixty-five 2167-9/2160-154 control plants and ninety-five S-2 plants were grown to maturity in the field. Plants were either self-pollinated or cross-pollinated to inbred maize lines A188, A619, A641, A661, A665, B37, B73, R806, and W153R. Control seed were produced by selfing 2167-9/2160-154 regenerated plants, or by crossing them with the inbreds listed above.
D. Expression of Herbicide Tolerance in Progeny of Regenerated Plants
Seeds obtained by the crossing procedure described above were viable and germinated normally. Seeds from thirty S-2 selfed plants and fifteen 2167-9/2160-154 control plants were planted in 25.times.50 cm trays of soil (28 seeds from each plant in one tray) and grown in the greenhouse. Seedlings at the 3-4 leaf stage were treated with 0.1, 0.44, and 1.1 kg/ha sethoxydim and evaluated for visual herbicide damage and shoot height. Based on visual rating of herbicide damage two weeks after treatment, selfed progeny of S-2 plants segregated approximately 1:2:1 for healthy, uninjured plants: to plants showing partial injury: to dead plants, respectively, at 0.44 and 1.1 kg/ha sethoxydim treatments. All control progeny of 2167-9/2160-154 control plants were killed by 0.1 kg/ha and greater levels of sethoxydim. These results demonstrate dominant expression of sethoxydim tolerance indicating that sethoxydim tolerance in S-2 plants is a heritable trait. Similar tests were conducted on progeny of S-2 plants crossed to the other inbreds. In all cases, these test cross progeny treated with 0.44 kg/ha sethoxydim segregated 1:1 for growing shoots versus dead shoots whether S-2 plants were used as male or female parents. These results confirm that sethoxydim tolerance is controlled by a single dominant nuclear gene. In all cases, control plants crossed to the other inbreds were killed and therefore sethoxydim-sensitive.
E. Method for Obtaining Uniform Herbicide-Tolerant Seed
Progeny of S-2 plants surviving sethoxydim treatments of 0.44 and 1.1 kg/ha and showing no herbicide injury were transferred to the greenhouse and grown to maturity. These plants may be selfed and their progeny evaluated for sethoxydim and haloxyfop tolerance to identify pure breeding herbicide-tolerant maize lines.
Progeny of S-2 plants crossed to inbred lines and exhibiting sethoxydim tolerance may be recurrently backcrossed to the same inbreds. Progeny of each cross may be screened for sethoxydim-tolerance, and tolerant plants grown to maturity and again crossed to the recurrent parent. After six or seven cycles of backcrossing, sethoxydim-tolerant plants may be selfed and progeny screened for tolerance to produce homozygous sethoxydim tolerant maize inbreds.
EXAMPLE IV
Selection of Additional Herbicide-Tolerant Maize Cell Lines
One primarily sethoxydim-tolerant maize cell line, 2167-9/2160-154 S-1, and two haloxyfop-tolerant maize cell lines, 2167-9/2160-154 H-1 and 2167-9/2160-154 H-2, were selected and characterized as follows:
A. Selection of Maize Cell Line 2167-9/2160-154 S-1
Maize cell line 2167-9/2160-154 S-1 was selected from maize cell culture using the protocol described in detail above for the selection of Line 2167-9/2160-154 S-2. Approximately 70 plants were regenerated from Line 2167-9/2160-154 S-1, and either self-pollinated or cross-pollinated to the inbred maize lines A188, A619, A641, A661, A665, B37, B73, R806, and W153R.
B. Selection of Maize Cell Line 2167-9/2160-154 H-1
Line 2167-9/2160-154 H-1 was selected from maize cell culture using a similar protocol described in detail above except maize callus tissue was selected using the herbicide haloxyfop. Maize callus tissue was initially plated on 0.01 .mu.M haloxyfop. At two-week intervals, surviving tissue was subcultured onto 0.05, 0.10 and 0.20 .mu.M haloxyfop. Approximately 50 plants were regenerated from Line 2167-9/2160-154 H-1, and were self-pollinated.
C. Selection of Maize Cell Line 2167-9/2160-154 H-2
Line 2167-9/2160-154 H-2 was selected from maize cell culture using a similar protocol described in detail for line 2167-9/2160-154 H-1. No plants have been successfully regenerated from this line.
D. Characterization of Lines 2167-9/2160-154 S-1, H-1 and H-2
The tolerant callus cultures were characterized to determine the magnitude of sethoxydim and haloxyfop tolerance. Callus tissue from these lines was evaluated in experiments as described above in the characterization of line 2167-9/2160-154 S-2. Table II summarizes the results of these studies. Line 2167-9/2160-154 S-1 and Line 2167-9/2160-154 H-2 showed a four-fold increase in haloxyfop tolerance, while Line 2167-9/2160-154 H-1 exhibited approximately a 60-fold increase in haloxyfop tolerance. Neither haloxyfop selected line showed a significant degree of sethoxydim tolerance, while the sethoxydim selected line S-1 exhibited approximately a 100-fold increase in sethoxydim tolerance.
E. Herbicide Inhibition of Acetyl Coenzyme A Carboxylase of Maize Cell Lines S-1, H-1 and H-2
Acetyl Coenzyme A Carboxylase (ACCase) was extracted from cell lines S-1, H-1 and H-2 and assayed as described in detail for maize cell line S-2, above. Table III below summarizes the results of these studies. The ACCase from line S-1 was more tolerant of both sethoxydim and haloxyfop, while the ACCase from line H-1 was more tolerant of haloxyfop, but not of sethoxydim. The ACCase from line H-2 showed no difference from the unselected parent line 2167-9/2160-154 in sensitivity to either herbicide.
However, cell line H-2 exhibited approximately a five-fold higher level of ACCase activity as compared to the unselected parent line 2167-9/2160-154. Thus, selection for sethoxydim or haloxyfop tolerance resulted in a less sensitive ACCase in cell line S-1 and H-1, as well as a higher level of ACCase activity in cell line H-2.
Deposit of Seeds
Seeds from representative S-2 plants (Ex. III (B)) and H-1 plants (Ex. IV(B)) have been deposited with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md. 20852 USA on Oct. 25, 1988 and assigned accession numbers ATCC 40507, and ATCC 40508, respectively.
