Constructs Containing Impatiens Necrotic Spot Tospovirus RNA and Methods of Use Thereof

The present invention relates to plants having reduced susceptibility to infection from tospoviruses, genetic material capable of generating tolerance to tospoviruses, probes suitable for isolating and diagnosing, and processes for obtaining such plants and genetic material and probes.

Viral infections in plants are frequently responsible for detrimental effects in growth, undesirable morphological changes, decreased yield and the like. Such infections often result in a higher susceptibility to infection in infected plants to other plant pathogens and plant pests. Transmission of plant viruses generally occurs via insect or fungal carriers or may occur through mechanical means.

Plant breeders continuously look to develop varieties of crop plant species tolerant to or resistant to specific virus strains. In the past, virus resistance conferring genes have been transferred from wild types related to commercial plants into commercial varieties through breeding. The transfer of an existing resistance in the wild from the wild type gene pool to a cultivar is a tedious process in which the resistance conferring gene(s) must first be identified in a source (donor) plant species and then combined into the gene pool of a commercial variety. Resistance or tolerance generated in this way is typically active only against one or at best a few strains of the virus in question. One disadvantage of breeding cultivars for resistance to a particular virus species is that there is often a lack of a gene source suitable for conferring disease resistance within the crop species.

Other approaches to limit the effect of virus induced disease on plants include the use of chemicals such as insecticides, fungicides and the like which act against virus carriers, and/or rely on the employment of preventative methods such as efficient phytosanitary working conditions. However, the use of chemicals to combat virus disease by killing the carrier is subject to increasingly tougher governmental regulations which present growers with a decreasing scala of permitted chemical plant-protectants.

In an alternative, a system referred to as "cross-protection" may be employed. Cross-protection is a phenomenon in which infection of a plant with one strain of a virus protects that plant against superinfection with a second related virus strain. The cross-protection method preferentially involves the use of avirulent virus strains to infect plants, which act to inhibit a secondary infection with a virulent strain of the same virus. However, the use of a natural cross-protection system can have several disadvantages. The method is very labour intensive because it requires inoculation of every plant crop, and carries the risk that an avirulent strain may mutate to a virulent strain, thus becoming a causal agent for crop disease in itself. A further possible hazard is that an avirulent virus strain in one plant species can act as a virulent strain in another plant species.

Several studies have indicated that the viral coat protein of the protecting virus plays an important role in cross-protection and that protection occurs when the resident virus and the challenging virus have the same or closely related coat protein structures.

Recent developments in gene manipulation and plant transformation techniques have given rise to new methods for generating virus resistance in plants. Genetically engineered cross-protection is a form of virus resistance which phenotypically resembles natural cross-protection, but is achieved through the expression of genetic information of a viral coat protein from the genome of a genetically manipulated plant. Generation of virus resistance via genetic engineering has been described in for instance, EP 223 452 and reported by Abel et al ›(1986) Science 232:738-743!. It was shown that expression of the tobacco mosaic virus strain U1 (TMV-U1) coat protein gene from the genome of a transgenic plant resulted in a delay of symptom development after infection with any TMV strain. Similar results with respect to coat protein-mediated protection have also been obtained for alfalfa mosaic virus (AMV), potato virus X (PVX) and cucumber mosaic virus (CMV).

Although TMV, CMV, AMV and PVX belong to different virus groups, they share a common architecture: in all such viruses the viral RNA is a positive strand RNA encapsidated by a viral coat consisting of many individual but identical viral coat proteins.

However, tospoviruses are essentially different from the plant viruses mentioned above. The genus tospovirus belongs to the family Bunyaviridae. All tospoviruses are transmitted by thrips. The virus particles are spherical in shape (80-120 nm in diameter) and contain internal nucleocapsids surrounded by a lipid envelope studded with glycoprotein surface projections. The multipartite genome consists of linear single stranded RNA molecules of negative or ambisense polarity. The terminal nucleotides of these RNA molecules are characterised by a consensus sequence as follows: 5' AGAGCAAUX....................GAUUGCUCU 3', wherein X is C or U. Members of the tospovirus group include tomato spotted wilt virus (TSWV), Impatiens necrotic spot virus (INSV), and tomato chlorotic spot virus (TCSV), also known as tomato mottled spot virus (TMSV) or TSWV-like isolate BR-O3. A general description of a tospovirus, using TSWV as a representative of the genus tospoviruses can be found in our co-pending application EP 426 195 herein incorporated by reference.

The tospovirus particle contains at least 4 distinct structural proteins: an internal nucleocapsid protein N of 29 kd and two membrane glycoproteins: G1, approximately 78 kd, and G2 approximately 58 kd. In addition, minor amounts of a large protein, L, approximately 260 kd have been detected in virus particles. Tospoviral genomes consist of three linear single stranded RNA molecules of about 2900 nucleotides (nt) (S RNA), about 5000 nt, (M RNA) and about 8900 nt (L RNA), each tightly associated with nucleocapsid proteins and a few copies of the L protein to form circular nucleocapsids. A schematic structure outlining most properties of an INSV is given in FIG. 1. Based on the above and other properties, INSV (like TSWV) has been classified as a member of the tospovirus genus.

Circumstantial evidence has been presented which suggests that an M RNA encoded gene is directly or indirectly involved in the synthesis of the G1 membrane glycoprotein ›Verkleij and Peters, (1983) J. Gen. Virol. 64:677-686!.

As mentioned above, tospoviruses such as TSWV, INSV and the like are transmitted by certain species of thrips. These tospovirus carriers belong to the family Tripidae and include tobacco thrips (Frankliniella fusca (Hinds.)), western flower thrips (F. occidentalis (Pergande)), common blossom thrips (F. Schultzei (Trybom)), chilli thrips (Scirtothrips dorsalis (Hood)), Thrips setosus (Moulton), onion thrips (T. tabaci (Lindeman)), F. intonsa and melon thrips (T. palmi (Karny)). The tospovirus is acquired by thrips only during their larval stages. Larvae can transmit the virus before they pupate but adults more commonly transmit the virus. Adult thrips can remain infective throughout their lives.

Tospoviruses are widespread in temperate, subtropical and tropical climate zones throughout the world. The current distribution of tospoviruses covers all continents and makes them one of the most widely distributed of groups of plant viruses. At least 370 plant species representing 50 plant families, both monocotyledons and dicotyledons, are naturally infected by tospoviruses of the Bunyaviridae. Tospoviruses seriously affect the production of food and ornamental crops. Symptoms of tospovirus infection in plants include stunting, ringspots, dark purple-brown sunken spots, stem browning, flower breaking, necrotic and pigmental lesions and patterns, yellows and non-necrotic mottle, mosaic in greens or even total plant death. Most plant hosts display only a few of these symptoms, however, the wide range of symptoms produced by tospovirus infection has complicated diagnosis of the disease and has led to individual diseases being given several different names. A further complication is that tospovirus symptoms within the same plant species may vary depending on the age of the plant, time of infection during the life-cycle of the plant, nutritional levels, environmental conditions, such as temperature, and the like.