EXAMPLE V
Formation of cDNA Clones Encoding ACCase
A. ACCase Purification
The acetyl CoA carboxylase enzyme was isolated and purified from plant tissues and characterized. The purified enzyme was used to generate antibody reagents useful in identifying cDNA clones encoding the gene or portions of the gene for ACCase.
ACCase was extracted from frozen shoots of 7-d-old maize (Zea Mays L. inbred A619 ro B73) seedlings grown in a growth chamber (24.degree. C., 90% RH, 16-h daylength at 210 .mu.E m.sup.-2 s.sup.-1). The outermost leaf and blade were removed and the remainder of the shoot was frozen in liquid N.sub.2. Embryos and endosperm tissue from developing kernels were harvested from field-grown ears at 36 to 40 days after pollination (DAP). Black Mexican Sweetcorn (BMS) maize suspension cells were obtained from cultures as previously described (Parker et al., 1990b). Tissues were stored in liquid N.sub.2 until used.
Extraction and purification steps were performed at 0.degree. to 4.degree. C. Crude extracts of leaf, bundle sheath strands, embryo, endosperm, and BMs cells were prepared from frozen tissue as described by Parker et al. (1990a), except that extraction buffer contained 0.1M Tricine-KOH, pH 8.3, 0.3M glycerol, 5 mM DTT, 2 mM Na.sub.2 EDTA, and 0.5 mM phenyl methonyl sulfonyl fluoride (PMSF). Triton X-100 (0.01% v/v) was added to bundle sheath strand extracts and to some whole leaf extracts. For some experiments, additional protease inhibitors (leupeptin, 2 .mu.g mL; pepstatin A, 100 .mu.g mL.sup.-1 ; benzamidine, 1 mM; .epsilon.-amino-n-caproic acid, 5 mM; and soybean trypsin inhibitor, 10 .mu.g mL.sup.-1) were included. Filtered homogenates were centrifuged 20 min at 30,000 g. A portion of the crude supernatant fraction was immediately boiled 5 min in 1 vol SDS sample buffer (Parker et al., 1990b) for SDS-PAGE analysis; the remainder was desalted on a 10-mL Sephadex G-25 column into extraction buffer minus PMSF.
ACCase was purified from the crude extract supernatant in four steps. This fraction was brought to 30% saturation with solid (NH.sub.4).sub.2 SO.sub.4, stirred 15 min, and centrifuged 20 min at 20,000 g. The supernatant was then brought to 40% saturation with (NH.sub.4).sub.2 SO.sub.4 solution, stirred 30 min, and centrifuged. The pellet was dissolved in 5 mL extraction buffer, microfuged 5 min, and the resulting supernatant was applied to a Sephacryl S-300 gel filtration column (Pharmacia; 2.5.times.46 cm) equilibrated with S-300 buffer (0.1M Tricine-KOH, pH 8.3, 0.5M glycerol, 0.5 mM DTT, 2 mM Na50 mM KCI). In later experiments a Sephacryl S-400 column was used. Fractions (2.5 mL) were eluted at 0.75 mL min.sup.-1. ACCase activity eluted shortly after the void A.sub.280 peak (V.sub.0 =75 mL). Active fractions were pooled, brought to 4.25 mM MgCl.sub.2 (from a 0.5M solution), and applied at 0.2 mL min.sup.-1 to a Blue Sepharose CL-6B (Pharmacia; 1.5.times.15 cm) equilibrated with Blue sepharose buffer (S-300 buffer containing 4.25 mM MgCl.sub.2 and 10 mM NaHCO.sub.3). The column was washed overnight with 150 mL buffer (0.45 mL min.sup.-1). ACCase activity was then eluted with 50 mL buffer plus 10 mM ATP (0.45 mL min.sup.-1). Active fractions were pooled and applied to an FPLC Mono-Q HR 5/5 anion-exchange column (Pharmacia) equilibrated with S-300 buffer minus KCl. The column was washed with 30 mL S-300 buffer minus KCl and then with a 48-mL, 0 to 500 mM KCl gradient in S-300 buffer (0.25 mL min.sup.-1). Fractions (1 mL) from the two peaks of ACCase activity were pooled separately. All purification fractions were desalted into S-300 buffer and assayed for ACCase activity and protein.
ACCase was also analyzed from mesophyll chloroplasts and bundle sheath strands. Mesophyll chloroplasts from homogenates of 7- to 8-d-old seedlings that were kept in the dark 24 h prior to harvesting were isolated on a linear Percoll gradient according to Burton et al. (1989), except that buffers contained 0.6M sorbitol and centrifugation g-forces were reduced by 25%. Intact chloroplasts were taken from the discrete lower green band present after Percoll gradient centrifugation (Morioux and Douce, 1981). Pelleted chloroplasts were lysed by resuspending them in ACCase extraction buffer plus PMSF and 0.01% (v/v) Triton X-100. Bundle sheath strands were obtained from the original leaf homogenate material retained on a 70-.mu.m filter after re-homogenizing the retentate five times in a total of 2L buffer. Triton X-treated, desalted leaf, mesophyll chloroplast, and bundle sheath strand extracts were assayed for activities of Rubisco (Zhu and Jensen, 1991), NADP-dependent malate dehydrogenase (Hatch and Slack, 1969), phosphoenolpyruvate carboxylase (Leegood and Walker, 1983), catalase (Worthington Biochemicals, 1972), and fumarase (Hill and Bradshaw, 1969), and for total chlorophyll (Arnon, 1949). Mesophyll chloroplast preparations were judged to be relatively free of contamination by bundle sheath chloroplasts because they contained 3-fold greater NADP-dependent malate dehydrogenase and one-tenth as much Rubisco activity (mg.sup.-1 chlorophyll) than bundle sheath strand extracts. Mesophyll chloroplast preparations also contained .ltoreq.2.6% as much catalase, fumarase, and phosphoenolpyruvate carboxylase activities (mg.sup.-1 chlorophyll) as did whole-leaf extracts, indicating they were relatively free of peroxisomal, mitochondrial, or cytoplasmic components.
ACCase activity as measured by acetyl-CoA-dependent H.sup.14 CO.sub.3 -(ICN, 2.07 GBq mmol.sup.-1) incorporation into acid-stable product previously shown to be malonyl-CoA (Burton et al., 1989). Assays of desalted purification fractions or crude, desalted tissue extracts contained up to 50 and 25% (v/v) enzyme, respectively. In some experiments methylcrotonyl-CoA or propionyl-CoA were substituted for acetyl-CoA (Wurtele and Nikolau, 1990). Avidin (10 U mL.sup.-1) was included in some assays. Herbicide inhibition assays contained 1% (v/v) ethanol plus or minus 1 .mu.M haloxyfop (2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid, Dow Chemical Co. analytical grade racemic mixture) or 10 .mu.M sethoxydim (2-[1[(ethoxylmino)butyl]-5-[2-(ethylthio)-propyl]-3-hydroxy-2-cyclohexene -1-one, Li salt, BASF Corp. technical grade). Data are means plus standard error of three assays.
Protein concentrations were determined in duplicate with the Bio-Rad Coomassie blue dye-binding assay as described by the manufacturer, using BSA as the standard.
Centrifuged crude extracts and proteins in purification fractions and immunoprecipitation supernatants were separated by SDS-PAGE in 6 or 7.5% gels as previously described (Parker et al., 1990b). Purification fractions were precipitated in 10% (v/v) TCA, washed with 80% (v/v) acetone, and air-dried 10 min prior to electrophoresis. Proteins in gels were stained with silver (Heukeshoven and Dernick, 1985). High molecular weight protein standards for SDS-PAGE (Pharmacia) were used to estimate polypeptide masses.
The four-step purification procedure shown in Table IV typically yielded 30 to 190 .mu.g of highly purified ACCase from 50 g (fresh weight) of maize inbred A619 or B73 seedling leaves. ACCase activity in the crude supernatant fraction precipitated between 30 and 40% saturation with (NH.sub.4).sub.2 SO.sub.4, which appeared to increase total ACCase activity approximately 38%. Crude extract components might have depressed the reaction rate shown in Table IV because the assay mixture contained 50% enzyme (v/v). In tests of fractions from another purification, enzyme velocity was proportional to enzyme concentration in assay mixtures containing up to 25% (v/v) crude extract, but 50% (v/v) mixtures were not tested. ACCase activity eluted from the Sephacryl S-300 gel filtration column slightly after the green void peak. Approximately 56% of the S-300 fraction ACCase activity was recovered from the Blue Sepharose column, primarily in the initial ATP-containing fractions (12.5 mL). Both 10 mM NaHCO.sub.3 and 4.25 mM MgCl.sub.2 (1- and 0.85-fold standard assay concentrations, respectively) were included in the Blue Sepharose buffer because they improved the total and specific ACCase activity remaining after batch absorption to Blue Sepharose beads, elution with ATP, and desalting into extraction buffer minus PMSF. Neither NaHCO.sub.3 nor MgCl.sub.2 improved enzyme stability of crude extracts. Mono-Q anion-exchange chromatography resulted in separation of two ACCase activity peaks which eluted at approximately 210 mM (designated ACCase II) and 250 mM KCl (designated ACCase I), as previously observed for a hybrid maize variety (Howard and Ridley, 1990). ACCase I comprised about 85% of the total activity recovered from the column (29% of the original crude extract activity) and had high specific activity (Table IV). The specific activity of ACCase II was less than 30% that of ACCase I (data not shown). Both activities were inhibited >90% by avidin, as previously reported (Howard and Ridley, 1990). The mass of native ACCase I was estimated to be approximately 490 kD by gel filtration on Superose 6 (data not shown).
B. Formation and Specificity of Antibodies to ACCase
Antibodies are sensitive reagents that allow for the identification of gene products from cDNA and other cloned genes. Antibodies to purified ACCase were prepared and used to screen for cDNA clones encoding all or a portion of a gene for ACCase.
Antiserum to maize ACCase was obtained by immunizing a female New Zealand White rabbit. An intramuscular injection of 100 .mu.g of Mono-Q-purified, SDS-denatured ACCase I in Freund's complete adjuvant was followed by subcutaneous injections of 20 to 100 .mu.g of gel-purified ACCase I polypeptide in acrylamide plus incomplete adjuvant every 4 to 6 weeks, for a total of six injections. Serum was stored at -20.degree. C. in 0.02% (w/v) NaN.sub.3.
For Western blots, proteins in SDS gels were electrophoretically transferred to Immobilon (Millipore; Pareter et al. (1990b)) for 1 hr at 20 V in a Bio-Rad Transphor semi-dry blotter and then stained with Ponceau S (Harlow and Lane, 1988). Destained blots were blocked with Tris-buffered saline plus 0.5% (v/v) Tween-20 (Bio-Rad), and 10% (w/v) bovine serum (for antiserum blots only). ACCase and biotinylated proteins were detected with immune serum (1/10,000) plus goat anti-rabbit IgG-alkaline phosphatase conjugate or with avidin-alkaline phosphatase (Parker et al., 1990b). Blots were repeated at least three times.
For immunoprecipitations, equal ACCase activities (0.58 nmol min.sup.-1) in crude extracts were desalted into S-300 buffer containing 0.1M KCl and incubated 1 hr at 25.degree. C. with 16 .mu.L buffer or with 16 .mu.L serum consisting of 0 to 100% ACCase antiserum in preimmune serum. Immune complexes were incubated 1 hr at 25.degree. C. with a 2-fold (IgG binding) excess of Protein A-agarose and then microfuged 5 minutes to obtain immunoprecipitation supernatant fractions. ACCase activity of supernatants was expressed as a percent of the 100% preimmune serum control. Data are means plus SE of three replicate assays for each of two sets of extracts.
Western blots and silver-stained gels of purification fractions separated by 7.5% SDS-PAGE showed that neither ACCase I nor ACCase II Mono-Q fractions contained biotinylated polypeptides smaller than 212 kD. A polypeptide >212 kD was the primary protein component of the ACCase I Mono-Q fragment (FIG. 5). The ACCase II fraction contained a biotinylated polypeptide >212 kD and a large amount of a 55 kD non-biotinylated polypeptide. Fractions from earlier purification steps contained additional biotinylated proteins of approximately 74, 75, and 125 kD (FIG. 5).