Although TSWV has been known for many years, is widely distributed, and is the causal agent of a disease which leads to significant loss in yield in crops and ornamentals, limited progress has been made in identifying sources of genes capable of conferring resistance to TSWV or other tospoviruses. A monogenic TSWV tolerance has been identified in Lycopersicon peruvianum, but this trait has not been transferred to cultivated tomatoes so far, nor has a resistance source been identified for other crop species. The use of natural cross-protection systems to decrease the invasive effects by tospovirus strains capable of causing damage is not well documented. Limited positive results have been reported for tomato and lettuce.

The introduction of genetic information capable of conferring resistance or tolerance to tospoviruses into plant gene pools by means of genetic manipulation provides the breeder and grower alike with a new method for combatting tospovirus induced disease. In particular, it has been found that genetic manipulation techniques may be employed to confer resistance to INSV related disease in plants.

According to the present invention there is provided a recombinant INSV DNA construct comprising a DNA sequence coding for transcription into

a) an RNA sequence of an INSV or an RNA sequence homologous thereto;

b) an RNA sequence of an INSV or an RNA sequence homologous thereto capable of encoding for an INSV protein or a part thereof, in which one or more codons have been replaced by synonyms, or an RNA sequence homologous thereto; or

c) an RNA sequence complementary to an RNA sequence according to a) or b),

which INSV DNA is under expression control of a promoter capable of functioning in plants and includes a terminator capable of functioning in plants.

The DNA sequences defined under a), b) and c) above, for the purposes of the present invention will be referred to as "INSV Related DNA Sequences" hereinafter. An INSV Related DNA Sequence according to the invention may be modified as appropriate to create mutants or modified sequences homologous to such INSV Related DNA Sequences from which they are derived, using methods known to those skilled in the art such as site-directed mutagenesis and the like. Such mutants or modified coding sequences are embraced within the spirit and scope of the invention.

The term "RNA sequence of an INSV" may refer to a sequence of the S, M or L RNA strand, preferably an S or M RNA strand, more preferably to an S RNA strand of an INSV.

The term "RNA sequence homologous to an RNA sequence of an INSV" refers to an RNA sequence of an INSV wherein a number of nucleotides have been deleted and/or added but which is still capable of hybridization to a nucleotide sequence complementary to an RNA sequence of an INSV under appropriate hybridization conditions. For the purposes of the present invention appropriate hybridization conditions may include but are not limited to, for example, an incubation for about 16 hours at 42.degree. C., in a buffer system comprising 5.times.standard saline citrate (SSC), 0.5% sodium dodecylsulphate (SDS), 5.times.Denhardt's solution, 50% formamide and 100 .mu.g/ml carrier DNA (hereinafter the buffer system), followed by washing 3.times. in buffer comprising 1.times.SSC and 0.1% SDS at 65.degree. C. for approximately an hour each time

Preferably, hybridization conditions employed in the present invention may involve incubation in a buffer system for about 16 hours at 49.degree. C. and washing 3.times. in a buffer comprising 0.1.times.SSC and 0.1% SDS at 55.degree. C. for about an hour each time. More preferably, hybridization conditions may involve incubation in a buffer system for about 16 hours at 55.degree. C. and washing 3.times. in a buffer comprising 0.1.times.SSC and 0.1% SDS at 65.degree. C. for approximately an hour each time.

The length of the INSV Related DNA Sequence will i.a. depend on the particular strategy to be followed, as will become apparent from the description hereinafter. In general, the INSV Related DNA Sequence may comprise at least 20, and suitably 50 or more nucleotides.

The term "promoter" refers to the nucleotide sequence upstream from the transcriptional start site and which contains all the regulatory regions required for transcription, including the region coding for the leader sequence of mRNA (which leader sequence comprises the ribosomal binding site and initiates translation at the AUG start codon).

Examples of promoters suitable for use in DNA constructs of the present invention include viral, fungal, bacterial, animal and plant derived promoters capable of functioning in plant cells. The promoter may express the DNA constitutively or differentially. Suitable examples of promoters differentially regulating DNA expression are promoters inducible by disease carriers, such as thrips, e.g. so-called wound-inducible promoters. It will be appreciated that the promoter employed should give rise to the expression of an INSV Related DNA Sequence at a rate sufficient to produce the amount of RNA necessary to decrease INSV susceptibility in a transformed plant. The required amount of RNA to be transcribed may vary with the type of plant. Particularly preferred promoters include the cauliflower mosaic virus 35S (CaMV 35S) promoter, derivatives thereof, and a promoter inducible after wounding by a disease carrier such as thrips, e.g. a wound inducible promoter. Examples of further suitable promoters include nopaline synthase, octopine synthase and the like.

The term "terminator" refers to a DNA sequence at the end of a transcriptional unit which signals termination of transcription. Terminators are DNA 3'-non-translated sequences that contain a polyadenylation signal, that causes the addition of polyadenylate sequences to the 3'-end of a primary transcript. Terminators active in plant cells are known and described in the literature. They may be isolated from bacteria, fungi, viruses, animals and/or plants. Examples of terminators particularly suitable for use in the DNA constructs of the invention include the nopaline synthase terminator of A. tumefaciens, the 35S terminator of CaMV and the zein terminator from Zea mays.

In accordance with the present invention, an RNA sequence is complementary to another RNA sequence if it is able to form a hydrogen-bonded complex therewith, according to rules of base pairing under appropriate hybridization conditions (as described hereinabove).

The present invention also provides a vector capable of introducing the DNA construct of the invention into plants and methods of producing such vectors.

The term "vector" as employed herein refers to a vehicle with which DNA constructs of INSV or fragments thereof may be incorporated into the cells of a host organism.

The term "plants" refers to differentiated plants as well as undifferentiated plant material such as protoplasts, plant cells, including cybrids and hybrids, seeds, plantlets and the like which under appropriate conditions can develop into mature plants, progeny thereof and parts thereof such as cuttings, fruits of such plants and the like.

The invention further provides plants comprising in their genome a DNA construct of the invention, and methods of producing such plants. Such methods include plant breeding, plantlets derived from protoplast fusion and the like.

The plants according to the invention have reduced susceptibility to diseases induced by INSV or diseases related to INSV infection and suffer from substantially fewer or none of the disadvantages and limitations of plants obtained by classical methods as mentioned hereinabove.

Many types of plants are susceptible to INSV infection however only in some types is INSV infection known to give rise to a disease state directly attributable to the virus. Such types of plants include the ornamental or flowering plants. Examples of such plants include but are not limited to Ageratum, Amaranthus, Anthirrhinum, Aquilegia, Begonia, Chrysanthemum, Cineraria, clover, Cosmos, cowpea, Cyclamen, Dahlia, Datura, Delphinium, Gerbera, Gladiolus, Gloxinia, Hippeastrum, Impatiens, Mesembryanthemum, petunia, Primula, Saint Paulia, Salpiglossis, Tagetes, Verbena, Viola, Vinca, Zinnia, Pelargonium and the like.