To better compare the biotinylated polypeptides >212 kD in ACCase fractions I and II, we used 6% SDS-PAGE, which showed that the mass of ACCase II was approximately 8 kD less than that of ACCase I. Molecular masses were estimated to be 219 kD (ACCase II) and 227 kD (ACCase I), based on comparisons with polypeptide standards and the observation (N. R.Palosaari, personal communication) that, on Phastgels (Pharmacia), ACCase I polypeptide was slightly smaller than dodecameric horse spleen ferritin (238 kD; Heusterspreute and Crichton, 1981). All purification fractions through the Blue Sepharose step contained both ACCase I and II polypeptides. Rapid extraction of leaves in buffer containing five additional protease inhibitors, or a 4 hr incubation of extracts at 25.degree. C., had little or no effect on the relative amounts of the two polypeptides, suggesting that ACCase II is not a breakdown product of ACCase I.
Antiserum to ACCase I strongly recognized the ACCase I polypeptide in crude extracts and showed little or no recognition of ACCase II polypeptides. No bands were recognized by preimmune serum. Assuming that avidin binds similarly to ACCase I and II polypeptides, it appears that the amount of ACCase II on the Western blot was slightly less than the amount of ACCase I. However, the relative staining with antibody compared to avidin indicated that the antibody had significantly less affinity for ACCase II than ACCase I.
To determine whether the same ACCase polypeptides were expressed in different maize cell types, proteins in mesophyll chloroplasts and crude extracts of leaves, endosperm tissue, embryos, and BMS cells were separated by SDS-PAGE. All preparations contained a predominant biotinylated polypeptide of approximately 227 kD (ACCase I) that was strongly recognized by ACCase antiserum or avidin. Similar 227 kD band densities were observed when gel lanes were probed with either avidin or ACCase antiserum. The 219 kD ACCase II polypeptide was readily detected in leaves only by avidin binding, but was in low abundance or not detected in extracts from other tissues. Only the 227 kD ACCase I polypeptide was detected in purified mesophyll chloroplasts, however, suggesting that the 219 kD ACCase II polypeptide is localized elsewhere in mesophyll cells or in other cell types of young leaves. ACCase activity and a >212 kD biotinylated polypeptide(s) were also found in bundle sheath strand extracts, but low yields prevented us from determining the type of ACCase present. Two other major biotinylated polypeptides of 75 and 74 kD were found in all tissues. Other non-biotinylated proteins of 66 kD (faint) and 55 kD were also recognized by ACCase antiserum. The 55 kD polypeptide was only found in leaves; it was also present in both ACCase I and II Mono Q fractions (FIG. 5) and was identified as the Rubisco large subunit based on its comigration with protein immunoprecipitated by spinach Rubisco antiserum.
ACCase antiserum immunoprecipitated at least 75% of ACCase activity from crude, desalted extracts of leaves, endosperm tissue, embryos, and BMS cells (FIG. 6), indicating that most of the ACCase activity in these tissues is immunologically related to the ACCase I polypeptide of leaves. Less activity was precipitated from leaves (75%) than from other tissues, particularly embryos (98%). Compared to immunoprecipitation, inhibition of ACCase activity by antiserum in solution was less than 20% as effective in reducing ACCase activity.
The substrate specificity of ACCase from different purification fractions was examined to compare [.sup.14 C]HCO.sub.3.sup.- -incorporation in the presence of different acyl-CoA substrates. Both ACCase I and II utilized propionyl-Co-A 40 to 50% as rapidly as acetyl-CoA at 50 to 500 .mu.M substrate (data not shown) even though they contained no biotinylated polypeptides (FIG. 5) the size of known propionyl CoA carboxylases (70 to 75 kD; see Wurtele and Nikolau, 1990). Activities in the presence of both acetyl-CoA and propionyl-CoA (250 or 500 .mu.M each) were approximately 90 (ACCase I) to 130% (ACCase II) that of 500 .mu.M acetyl-CoA alone. Crude leaf extracts utilized propionyl-CoA and methylcrotonyl-CoA 60% as efficiently as acetyl-CoA. Methylcrotonyl CoA carboxylase activity was reduced 85% by gel filtration and was completely removed by Blue Sepharose affinity chromatography.
ACCase I and II differed significantly in their inhibition by either haloxyfop or sethoxydim (FIG. 7). Acetyl-CoA or propionyl-CoA-dependent H.sup.14 CO.sub.3.sup.- -incorporation by ACCase I was strongly inhibited (65 to 80%) by 1 .mu.M haloxyfop or 10 .mu.M sethoxydim, while ACCase II activity was inhibited less than 50% for all herbicide/substrate combinations examined.
C. Cloning and Identification of Maize cDNA Clones Encoding ACCase
Maize cDNA clones encoding a portion of the ACCase game were identified by screening a DNA library generated from maize. The cDNA clones were used to identify the sequence of the ACCase gene and to identify the genomic DNA fragments encoding the gene or genes for ACCase.
A .lambda. gt11 cDNA library from maize inbred A188 seedlings was prepared by standard method for oligo-dT priming, as described for pea cDNA. (Gantt and Key, Eur. J. Biochem., 166:119-125 (1987). Plaque lifts of the maize cDNA library were screened with maize ACCase antiserum to identify plaques expressing ACCase-like proteins, as described by Sambrook et al., cited supra. (1989). The initial screen of 800,000 plaques yielded 120 positives. Rescreening and plaque purification reduced the number of positives to 14. All 14 clones bound ACCase antibodies that, when eluted from plaque lifts (J. Hammarback et al., J. Biol. Chem., 265:12763 (1990)), recognized a 227-kD biotinylated polypeptide on SDS-PAGE western blots of embryo and leaf crude extracts. The strongest western blot reaction was obtained with cDNA clone #15-14. The six best clones were digested with EcoRI to excise maize cDNA inserts. Total insert sizes ranged from 1.2 to 5.1 kb indicating the clones most likely did not contain the full coding sequences for the mature 219-kD and 227-kD ACCase polypeptides (minimum estimates of 6.1 and 6.3 kb, respectively).
Clone #15-14 contained three EcoRI fragments of 2.0, 1.2 and 0.23 kb shown in FIG. 8. Southern blots showed that the 1.2 and 2.0-kb fragments of clone #15-14 each hybridized to different fragments in the other five clones, with the exception of clone #4-4 which only contained a 1.2-kb fragment. The six maize cDNA clones contained EcoRI fragments that hybridized to a large transcript (ca. 7.8 kb) on northern blots of total RNA from maize leaves, embryos and endosperm (FIG.9). BMS cell culture RNA also contained a 7.8 kb transcript, but the hybridization signal is not evident on this exposure (FIG. 9). The relative abundance of the 7.8-kb transcript in embryos was higher than the other sources which is consistent with their ACCase activity (unpublished data).
The three EcoRI fragments were subcloned from cDNA clone #15-14 into BlueScript vector and sequenced by the dideoxy chain termination method (Sequenase 2.0 USB) initially using T3 and T7 primers and then oligonucleotide primers based on insert sequence. A clone #16-6 was also sequenced in a similar manner. Clone #16-6 included three EcoRI fragments of 3.1 kb, 1.2 kb, and 0.23 kb and had additional sequence located upstream from that of clone #15-14. After comparing the sequence and determining that the sequence was the same, the additional 1.2 kb sequence at the 5' end was sequenced.
Clone #18-5 was sequenced in a similar manner. Clone #18-5 included 3.9 kb, 1.2 kb, and 0.23 kb EcoRI fragments and contains an additional 1.9 kb 5' sequence upstream from clone #15-14. Subclone #18-5I (3.9 kb EcoRI fragment) has been deposited with the American Type Culture Collection and given Accession No. 69236.
Gen Bank, PIR-29, and Swiss-Prot 19 data banks have been searched for amino acid homology with the corresponding amino acid sequences of the three subclones of clone #18-5. Peptide sequences corresponding to the maize cDNA subclones had higher similarity to chicken, rat, and yeast ACCases than to any other peptide sequence in the data banks. FIG. 8 illustrates the relative organization of the 3.9, 1.2 and 0.23-kb EcoRI fragments of clone #18-5 and their co-linearity and extent of amino acid identity with chicken ACCase cDNA sequence. This comparison shows that the maize clone #18-5 has a large region near the 3' end with high amino acid identity (40 to 61%) to chicken ACCase, a longer region with 23% identity in the middle of the 3.9-kb sequence, and a short region with 52% identify near the 5' of the 3.9 kb sequence.
Portions of the sequence of the #18-5I subclone have been identified as encoding domains of the ACCase enzyme of functional significance. Those functional regions include a fragment that spans the presumed transcarboxylase active site in the enzyme having the following presumed sequence (SEQ ID No:2): ##STR1## This functional domain is contained in the sequence 1112 to 856 bp from the 3' stop codon or carboxy terminus region of the ACCase coding sequence of maize. This transcarboxylase active sequence is also present in clone #15-14.
Another functional region that has been identified spans the 12 bp sequence encoding the biotin binding site having the following peptide sequence (SEQ ID No:3): ##STR2## The biotin binding site is encoded approximately 30% in from the 5' (N-terminus) end of rat, chicken and yeast ACCase genes. These functional domains are useful in mapping and further identifying other cDNA and/or genomic fragments encoding ACCase genes.
The cDNA clones encoding portions of the acetyl CoA carboxylase genes are useful to identify the sequence of the gene or genes and are useful as probes to locate the genomic copies of the gene or genes. Because the ACCase antibodies used to screen the .lambda. gt11 library recognize both the 219 and 227 kD ACCase polypeptides, it has not been determined which polypeptide is encoded by these less than full length clones. It is likely that the majority of the clones encode the 227 kD polypeptide since that polypeptide is more abundant in the leaf tissue source of the DNA library and the antibodies have a higher affinity for the 227 kD ACCase polypeptide.
EXAMPLE VI
Isolation and Sequencing of Genomic Encoded ACCase Genes and Strategy for Getting the Complete Gene Sequence
The maize genome has been analyzed to identify copy number and location of the genomic copies of ACCase gene or genes. Portions of the genomic copies of the acetyl coA carboxylase genes from maize have been cloned and sequenced.
A maize genomic lambda library (Clontech, Palo Alto, Calif.) was screened with the 2 kb subclone from #15-14 and several clones of about 15 kb were identified as having homology to the ACCase cDNA. Restriction mapping and partial sequence analysis revealed two types of genomic clones (Type I and Type II) that differed in restriction sites and in their position relative to the ACCase partial cDNA sequence as shown in FIG. 8.
The 2.5 kb EcoRI-SaII fragment (#16) from the Type I genomic clone and the 3.0 kb EcoRI-EcoRI fragment (#34) from the Type II genomic clone were shown to hybridize to the 3.9 kb probe from #18-5 and were subcloned into the Bluescript vector and sequenced. Approximately 1.5 kb of DNA sequence from the genomic I 2.5 kb fragment were 100% identical to coding sequence from the 3.9 kb cDNA subclone #18-5I described in Example V; the remaining sequence exhibited no identity with the cDNA clone and presumably represents noncoding intron sequence. A 350 nucleotide sequence derived from the genomic II 3.0 kb fragment was about 95% identical to the cDNA clone indicating that its coding sequence differs from that of genomic Type I. These results also indicate that the genome could carry at least two different genes encoding acetyl CoA carboxylase activity.
To identify and clone the remainder of the gene representing the amino-terminus of maize ACCase, additional regions from the Type I genomic clone have been subcloned and partly sequenced. The 3.5 kb HindIII-HindIII fragment (#28) has been sequenced for about 400 nucleotides from each end. The 3' end of #28 shows significant homology to the amino acid sequence from the chicken sequence located about 0.5 kb from the start of the chicken gene. The complete sequence for fragment #28 will be obtained and analyzed to determine whether it contains the amino-terminus (5' end) of the ACCase coding region. The start of the transcribed region, and thus the likely start of the coding region for ACCase, can be identified by using the genomic clones in RNAse protection analysis (Sambrook et al., 1989). Based on sequence data from the genomic clone, alignment, as shown in FIG. 8, with sequences of other ACCases and identification of potential open reading frames, oligonucleotide primers will be constructed to synthesize cDNA molecules representing the amino terminus of the ACCase gene. These molecules will be hybridized to genomic I DNA fragments such as #28 and the nonhybridizing portions digested with S1 nuclease. The end of the protected fragment will be determined by analysis on a DNA sequencing gel.