Other types of plants may be susceptible to INSV infection but these plants may not present disease symptoms directly associated with INSV infection, however such plants may present symptoms of a disease as a result of a secondary infection by a different organism made possible as a result of an initial infection by INSV. Such plants may therefore be viewed as being the subject of an INSV infection related disease and may include plants selected from a wider group of plant types. Further examples of this group of plant types may include vegetable and other crops. Such crop types include alfalfa, aubergine, beet, broad bean, broccoli, brussels sprouts, cabbage, cauliflower, celery, chicory, cow pea, cucumber, endive, gourd, groundnut, lettuce, melon, onion, papaya, pea, peanut, pepper, pineapple, potato, safflower, snap bean, soybean, spinach, squash, sugarbeet, sunflower, tobacco, tomato, water melon and the like.

The invention relates in particular to ornamental plants and preferably to those listed ornamental plants comprising in their plant genome a DNA construct of the invention.

The particular features of tospoviruses including those of INSV are illustrated hereinafter.

The S, M and L RNA are single stranded RNA molecules. The S RNA of INSV is about 3000 nucleotides long(SEQ. ID No.1; SEQ ID No. 2) and comprises two genes, one (SEQ ID No.3) encoding a non-structural protein (NSs) in viral sense, the other one (SEQ ID No.11) encoding the nucleocapsid protein (N) in viral complementary sense. The intergenic region between the NSs- and N-gene can be folded into a secondary structure (Seq ID No. 7 and SEQ ID No.8). The 5'- and 3'-terminal sequences of the S RNA are capable of hybridizing to each other such that the first nucleotide is opposite (and complementary) to the last nucleotide of said S RNA strand. For the purposes of the description the double-stranded structure obtained by hybridizing both RNA termini will be referred to as a "pan-handle" (SEQ ID No.5 and SEQ ID NO. 6) hereinafter.

The M RNA strand of INSV comprises about 5000 nucleotides (SEQ ID No. 14). It contains at least two open reading frames, one encoding a non-structural protein (NSm) in viral sense (SEQ ID No.15), and another open reading frame (SEQ ID No.21) in viral complementary sense. This open reading frame is translated on polysomes located on the endoplasmic reticulum where the nascent polypeptide chain is cleaved co-translationally to form the spike proteins G1 and G2 respectively. As with S RNA, the termini of the M RNA strand are complementary to each other and may likewise hybridize to form a "pan-handle" (SEQ ID No.18 and SEQ ID No.19).

The L RNA strand of INSV comprises about 8900 nucleotides. It contains complementary 3' and 5' ends for a length of from about 50 to about 80 nucleotides. The RNA has a negative polarity, with one open reading frame (ORF) located as the viral complementary strand. This ORF corresponds to a primary translation product of about 2875 amino acids in length with an anticipated Mw of between about 300,000 to about 350,000. Comparison with the polymerase proteins of other negative strand viruses indicates that this protein probably represents a viral polymerase. In some mutant strains, shortened L RNA molecules have been found in addition to the wild type, full length L RNA. These shortened L RNAs however are observed to possess the characteristic terminal nucleotide sequences and thus are capable of forming "pan handle" structures. They are also encapsidated with nucleocapsid protein and are included in virus particles. Their presence suppresses symptom development resulting in less severe detrimental effect. Thus, these shortened L RNA molecules can be regarded as defective interfering (DI) RNAs. A defective interfering RNA is one which is capable of interfering in replication by competing with other genomic RNAs for polymerases and therefore is capable of being replicated, and by so doing inhibits the replication and/or expression of other genomic RNA's with which it is competing. Thus, a DI RNA may comprise any RNA sequence which is capable of being replicated and may be an L, S, or M RNA within the context of the present invention. Such DI RNA sequences may comprise RNA sequences which have had nucleotides either deleted from or added thereto provided that they are capable of competing for polymerases and of replicating.

A preferred embodiment of the invention relates to DNA constructs of the invention coding for transcription into INSV RNA sequences of a "pan-handle" (SEQ ID No.5, SEQ ID No.6; SEQ ID No.18, SEQ ID No.19), or into INSV RNA sequences homologous thereto.

Another preferred embodiment of the invention relates to DNA constructs of the invention coding for transcription into INSV-RNA sequences of an open reading frame in viral complementary sense i.e. having negative polarity, or into corresponding RNA sequences in which one or more codons have been replaced by their synonyms, or into RNA sequences homologous thereto.

A further preferred embodiment of the invention relates to DNA constructs of the invention coding for transcription into INSV-RNA sequences of a hairpin (SEQ ID No.7, SEQ ID No.8; SEQ ID No.13, SEQ ID No.16) or into RNA sequences homologous thereto.

Preferably, the INSV-RNA sequence referred to hereinabove has at least 20 nucleotides. Preferably, the INSV-RNA sequence has at least 50 nucleotides.

Examples of DNA constructs suitable for use according to the invention include INSV-Related DNA Sequences coding for transcription into (reference is made to the sequence listing);

i) the viral S RNA nucleotide sequence from 1 to 3017 (SEQ. ID No.1)

ii) the viral S RNA nucleotide sequence from position 25 to 3017 (SEQ. ID No.2);

iii) the viral S RNA nucleotide sequence from 87 to 1436 (SEQ. ID No.3);

iv) the viral S RNA nucleotide sequence from 2080 to 2868 (SEQ. ID No.4);

v) the viral S RNA "pan-handle" structure comprising:

a) a first nucleotide sequence of from about 30 to about 36 nucleotides in length from the 5' end of the viral S RNA

and

b) a second nucleotide sequence of from about 30 to about 36 nucleotides in length from the 3' end of the viral S RNA

vi) the viral S RNA nucleotide sequence from 1437 to 2079; (SEQ ID No. 7)

vii) the viral S RNA nucleotide sequence from 1440 to 2041; (SEQ ID No.8)

viii) the viral complementary S RNA nucleotide sequence from 1 to about 3017; (SEQ ID No.9)

ix) the viral complementary S RNA nucleotide sequence from 1 to 2993; (SEQ ID No.10)

x) the viral complementary S RNA nucleotide sequence from 150 to 938; (SEQ ID No.11)

xi) the S RNA nucleotide sequence from 1581 to 2930 of the viral complementary S RNA strand; (SEQ ID No.12);

xii) the viral complementary S RNA secondary structure having a nucleotide sequence of 642 nucleotides from 939 to 1580; (SEQ ID No.13)

xiii) S RNA nucleotide sequence from 87 to 1436 in which one or more codons have been replaced by their synonyms;

xiv) S RNA nucleotide sequence from 2080 to 2868 in which one or more codons have been replaced by their synonyms;

xv) the M RNA nucleotide sequence from 1 to 4970 (SEQ ID No.14);

xvi) the M RNA sequence from 86 to 997 (SEQ ID No.15);

xvii) the M RNA sequence of the intergenic region from 998 to 1470 (SEQ ID No.16);

xviii) the M RNA sequence from 1471 to 4884; (SEQ ID No. 17)

xix) the M RNA "pan-handle" structure comprising: a) a first nucleotide sequence of from about 30 to about 36 nucleotides in length from the 5' end of the viral M RNA

and

b) a second nucleotide sequence of from about 30 to about 36 nucleotides in length from the 3' end of the viral M RNA

xx) the complementary viral M RNA sequence from 1 to 4970; (SEQ ID No.20)

xxi) the complementary viral M RNA sequence from position 87 to position 3500 of the complementary viral M RNA sequence; (SEQ ID No.21)

xxii) the complementary viral M RNA sequence from position 3974 to 4885 (SEQ ID No.22)

xxiii) RNA sequences homologous to the nucleotide sequences defined under i) to xii) and xv) to xxii) hereinabove.

xxiv) fragments of sequences defined under i) to xxii) hereinabove.

Preferred INSV-Related DNA Sequences code for transcription into the RNA sequences according to sequences iv) to xii) and xv) to xxii) as defined above, or into RNA sequences homologous thereto, or into fragments thereof comprising at least 15 nucleotides, more preferably at least 20 nucleotides, and most preferably at least 50 nucleotides.

According to another preferred embodiment of the invention the DNA constructs of the invention comprise INSV Related DNA Sequences coding for transcription into a combination of the 5' and 3' terminal sequences (ie "pan-handles) of viral S, M or L RNA respectively, more preferably of S or M RNA, and most preferably of S RNA. Examples of S RNA and M RNA terminal sequences include

i) a first nucleotide sequence 36 nucleotides in length from the 5' end of the viral S RNA:

(SEQ ID No.5 i.e. nucleotides from position 1 to 36 of SEQ ID No.1, where N stands for A,T,G, or C)

and

a second nucleotide sequence 36 nucleotides in length from the 3' end of the viral S RNA:

(SEQ ID No.6 ie nucleotides from position 2981 to 3017 of SEQ ID No.1)

ii) a first nucleotide sequence of 36 nucleotides in length from the 5' end of the viral M RNA:

(SEQ ID No.18 ie nucleotides from position 1 to 36 of SEQ ID No.13)

and

b) a second nucleotide sequence 36 nucleotides in length from the 3' end of the viral M RNA

(SEQ ID No.19 ie nucleotides from position 4941 to 4970 of SEQ ID No. 13)

In the case of the terminus at the 5' end of the S RNA it is not known whether or not there are sixteen or seventeen nucleotides in the unknown region demarked by a series of "N" s, however the exact number of nucleotides in this region is not considered to be critical to the formation of "pan-handle" structures so long as the 5' end of the S RNA is capable of complementing the 3' end of the S RNA thus enabling the formation of a "pan-handle" structure.

The invention further provides probes suitable for use as diagnostic tools for the diagnosis of disease in plants suspected of being infected with INSV tospoviruses. Such probes comprise a labeled oligonucleotide (RNA or DNA) sequence complementary to an RNA sequence of an INSV tospovirus. The desired length of the sequence and appropriate method for diagnostic use of probes are known by those skilled in the art. A suitable probe may comprise a nucleotide sequence of at least 12 to about 800 nucleotides, preferably at least 15, more preferably more than 30 nucleotides, and most preferably from about 400 to 600 nucleotides complementary to an RNA sequence of an INSV tospovirus.

Probes according to the invention are helpful in identifying INSV tospovirus RNA or parts thereof in infected plant material i.a. for diagnostic purposes prior to full presentation of disease symptoms in plants.

The invention accordingly also provides a diagnostic method of determining INSV tospovirus infection in plants which comprises detecting INSV tospovirus replicative forms employing the probes of the invention in dot-blot type assays.

Probes according to the invention are useful in the construction of and use of chimeric genes comprising a DNA sequence corresponding to an RNA sequence of an INSV tospovirus.

The DNA constructs of the invention may be obtained by insertion of an INSV Related DNA Sequence in an appropriate expression vector, such that the sequence is brought under expression control of a promoter capable of functioning in plants and its transcription is terminated by a terminator capable of functioning in plants.

The term "appropriate expression vector" as used herein refers to a vector containing a promoter region and a terminator region which are capable of functioning in plant cells.

The insertion of an INSV Related DNA Sequence into an appropriate expression vector may be carried out in a manner known per se. Suitable procedures are illustrated in the examples hereinafter.

Likewise the construction of an appropriate expression vector may be carried out in a manner known per se.

Plants according to the invention may be obtained by

a) inserting into the genome of a plant cell a DNA construct as hereinbefore defined;

b) obtaining transformed cells; and

c) regenerating from the transformed cells genetically transformed plants.

DNA vectors of the present invention may be inserted into the plant genome of plants susceptible to INSV infection. Such plant transformation may be carried out employing techniques known per se for the transformation of plants, such as plant transformation techniques involving Ti plasmids derived from Agrobacterium tumefaciens, A. rhizogenes or modifications thereof, naked DNA transformation or electroporation of isolated plant cells or organized plant structures, the use of micro-projectiles to deliver DNA, the use of laser systems, liposomes, or viruses or pollen as transformation vectors and the like.

Plants of the invention may be monitored for expression of an INSV-Related DNA Sequence by methods known in the art, including Northern analysis, Southern analysis, PCR techniques and/or immunological techniques and the like. The plants of the invention show decreased susceptibility to INSV infection as demonstrated by tests whereby the plants are exposed to INSV preferentially at a concentration in the range at which the rate of disease symptoms correlates linearly with INSV concentration in the inoculum.

Methods suitable for INSV inoculation are known in the art and include mechanical inoculation, and in particular, the use of appropriate vectors.

Plants of the invention may also be obtained by the crossing of a plant obtained according to the methods of the invention with another plant to produce plants having in their plant genome a DNA construct of the invention.

The invention is illustrated by the following non-limiting examples and accompanying figures.

FIG. 1: Schematic representation of an INSV particle.

FIG. 2: Sequence strategy for INSV viral S RNA.

FIG. 3: Open reading frame analysis of the INSV S RNA, full bars represent translational stop codons (TAA, TAG, TGA), half size bars indicate start codons (ATG).

FIG. 4: Schematic review of the construction of a suitable expression vector (pZU-B).

FIG. 5: Schematic review of the construction of a suitable plasmid comprising the INSV N protein-coding sequence.

FIG. 6: Schematic review of the construction of a suitable plasmid comprising the INSV NSs protein-coding sequence.

FIG. 7: Schematic review of the construction of a suitable plasmid comprising the INSV NSm protein-coding sequence.

FIG. 8: Schematic review of the construction of a suitable plasmid comprising the INSV G1/G2 glycoprotein precursor-coding sequence.

FIG. 9: Schematic review of the construction of a INSV N gene-containing plant transformation vector.

FIG. 10: Schematic review of the construction of a INSV NSs gene-containing plant transformation vector.

FIG. 11: Schematic review of the construction of a INSV G1/G2 glycoprotein precursor gene-containing plant transformation vector.

FIG. 12: Schematic review of the construction of a INSV NSm gene-containing plant transformation vector.

FIG. 13: The secondary structure located at the intergenic region of INSV S RNA.

Suitable examples of preferred INSV Related DNA Sequences coding for transcription into a sequence of the secondary structure of the intergenic region of S RNA or of RNA sequences homologous thereto are sequences coding for the 1437 to 2079 nucleotide sequence of S RNA or for a sequence homologous to such sequences.