To synthesize the remaining coding region between the end of the cDNA cone #18-5 and the start of transcription, two oligonucleotide primers will be synthesized. Primer 1 is complementary to the DNA sequence (SEQ ID No:4):
near the 5' end of cDNA subclone #18-5I and will be used as a primer for synthesis of cDNA molecules from maize seedling leaf or embryo RNA. Primer 2 will correspond to the DNA sequence near the transcription start site and will be used in combination with primer 1 for the amplification of DNA according to the polymerase chain reaction (PCR) procedure. The PCR products will be ligated into the Bluescript vector. Several independent clones will be sequenced and their sequences compared to the known sequence of the type I genomic clone to determine the exact coding sequence corresponding to that maize gene for ACCase. A similar strategy may be used to obtain the complete coding sequence for genomic type II ACCase.
The initial restriction fragment length polymorphism (RFLP) analysis of EcoRI-digested total DNA from three maize inbred lines showed one band when probed with the 2 kb subclone from #15-14 (internal to gene) and two bands when probed with the 1.2 kb subclone (near the 3' end of the gene). Fragments homologous to the 2 kb probe were monomorphic and the more intense of the two bands hybridizing with the 1.2 kb probe was dimorphic. As discussed in Example V, these results support the view that maize contains at least two distinguishable ACCase genes and that they may be quite similar for much of the coding region. Additional genomic Southern blots of a set of recombinant inbred lines (University of Missouri) were used to map polymorphisms for the ACCase probes to maize chromosomes. One polymorphism was mapped to the short arm of chromosome 2; other polymorphisms were not evident in these initial tests to identify a chromosomal location for other maize ACCase genes. The chromosomal location of different ACCase genes will be verified by additional RFLP mapping in recombinant inbreds using gens-specific probes obtained from Type I and Type II genomic clones. The copy number for each locus will be determined by Southern blot restriction comparisons or quantitation on DNA slot blots.
EXAMPLE VII
Expression of a cDNA Clone or Genomic Clones Encoding the ACCase Gens
The cDNA and genomic clones encoding all or a portion of the ACCase gens can be subcloned into a known expression system and the gens products reactive with the antibodies specific for maize ACCase can be identified using a Western blot. The gens products can also be further characterized structurally and/or enzymatically. This will ensure that the genomic and cDNA clones encode acetyl CoA carboxylase genes and provide a system for screening for promoters that provide for overproduction of the native or herbicide tolerant ACCase enzyme in plants.
For example, the 2 kb EcoRI fragment from clone #15-14 can be subcloned into a plant transformation plasmid pBI121 or pBI221 downstream from the 35S CaMV promoter and upstream from the nopaline 3' polyadenylation signal sequence, as described in Jefferson, Plant Molec. Biol. Reptr., 5:387-405 (1987). This plasmid can then be used to transform plant cells such as tobacco, Brassica and Arabidopsis cells using protoplast or biolistic transformation, as described by Gordon-Kamm et al. (1990); Fromm et al. (1990); An, Methods in Enzymology, 153:292 (1987); and D'Hafluin, The Plant Cell, 4:1495 (19920. An increase in transient expression can be detected using quantitative Western blotting with antibodies specific for the ACCase enzymes. Polyclonal antibodies to maize ACCase most likely do not substantially crossreact with ACCase from dicots like tobacco or Arabidopsis.
Alternatively, the ACCase gene can be subcloned along with the 35S CaMV promoter into a binary Ti vector pGA482, as described in An, cited supra., which is a binary Ti vector system and can be used to transform plant cells via Agrobacterium. Stable transformed plants can be generated by standard methods as described in Example III, and levels of expression of ACCase genes can be determined by quantitative Western blots, as described in Harlow and Lane, Antibodies, Cold Spring Harbor Laboratories (1988). The ability to monitor expression of cloned ACCase genes will allow for the identification of promoters that provide for an enhanced expression of the ACCase gene. The expression system can be used to screen for those promoters that enhance gene expression of the ACCase gene at least about 5 to 10-fold over the endogenous levels of ACCase produced normally in the plant cells. Because the 35S CaMV promoter is known as a strong promoter, it is likely this promoter will provide for at least a 5-fold increase in the expression of ACCase over that normally produced in the plant cell.
In addition, this expression system can be used to screen antisense DNA sequences. For example, an antisense sequence can be obtained that is complementary to an about 0.5 kb region of the maize ACCase cDNA that has high homology with a portion of the chicken ACCase gene and contains the sequence for the presumed transcarboxylase active site domain, as shown in FIG. 8. The antisense sequence could be subcloned into a pBI121 or pBI221 expression under the control of an inducible plant promoter, such as nitrite reductase promoter (Back et al., Plant Molec. Biol., 17:9-18 (1991)). The ability of the antisense sequence to inhibit expression of the native ACCase gene can be evaluated in transformed cells, for example as described in Hamilton et al., Nature, 346:284-287 (1990).
EXAMPLE VIII
Identification and Cloning of the Gene From Herbicide Resistant Maize Cell Lines
Herbicide resistant maize cell lines were generated as described in Examples I, II, and IV. These herbicide resistant cell lines have been shown to produce an enzyme that is less sensitive to inhibition by sethoxydim or haloxyfop. The genes encoding the herbicide resistant forms of the ACCase gene will be identified and cloned using standard methods as described in Sambrook et al., Guide to Molecular Cloning: A Laboratory Manual (1989). The genes encoding the herbicide resistant forms of ACCase can then be introduced into herbicide sensitive plant species to confer herbicide resistance by standard methods.
For example, the ACCase enzyme in the maize cell line 2167-9/2160-154 S-1 was at least 100-fold less sensitive to sethoxydim than the wild-type. DNA from the cell line or plants will be obtained and digested with EcoRI and/or other appropriate restriction enzymes, according to standard methods. The restriction enzyme digest will be separated out by agarose gel electrophoresis and probed with either the 2 kb or the 3.9 kb cDNA ACCase probe described in Example V. Fragments hybridizing to the 2 kb or 3.9 kb probe will be subcloned into a Bluescript vector and portions of the gene will be sequenced, as described in Example V, to verify that the entire ACCase gene has been isolated. To confirm that the clone encodes the ACCase gene, it will be subcloned into the pBI121 or pBI221 expression vector, as described in Example VIII. The ACCase gene product expressed by the clone in either Black Mexican sweet corn cells or tobacco cells will be evaluated for reactivity with ACCase specific antibodies, enzyme activity, and/or resistance of the enzyme activity to inhibition with sethoxydim and/or haloxyfop. It is likely that the cloned gene encodes an ACCase which is resistant to inhibition by sethoxydim and haloxyfop.