Other advantageous features of the present invention will be apparent from the following examples.

MATERIAL AND METHODS

All INSV RNA-derived sequences presented here are depicted as DNA sequences for the sole purpose of uniformity. It will be appreciated that this is done for convenience.

Cultivars of Nicotiana tabacum and Petunia hybrida, used in plant transformation studies, are grown under standard greenhouse conditions. Axenic explant material is grown on standard MS media ›Murashige and Skoog, (1962) Physiol Plant 15:473-497! containing appropriate phytohormones and sucrose concentrations.

E. coli bacteria are grown on rotary shakers at 37.degree. C. in standard LB-medium. Agrobacterium tumefaciens strains are grown at 28.degree. C. in MinA medium supplemented with 0.1% glucose ›Ausubel et al., (1987) Current Protocols in Molecular Biology, Green Publishing Associates and Wiley Intersciences, New York, Chichester, Brisbane, Toronto, and Singapore!.

In all cloning procedures the E. coli strain JM83, (F.sup.-, .DELTA.(lac-pro), ara, rpsL, .O slashed.80, dlacZM15) is used as a recipient for recombinant plasmids.

Binary vectors are conjugated to Agrobacterium tumefaciens strain LBA 4404, a strain containing the Ti-plasmid vir region, ›Hoekema et al., (1983) Nature 303:179-180! in standard triparental matings using the E. coli HB101, containing the plasmid pRK2013 as a helper strain. ›Figurski and Helinski, (1979) Proc. Natl. Acad. Sci.USA 76:1648-1652! Appropriate Agrobacterium tumefaciens recipients are selected on media containing rifampicin (50 .mu.g/ml) and kanamycine (50 .mu.g/ml).

Cloning of fragments in the vectors pUC19 ›Yanish-Perron et al. (1985) Gene 33:103-119!, pBluescript (Stratagene), pBIN19 ›Bevan et al., (1984) Nucl Acids Res. 12:8711-8721! or derivatives, restriction enzyme analysis of DNA, transformation to E. coli recipient strains, isolation of plasmid DNA on small as well as large scale, nick-translation, in vitro transcription, DNA sequencing, Southern blotting and DNA gel electrophoresis are performed according to standard procedures ›Maniatis et al., (1982) Molecular Cloning, a Laboratory Manual. Cold Spring Harbor Laboratory, New York; Ausubel et al. supra, (1987)!.

DNA amplification using the polymerase chain reaction (PCR) were performed as recommended by the supplier of the Taq polymerase (Perkin Elmer Cetus).

Amplifications of RNA by reverse transcription of the target RNA followed by standard DNA amplification were performed using the Gene Amp RNA PCR Kit as recommended by the supplier (Perkin Elmer Cetus).

Isolation of INSV particles and genetic material therein

INSV isolate NL-07, an isolate from Impatiens, is maintained on Impatiens by grafting. Virus is purified from systemically infected Nicotiana rustica leaves, after mechanical inoculation essentially as described by Tas et al. ›(1977) J. Gen. Virol. 36:81-91!. All material used in the isolation procedure should be maintained at a temperature of 4.degree. C. Twelve days after inoculation 100 grams of infected leaves are harvested and ground for 5-10 seconds at a low speed setting in 5 volumes extraction buffer (0.1M NaH.sub.2 PO.sub.4, 0.01M Na.sub.2 SO.sub.3, pH 7) in a Waring blender. The suspension is filtered through cheesecloth and the filtrate is centrifuged for 10 minutes at 16,000.times.g. The resulting pellet is resuspended in three volumes resuspension buffer (0.01M NaH.sub.2 PO.sub.4, 0.01M Na.sub.2 SO.sub.3, pH 7). The pellet is dissolved by stirring carefully at 4.degree. C. After centrifuging for 10 minutes at 12,500.times.g the pellet is discarded and the supernatant centrifuged again for 20 minutes at 50,000.times.g. The pellet is resuspended in 0.2 volume of resuspension buffer (0.01M NaH.sub.2 PO.sub.4, 0.01M Na.sub.2 SO.sub.3, pH 7) and kept on ice for 30 minutes. Anti-serum raised in rabbits against material from non-infected Nicotiana rustica is added to the solution and carefully stirred for 1 hour. Non-viral complexes are pelleted after 10 minutes centrifuging at 16,000.times.g. The cleared supernatant is loaded on a linear 5%-40% sucrose gradient in resuspension buffer(0.01M NaH.sub.2 PO.sub.4, 0.01M Na.sub.2 SO.sub.3, pH 7), and spun for 45 minutes at 95,000.times.g. The opalescent band containing INSV particles is carefully collected with a syringe and diluted 4 times with resuspension buffer. Washed viruses are pelleted by centrifugation for 1.5 hours at 21,000.times.g and resuspended in one volume of resuspension buffer. Generally, 100 grams of leaf material yields approximately 0,5 mg of INSV viruses. INSV RNA is recovered preferentially from purified virus preparations by SDS-phenol extractions followed by ethanol precipitation. From 1 mg INSV, 1-5 .mu.g of RNA is extracted. The isolated RNA molecules are analysed for intactness by electrophoresis on an agarose gel. Three distinct RNA molecules are identified with apparent sizes of about 3000 nucleotides (S RNA), about 4900 nucleotides (M RNA) and about 8900 nucleotides (L RNA) respectively.

Sequence determination of the 3'-termini of the INSV viral RNAs

In order to perform direct RNA sequencing, INSV RNA is extracted from purified nucleocapsids essentially according to Verkleij et al. (1983) supra. Twelve days after inoculation 100 grams of infected leaves are harvested and ground for 5-10 seconds at a low speed setting in four volumes of TAS-E buffer (0.01M EDTA, 0.01M Na.sub.2 SO.sub.3, 0.1% cysteine, 0.1M TRIS pH 8.0) in a Waring blender. The suspension is filtered through cheesecloth and centrifuged for 10 minutes at 1,100.times.g. Nucleocapsids are recovered from the supernatant after 30 minutes of centrifuging at 66,000.times.g. The pellet is carefully resuspended in one volume of TAS-R buffer (1% Nonidet NP-40, 0.01M EDTA, 0.01M Na.sub.2 SO.sub.3, 0.1% cysteine, 0.01M glycine, 0.01M TRIS, pH 7.9). The pellet is dissolved by stirring carefully for 30 minutes at 4.degree. C. The supernatant is cleared by centrifuging for 10 minutes at 16,000.times.g. Crude nucleocapsids are collected from the cleared supernatant by sedimentation through a 30% sucrose cushion for 1 hour at 105,000.times.g. The nucleocapsid pellet is resuspended in 400 .mu.l 0.01M Na-citrate pH 6.5, layered on a 20-40% sucrose (in 0.01M Na-citrate pH 6.5) and spun for 2 hours at 280,000.times.g. The three different opalescent bands, respectively L, M and S nucleocapsid, are collected separately. INSV RNA is recovered preferentially from purified nucleocapsid preparations by SDS-phenol extractions followed by ethanol precipitation. Generally, 100 .mu.g of RNA are obtained from 100 grams of infected leaves. The 3'-ends of the separate INSV RNAs are labeled using RNA ligase and 5'-›.sup.32 P!pCp. The end-labeled RNA molecules are separated on a low gelling temperature agarose gel ›Wieslander, (1979) Anal Biochem 98:305-309!. The enzymatic approach described by Clerx-Van Haaster and Bishop ›(1980) Virology 105:564-574! and Clerx-Van Haaster et al. ›(1982) J Gen Virol 61:289-292! is used to determine the 30 terminal nucleotides of the 3'- and 5'-ends of both S and M RNA.