This gene can then be introduced into an herbicide-sensitive plant cell, including maize cells, to confer herbicide resistance to that plant species.
The complete coding sequence encoding the herbicide resistant form of the ACCase enzyme will be cloned into a plant transformation vector such as pBI121 or pBI221 as described in Jefferson, Plant Molec. Biol. Reporter, 5:387-405 (1987). This vector contains the 35S CaMV constitutive promoter, the .beta.-glucuronidase structural gene, and the nopaline synthase 3' polyadenylation signals. The .beta.-glucuronidase gene is replaced with a cloned ACCase gene. Optionally, the cloned ACCase gene can be combined with natural or synthetically produced chloroplast transit peptide sequence from pea, as described in Keegstra & Olsen, Ann. Rev. Plant. Physiol./Mol. Biol., 40:471-501 (1989) and/or unique restriction sites so the cloned gene can be distinguished from the endogenous maize ACCase gene. Standard methods of subcloning will be utilized as described in Sambrook et al., cited supra.
For transformation of maize cells, type II calli can be transformed using biolistic transformation, as described by Gordon-Kamm et al. (1980); Fromm et al. (1990); and Walters et al. (1992). Alternatively, type I embryogenic calli can be transformed using electroporation after mechanically or enzymatically wounding calli, as described by D'Hafluin et al., The Plant Cell, 4:1495 (1992). Once the cloned gene is introduced in these calli and transformants are selected, typically by antibiotic resistance, fertile transgenic maize plants can be regenerated, as described by D'Hafluin et al. cited supra. Fertile transgenic plants can be evaluated for herbicide tolerance, as described in Example III. It is likely that the fertile transgenic plants having and expressing a cloned ACCase gene resistant to sethoxydim and/or haloxyfop will exhibit herbicide tolerance.
EXAMPLE IX
Generation of Transgenic Plants Having an Increase in Oil Content
Once identified and cloned, the gene or genes from maize acetyl CoA carboxylase can be introduced into plant species, including maize, with a promoter that provides for overexpression of the ACCase enzyme. The overexpression of the ACCase enzyme is likely to lead to an increase in the oil content of the plants and seeds.
Naturally occurring soybeans that have a high oil content and soybeans that have a low oil content have been identified. The acetyl CoA carboxylase from both types of soybeans was isolated, as described in Example V. The activity of the enzyme was measured as a function of the time of seed development and the results are shown in FIG. 11.
The results in the FIG. 11 indicate that higher oil content soybean is associated with a 2-fold increase in the ACCase activity during early to mid stages of development when compared with a low oil content soybean. Thus, increased expression of the ACCase gene correlates with an increase in the oil content of the seed. Total oil content of the seed was also measured at maturity (60 days). The high oil producing cell lines, Anoka and PI28C.134, have a total oil content of 21.8% and 19.9%, respectively. In contrast, the low oil soybean line of M76-395, has an oil content of 13.6% oil. Thus, the increase of ACCase expression early in seed development correlates with a higher total oil content in the seed at maturity.
A gene encoding a genomic maize acetyl CoA carboxylase can be isolated, as described in Example V, and used to transform plant species by protoplast and biolistic transformation. If the gene is combined with a strong promoter, such as the 35S cauliflower mosaic virus promoter, overexpression of the ACCase gene is likely. Alternatively, selecting transformed cells with multiple copies of the gene can also result in transformed cells overexpressing the ACCase gene. The gene can be cloned into a vector such as pBI121 or pBI221, as described by Jefferson, cited supra. This vector contains the 35S cauliflower mosaic virus promoter, the .beta.-glucuronidase structural gene, and the nopaline synthase 3' polyadenylation signals. The cloned ACCase gene can replace the .beta.-glucuronidase gene and then be used to transform plant cells, including maize, as described in Example VIII.
Transformed cells can be screened for overproduction of ACCase. The presence of the cloned gene can be verified by identifying the unique restriction enzyme sites incorporated into the cloned gene. ACCase levels can be assessed by standard enzyme assay methods and quantitative Western blots using antibodies specific for maize ACCase. Fatty acid and lipid content in cells lines overproducing ACCase are likely to be elevated and can be assessed using standard methodologies, as described in Clark & Snyder, JACS, 66:1316 (1989). Transformed cell lines exhibiting overproduction of ACCase and an increase in total oil content will be used to regenerate fertile transgenic plants and seeds, as described in D'Hafluin, cited supra.
While the present invention has been described in connection with the preferred embodiment thereof, it will be understood many modifications will be readily apparent to those skilled in the art, and this application is intended to cover any adaptations or variations thereof. It is manifestly intended this invention be limited only by the claims and equivalents thereof.
All patents and publication described herein are hereby incorporated by reference.
Claims
What is claimed is:
1. An expression cassette comprising: a DNA molecule comprising DNA SEQ. ID NO:1 coding for a maize acetyl CoA carboxylase enzyme operably linked to a promoter functional in a host cell.
2. An expression cassette according to claim 1 further comprising: a DNA molecule encoding an amino terminal maize chloroplast transit peptide, wherein the DNA molecule is operably linked between the promoter and the gene coding for the maize acetyl CoA carboxylase.
3. An expression cassette according to claim 1, wherein the promoter is a tissue specific promoter.
4. An expression cassette according to claim 1, which is on a plasmid.
5. An expression cassette according to claim 1, wherein the promoter provides for about a 2- to 20-fold increase in expression of the acetyl CoA carboxylase enzyme over that of a native enzyme.
6. An expression cassette according to claim 1, wherein the promoter is selected from the group consisting of 35S cauliflower mosaic virus promoter, and the nopaline synthase promoter.
Patent Citations (2)
Non-Patent Literature (39)
- Parker, et al, Third U of M Research Poster Session: Basic and applied bio-medical research in academic and industry (abstract) (May 25, 1988).
- Marshall, et al (May 1988) Agronomy Abstracts 170: Title Summary No. C7-51P.
- Roessler (1990) Plant Physiology 92: 73-78.
- Parker, et al (1987) NCWCC Proceedings, p. 56, presented Dec. 9, 1987.
- Parker, et al "Selection and characterization of corn cell lines tolerant to sethoxydim," 64 (abstract #180) Feb. 3, 1988.
- Bai, et al (1986) Journ. of Biol. Chem. 261(26): 12395-12399.
- Lamhonwah, et al (1987) Archives Biochem. Biophys. 254 (2): 631-636.
- An, "Binary Ti Vectors for Plant Transformation and Promoter Analysis", Methods in Enzymology, 153:292 (1987).
- Armstrong et al., "Establishment and Maintenance of Friable, Embryogenic Maize Callus and the Involvement of L-proline", Planta, 164:207 (1985).
- Arnon, "Copper Enzymes in Isolated Chloroplasts. Polyphenoloxidase in beta-Vulgaris", Plant Physiol., 24:1-15 (1949).
- Bachmann, "Linkage Map of Escherichia coli K-12, Edition 7", Microbiological Reviews, 47:180-230 (1983).
- Back et al., "Isolation of the Spinach Nitrite Reductase Gene Promoter Which Confers Nitrate Inducibility on GUS Gene Expression in Transgenic Tobacco", Plant Molec. Biol., 17:9-18 (1991).
- Berg et al., "The Prokaryotic Transposable Element Tn5", Biotechnology, 1:417-436 (1983).
- Burton et al., "Inhibition of Plant Acetyl-Coenzyme a Carboxylase by the Herbicides Sethoxydim and Haloxyfop", Biochem. Biophys. Res. Comm., 148:1039-1044 (Nov. 13, 1987).
- Burton et al., "Inhibition of Corn Acetyl-CoA Carboxylase by Cyclohexanedione and Aryloxyphenoxypropionate Herbicides", Pest. Biochem. Physiol., 34:76-85 (1989).
- Cline et al., "Precursors to Two Nuclear-encoded Chloroplast Proteins Bind to the Outer Envelope Membrane before Being Imported into Chloroplasts", J. Biol. Chem., 260:3691-3696 (1985).
- D'Hafluin et al., "Transgenic Maize Plants by Tissue Electroporation", The Plant Cell, 4:1495-1505 (1992).
- Egli et al., "Biochemical and Genetic Characterization of Maize Acetyl-CoA Carboxylase", Maize Genetics Conference, Abstract (Mar. 19-22, 1992).
- Fromm et al., "Inheritance and Expression of Chimeric Genes in the Progeny of Transgenic Maize Plants", Bio/Technol., 8:833-839 (1990).
- Gantt et al., "Molecular Cloning of a Pea H1 Histone cDNA", Eur. J. Biochem., 166:119-125 (1987).
- Gordon-Kamm et al., "Transformation of Maize Cells and Regeneration of Fertile Transgenic Plants", The Plant Cell, 2:603-618 (1990).
- Green, "Prospects for Crop Improvement in the Field of Cell Culture", Hort. Sci., 12, 7-10 (1977).
- Hamilton et al., "Antisense Gene that Inhibits Synthesis of the Hormone Ethylene in Transgenic Plants", Nature, 346:284-287 (1990).
- Hammarback et al., "Antibody Exchange Immunochemistry", J. Biol. Chem., 265:12763 (1990).
- Hoj et al., "Partial Separation of Individual Enzyme Activities of an ACP-Dependent Fatty Acid Synthetase from Barley Chloroplasts", Carlsberg Res. Commun., 47:119-141 (1982).
- Jefferson, "Assaying Chimeric Genes in Plants: The GUS Gene Fusion System", Plant Molec. Biol. Reporter, 5:387-405 (1987).
- Keegstra et al., "Chloroplastic Precursors and their Transport Across the Envelope Membranes", Ann. Rev. Plant. Physiol. Mol. Biol., 40:471-501 (1989).
- Liedvogel et al., "Fatty-acid Synthesis in Chloroplasts from Mustard (Sinapis alba L.) Cotyledons: Formation of Acetyl Coenzyme A by Intraplastid Glycolytic Enzymes and a Pyruvate Dehydrogenase Complex", Planta, 169:481-489 (1986).
- Mishina et al., "Yeast Mutants Defective in Acetyl-Coenzyme A Carboxylase and Biotin: Apocarboxyase Ligase", Eur. J. Biochem., 111:79-87 (1980).
- Mitra et al., "Three Distinct Regulatory Elements Comprise the Upstream Promoter Region of the Nopaline Synthase Gene", Molec. Gen. Genetic., 215:294 (1989).
- Nikolau et al., "Acetyl-Coenzyme A Carboxylase in Maize Leaves", Arch. Biochem. Biophys., 211:605-612 (1981).
- Ohlrogge et al., "Subcellular Localization of Acyl Carrier Protein in Leaf Protoplasts of Spinacia oleracea", Proc. Natl. Acad. Sci. USA, 76:1194-1198 (1979).
- Parker et al., "Selection and Characterization of Sethoxydim-tolerant Maize Tissue Cultures", Plant Physiol., 92:1220-1225 (1990).
- Parker et al., "Dominant Mutations Causing Alterations in Acetyl-Coenzyme A Carboxylase Confer Tolerance to Cyclohexanedione and Aryloxyphenoxypropionate Herbicides in Maize", Proc. Natl. Acad. Sci USA, 87:7175-7179 (1990).
- Post-Beittenmiller et al., "Regulation of Plant Fatty Acid Biosynthesis", Plant Physiol., 100:923-930 (1992).
- Smith et al., "Measurement of Protein Using Bicinchoninic Acid", Anal. Biochem., 150:76-85 (1985).
- Turnham et al., "Changes in the Activity of Acetyl-CoA Carboxylase During Rape-Seed Formation", Biochem. J., 212:223-229 (1983).
- Walters et al., "Transformation and Inheritance of a Hygromycin Phosphotransferase Gene in Maize Plants", Plant Mol. Biol., 18:189-200 (1992).
- Zhu et al., "Xylulose 1,5-Bisphosphate Synthesized by Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase During Catalysis Binds to Decarbamylated Enzyme", Plant Physiol., 97:1348-1353 (1991).