Synthetic oligonucleotides complementary to the 3'-termini are synthesized using a commercially available system (Applied Biosystems) and used for dideoxy-sequencing with reverse transcriptase.

cDNA cloning of INSV genetic material

Oligonucleotides complementary to the 3'-end of the S RNA are used for priming first strand cDNA synthesis. With these primers, double stranded DNA to INSV RNA is synthesized according to Gubler and Hoffman ›(1983) Gene 25:263-269!.

Two different approaches are used to generate cDNA clones to the INSV viral RNAs. A first series of clones is obtained by random priming of the INSV RNA using fragmented single stranded calf thymus DNA, followed by first and second strand cDNA synthesis. cDNA is made blunt-ended using T4-DNA polymerase and ligated with T4 ligase into the SmaI site of pUC19.

A second series of INSV cDNA clones is obtained by priming first strand DNA synthesis with the oligonucleotides complementary to the 20 terminal nucleotides at the 3'-ends of the INSV RNAs. Blunt ended cDNA fragments are cloned into the Sma I site of pUC19.

cDNA clones from both series containing viral inserts are selected via colony hybridization, essentially according to the method of Grunstein and Hogness ›(1975) Proc. Natl. Acad. Sci. USA 72:3961-3965! using ›.sup.32 !P-labeled, randomly primed first strand cDNA as a probe. Sets of overlapping cDNA clones are selected by Southern analysis followed by plasmid walking, in order to construct a restriction map, based on cDNA derived sequences of the S RNA (FIG. 2).

Sequence determination of the INSV S RNA

In order to determine the sequence of the S RNA 5 selected cDNA clones are subcloned into pBluescript, resulting in the plasmids pINSV-S2, pINSV-S15, pINSV-S61, pINSV-S60 and pINSV-S39, (FIG. 2). The clones are sequenced in both directions using the protocol of zhang et al. ›(1988) Nucl. Acids. Res. 16:1220!. The nucleotide sequence of the 3'-end of the S RNA is determined by primer extension of the synthetic oligonucleotide INSV-S60 (5' d(AGAGCAATTGTGTCA) which is complementary to the 15 nucleotides of the 3'-terminus. Sequence data from the INSV S RNA (3017 nt) is summarized in the sequence listing (SEQ ID No.1 to SEQ ID No.12).

Computer simulated translation of the 6 different reading frames on the viral strand and viral complementary strand reveals the presence of two putative open reading frames (FIG. 3). On the viral strand an open reading frame is found starting at position 87 and terminating at a UAA stopcodon at position 1436 encoding a protein of 449 amino acids with a predicted molecular mass of about 51.2 kd. This protein is a non-structural protein, tentatively designated NSs (FIG. 3/SEQ ID No.26). The other open reading frame is located on the viral complementary strand from position 2080 to 2868 (SEQ ID No. 11), encoding a 262 amino acid long polypeptide with a predicted molecular mass of about 28.7 kd. This open reading frame encodes the viral nucleocapsid protein N (FIG. 3/SEQ ID No 25). Thus FIG. 3 shows the coding capacities of the viral and the viral complementary strand of INSV S RNA, indicating the NSs and N protein genes are expressed from subgenomic mRNAs (SEQ ID No.3, SEQ ID No.11 respectively). Thus, the situation occurs that a plant virus RNA has an ambisense gene arrangement. Other important features of this S RNA sequence is the existence of complementary terminal repeats capable of forming so-called "pan-handle" structures. These structures play an important role in replication and transcription of viral RNA. Another putative regulatory element is the secondary structure in the intergenic region of the S RNA, which most likely contains the transcription termination signals for both subgenomic mRNAs, encoding respectively the N and NSs-protein.

The nucleotide sequence of the INSV M and L RNA is elucidated employing similar strategies and methods as used to determine the nucleotide sequence of the S RNA.

Construction of an expression vector pZU-B

The recombinant plasmid pZO347 is a derivative of pBluescript carrying a 496 bp BamHI-SmaI fragment containing a 426 bp 35S promoter fragment (HincII fragment) of CaMV strain Cabb-S, linked to a 67 bp fragment of the non-translated leader region, the so-called .OMEGA.-region, of the tobacco mosaic virus. This results in a chimeric promoter with a complete transcriptional fusion between the promoter of CaMV to the untranslated leader of TMV. By using in vitro mutagenesis the original position of the TMV ATG startcodon is mutated to a SmaI site.

The plasmid pZO008 carries the nopaline synthase (NOS) terminator as a 260 bp PstI-HindIII fragment. This PstI-HindIII fragment is excised from pZO008 and ligated using T4 ligase into PstI-HindIII linearized pZO347. The resulting recombinant plasmid pZU-B is another plant expression vector. The sequence of this 35S-.OMEGA. promoter as used in the plant expression vector pZU-B is shown as SEQ ID No.23. The resulting recombinant plasmid pZU-B contains the 35S HincII-TMV .OMEGA. fusion (35S-.OMEGA.), unique SmaI and PstI sites and the NOS terminator (FIG. 4). This expression vector is preferentially used in constructing translational fusions of the gene for expression downstream of the chimaeric promoter 35S-.OMEGA..

Subcloning of the INSV N protein gene

The INSV N protein coding sequence is obtained by fusion of the cDNA clones pINSV-S60 and pINSV-S39 (FIG. 5). The cDNA clone pINSV-S60 is subjected to SpeI digestion and the fragment containing the 3'-end of the INSV N protein gene is separated electrophoretically and purified from the gel using a DEAE membrane (NA-45, Schleicher and Schull) and cloned in the largest SpeI fragment of pINSV-S39 linearized resulting in the recombinant plasmid pINSV-N. Primers are designed homologous to the translational start and stop codon. Primer INSV-066 d(GCAGATATCATGAACAAAGC) creates an EcoRV site just proximal to the start codon.

Primer INSV-070 d(GCAACCTGCAGCTCAAATCTCTT) creates a PstI site just distal to the stop codon. These primers are used in standard PCR experiments in which pINSV-N is used as the template. The resulting PCR fragment is isolated from the gel using a DEAE membrane (NA-45, Schleicher and Schull) and cloned in the SmaI linearized pBluescript to generate plasmid pINSV-N2. The added restriction sites, EcoRV and PstI, facilitate the construction of further plasmids. (Alternatively, one may choose to add the sites in different ways such as but not limited to site-directed mutagenesis or by ligation of other synthetic oligonucleotide linkers. Such methods are all known to a person skilled in the art.)

Subcloning of the INSV non-structural protein genes (NSs gene) of INSV S RNA

The sequence of the gene corresponding to the non-structural protein NSs is isolated using RNA based PCR on isolated INSV S RNA. Two primers are designed which are homologous to regions spanning either the translational start codon or stop codon. The start codon primer contains an EcoRV site proximal to the ATG codon, the stop codon primer has a PstI site just distal thereto. Purified INSV S RNA is subjected to the Gene AMP RNA PCR. The resulting PCR fragment is isolated from the gel and cloned into SmaI linearized pBluescript yielding the recombinant plasmid pINSV-NSs (FIG. 6).

Subcloning of the INSV non-structural protein gene (NSm gene) of the INSV M RNA

The sequence of the gene corresponding to the non-structural protein NSm is isolated using RNA based PCR on isolated INSV M RNA. Two primers are designed which are homologous to regions spanning either the translational start codon or stop codon. The start codon primer contains an EcoRV site proximal to the ATG codon, the stop codon primer has a PstI site just distal thereto. Purified INSV S RNA is subjected to the Gene AMP RNA PCR. The resulting PCR fragment is isolated from the gel and cloned into SmaI linearized pBluescript yielding the recombinant plasmid pINSV-NSm (FIG. 7).

Subcloning of the INSV G1/G2 glycoprotein gene (G1/G2 gene) of the INSV M RNA

The sequence of the gene corresponding to the G1/G2 glycoprotein precursor is isolated using RNA based PCR on isolated INSV M RNA. Two primers are designed homologous to regions spanning either the translational start codon or stop codon. The start codon primer contains an EcoRV site proximal to the ATG codon, the stop codon primer has a PstI site just distal thereto. Purified INSV M RNA is subjected to the Gene AMP RNA PCR. The resulting PCR fragment is isolated from the gel and cloned into SmaI linearized pBluescript yielding the recombinant plasmid pINSV-G1/G2 (FIG. 8).

Construction of plant transformation vectors containing INSV sequences

N protein constructions in pZU-B

In order to make a fusion in which the ATG start codon from the N protein gene is fused directly to the 3'-end of the TMV untranslated leader of the 35S-.OMEGA. promoter the start codon of the N gene has to be mutated using the PCR approach as hereinbefore described. The N protein gene is excised from the plasmid pINSV-N2 via an EcoRV-PstI digestion. The fragment is isolated and inserted into the SmaI-PstI linearised pZU-B, resulting in recombinant plasmid pINSV-NB. The chimeric cassette containing the 35S-.OMEGA. promoter, the N gene and the NOS terminator is excised from the plasmid pINSV-NB via a BamHI/XbaI digestion. The isolated chimaeric gene cassette is then inserted into the BamHI/XbaI linearized pBIN19 to create the binary transformation vector pINSV-NBB. The resulting plasmid pINSV-NBB (FIG. 9) is used in plant transformation experiments using methods well known to a person skilled in the art.

NSs protein gene constructions in pZU-B

In order to create a fusion in which the ATG start codon from the NSs protein is fused directly to the 3'-end of the TMV leader of the 35S-.OMEGA. promoter the start codon of the NSs gene is mutated, using the PCR approach. The plasmid PINSV-Ns is digested with EcoRV and PstI and the NSs containing fragment is isolated from the gel and inserted into SmaI/PstI linearized pZU-B resulting in the recombinant plasmid pINSV-NSsB. The chimaeric cassette containing the 35S-.OMEGA. promoter, the mutated NSs protein gene and the NOS terminator is excised from the plasmid pINSV-NSsB via a BamHI/XbaI digestion. The isolated chimeric gene cassette is then inserted into the BamHI/XbaI linearized pBIN19 to create the binary transformation vector pINSV-NSsBB. The resulting plasmid pINSV-NSsBB (FIG. 10) is used in plant transformation experiments using methods well known to a person skilled in the art.

G1/G2 glycoprotein gene constructions in pZU-B

In order to create a fusion in which the ATG start codon from the G1/G2 glycoproteinprecursor is fused directly to the 3'-end of the TMV leader of the 35S-.OMEGA. promoter the start codon of the G1/G2 gene is mutated, using the PCR approach. The plasmid pINSV-G1/G2 is digested with EcoRV and PstI and the G1/G2 containing fragment is isolated from the gel and inserted into SmaI/PstI linearized pZU-B resulting in the recombinant plasmid pINSV-G1/G2B. The chimeric cassette containing the 35S-.OMEGA. promoter, the mutated G1/G2 glycoprotein gene and the NOS terminator is excised from the plasmid pINSV-G1/G2B via a BamHI/XbaI digestion. The isolated chimeric gene cassette is then inserted into the BamHI/XbaI linearized pBIN19 to create the binary transformation vector pINSV-G1/G2BB. The resulting plasmid pINSV-G1/G2BB (FIG. 11) is used in plant transformation experiments using methods well known to a person skilled in the art.

NSm protein gene constructions in pZU-B

In order to create a fusion in which the ATG start codon from the NSm protein is fused directly to the 3'-end of the TMV leader of the 35S-.OMEGA. promoter the startcodon of the NSm gene is mutated, using the PCR approach. The plasmid pINSV-NSm is digested with EcoRV and PstI and the NSm-containing fragment is isolated from the gel and inserted into SmaI/PstI linearized pZU-B resulting in the recombinant plasmid pINSV-NSmB. The chimeric cassette containing the 35S-.OMEGA. promoter, the mutated NSm protein gene and the NOS terminator is excised from the plasmid pINSV-NSmB via a BamHI/XbaI digestion. The isolated chimeric gene cassette is then inserted into the BamHI/XbaI linearized pBIN19 to create the binary transformation vector pINSV-NSmBB. The resulting plasmid pINSV-NSmBB (FIG. 12) is used in plant transformation experiments using methods well known to a person skilled in the art.

A DNA analysis programme is used to locate the "pan-handle" element of the loop in the viral INSV S RNA. The strongest "pan-handle" structure that is detected includes about the first 24-25 nucleotides at the 5'-end (1 to 24 or 25) of the viral S RNA and about the last 36 nucleotides at the 3'-end of the viral S RNA (SEQ ID Nos 5 and 6 respectively). The length of the pan-handle element of the loop is about 36 nucleotides long.

These regions are synthesized on a commercial DNA synthesizer and appropriate linker sequences are added. Construction of the "pan-handle" vectors of S and M RNA results in respectively: pINSV-termS and pINSV-termM. Using appropriate restriction enzyme combination these fragments are inserted between the 35S-.OMEGA. promoter and the NOS terminator of pZU-B yielding the chimeric cassettes: pINSV-termSA, pINSV-termMA, pINSV-termSB and pINSV-termMB. These cassettes are then transferred into the binary transformation vector pBIN19 using appropriate enzyme combinations yielding the following plasmids: pINSV-termSAB, pINSV-termMAB, pINSV-termSBB and pINSV-termMBB. Alternatively, it is possible to design "pan-handle" constructs including the 3'- and 5'-end termini that are larger than indicated above, or separated by any other DNA sequence in order to enhance the stability of the transcripts produced from these recombinant genes in plants. All "pan-handle" constructs resemble shortened tospovirus RNA molecules, specifically INSV RNA molecules and therefore can be regarded as defective interfering RNAs.

Construction containing INSV S RNA secondary structure region in pZU-B

A DNA analysis programme is used to locate a secondary structure in the viral INSV S RNA. The strongest secondary structure detectable starts at nucleotide 1440 and ends at nucleotide 2041 of SEQ ID No.1, (SEQ ID No 8).

The DNA fragment carrying the secondary structure region is isolated from pINSV-S61 using a PCR approach similar to that described earlier. The two primers used contain the sequences 1440-1460 and 2021-2041 of SEQ ID No.1. The PCR fragment is excised from an agarose gel and subsequently treated with T4 polymerase to create blunt ends and is subsequently cloned into the SmaI site of the expression vector pZU-B, resulting in the recombinant plasmid PINSV-HpSB. The plasmid pINSV-HpSB is digested with HindIII and the fragment containing the chimeric gene is excised from an agarose gel and ligated into XbaI linearized pBIN19, resulting in the transformation vector pINSV-HpSBB.

(It is clear to a person skilled in the art that other fragments can be isolated from the cDNA clones of the INSV S RNA containing the hairpin region as described above without interference to function. Also, a fragment containing the hairpin region may be synthesized using a DNA-synthesizer.)

Transformation of binary vectors to tobacco plant material

Methods to transfer binary vectors to plant material are well established and known to a person skilled in the art. Variations in procedures exist due to for instance differences in used Agrobacterium strains, different sources of explant material, differences in regeneration systems depending on as well the cultivar as the plant species used.

The binary plant transformation vectors as described above are used in plant transformation experiments according to the following procedures. The constructed binary vector is transferred by tri-parental mating to an acceptor Agrobacterium tumefaciens strain, followed by southern analysis of the ex-conjugants for verification of proper transfer of the construct to the acceptor strain, inoculation and cocultivation of axenic explant material with the Agrobacterium tumefaciens strain of choice, selective killing of the Agrobacterium tumefaciens strain used with appropriate antibiotics, selection of transformed cells by growing on selective media containing kanamycine, transfer of tissue to shoot-inducing media, transfer of selected shoots to root inducing media, transfer of plantlets to soil, assaying for intactness of the construct by southern analyses of isolated total DNA from the transgenic plant, assaying for proper function of the inserted chimeric gene by northern analysis and/or enzyme assays and western blot analysis of proteins.

Expression of INSV S RNA sequences in tobacco plant cells

RNA is extracted from leaves of regenerated plants using the following protocol. Grind 200 mg leaf material to a fine powder in liquid nitrogen. Add 800 .mu.l RNA extraction buffer (100 mM Tris-HCl (pH 8,0), 500 mM NaCl, 2 mM EDTA, 200 mM .beta.-Mercapto-ethanol, 0,4% SDS) and extract the homogenate with phenol, collect the nucleic acids by alcohol precipitation. Resuspend the nucleic acids in 0,5 ml 10 mM Tris-HCl (pH 8,0), 1 mM EDTA, add LiCl to a final concentration of 2M, leave on ice for maximal 4 hours and collect the RNA by centrifugation. Resuspend in 400 .mu.l 10 mM Tris-HCl (pH 8,0), 1 mM EDTA and precipitate with alcohol, finally resuspend in 50 .mu.l 10 mM Tris-HCl (pH 8,0), 1 mM EDTA. RNAs are separated on glyoxal/agarose gels and blotted to Genescreen as described by van Grinsven et al. ›(1986) Theor Appl Gen 73:94-101!. INSV S RNA sequences are detected using DNA or RNA probes labeled with ›.sup.32 P!, ›.sup.35 S! or by using non-radioactive labeling techniques. Based on northern analysis, it is determined to what extent the regenerated plants express chimaeric INSV S RNA sequences.

Plants transformed with chimaeric constructs containing an INSV N protein-encoding sequence are also subjected to western blot analysis. Proteins are extracted from leaves of transformed plants by grinding in sample buffer according to the method of Laemmli ›(1970) Nature 244:29-30!. A 50 .mu.g portion of protein is subjected to electrophoresis in a 12,5% SDS-polyacrylamide gel essentially as described by Laemmli (1970) supra. Separated proteins are transferred to nitrocellulose electrophoretically as described by Towbin et al. ›(1979) Proc. Natl. Acad. Sci. USA 76:4350-4354!. Transferred proteins are reacted with antiserum raised against purified INSV structural or non-structural proteins (Towbin et al. (1979) supra. Based on the results of the western analysis, it is determined that transformed plants do contain INSV N proteins encoded by the inserted chimaeric sequences.

Resistance of plants against INSV infection

Transformed plants are grown in the greenhouse under standard quarantine conditions in order to prevent any infections by pathogens. The transformants are self-pollinated and the seeds harvested. Progeny plants are analyzed for segregation of the inserted gene and subsequently infected with INSV by mechanical inoculation. Tissue from plants systemically infected with INSV is ground in 5 volumes of ice-cold inoculation buffer (10 mM phosphate buffer supplemented with 1% Na.sub.2 SO.sub.3) and rubbed in the presence of carborundum powder on the first two fully extended leafs of approximately 5 weeks old seedlings. Inoculated plants are monitored for symptom development during 3 weeks after inoculation.

Plants containing INSV Related DNA Sequences show reduced susceptibility to INSV infection as exemplified by a delay in symptom development, whereas untransformed control plants show severe systemic INSV symptoms within 7 days after inoculation.

Use of synthetic oligonucleotides for diagnostic purposes

RNA is extracted from leaves of suspected plants using the following protocol: grind 1 gram of leaf material, preferentially showing disease symptoms, in 3 ml 100 mM Tris-HCl, 50 mM EDTA, 1.5M NaCl and 2% CTAB (pH 8.0). After grinding, 1 ml of the homogenate is subjected to chloroform extraction and incubated at 65.degree. C. for 10 minutes. The inorganic phase is then collected and extracted with phenol/chloroform (1:1), followed by a last extraction with chloroform. The ribonucleic acids are isolated from the inorganic phase, containing the total nucleic acids, by adding LiCl to a final concentration of 2M. The preparation is incubated at 4.degree. C. for 1 hour, after which the ribonucleic acids are collected by centrifugation. The ribonucleic acid pellet is resuspended in 25 .mu.l 10 mM Tris-HCl, 1 mM EDTA (pH 8.0). The ribonucleic acids are recovered by standard alcohol precipitation. The ribonucleic acid pellet is resuspended in 25 .mu.l 10 mM Tris-HCl, 1 mM EDTA (pH 8.0).

1 .mu.l of the purified ribonucleic acids is spotted on a nylon blotting membrane (e.g. Hybond-N, Amersham UK). The presence of INSV in the plant is detected by standard hybridization, using any part or parts of the sequence isolated from virions or preferentially by designing synthetic oligomers on the basis of disclosed sequence information as a probe. (Alternatively, in vitro transcripts of regions of the INSV genome are used to detect INSV Related RNA Sequences in diseased plants.) A diseased plant is diagnosed by the occurrence of hybridization at the dot containing RNA material from the diseased plant.