PLANT PATHOGEN RESISTANCE GENES AND USES THEREOF

Abstract: The tomato Cf-5 gene has been cloned and its sequence provided, along with the encoded amino acid sequence. DNA encoding the polypeptide, and alleles, variants, mutants, derivatives and homologues thereof, may be introduced into plant cells and expressed, conferring pathogen resistance on plants comprising such cells and descendants thereof.

Description: PLANT PATHOGEN RESISTANCE GENES AND USES THEREOF The present invention relates to pathogen resistance in plants and more particularly the identification and use of pathogen resistance genes. It is based on cloning of the tomato Cf-5 gene. Plants are constantly challenged by potentially pathogenic microorganisms . Crop plants are particularly vulnerable, because they are usually grown as genetically uniform monocultures; when disease strikes, losses can be severe. However, most plants are resistant to most plant pathogens. To defend themselves, plants have evolved an array of both preexisting and inducible defences. Pathogens must specialize to circumvent the defence mechanisms of the host, especially those biotrophic pathogens that derive their nutrition from an intimate association with living plant cells. If the pathogen can cause disease, the interaction is said to be compatible, but if the plant is resistant, the interaction is said to be incompatible. Race specific resistance is strongly correlated with the hypersensitive response (HR) , an induced response by which (it is hypothesized) the plant deprives the pathogen of living host cells by localized cell death at sites of attempted pathogen ingress. It has long been known that HR-associated disease resistance is often (though not exclusively) specified by dominant genes (R genes) . Flor showed that when pathogens mutate to overcome such R genes, these mutations are recessive. Flor concluded that for R genes to function, there must also be corresponding genes in the pathogen, denoted avirulence genes (Avr genes) . To become virulent, pathogens must thus stop making a product that activates R gene-dependent defence mechanisms (Flor, 1971) . A broadly accepted working hypothesis, often termed the elicitor/receptor model, is that R genes encode products that enable plants to detect the presence of pathogens, provided said pathogens carry the corresponding Avr gene (Gabriel and Rolfe, 1990) . This recognition is then transduced into the activation of a defence response. Some interactions exhibit different genetic properties. Helminthosporiurn carbonum races that express a toxin (He toxin) infect maize lines that lack the Hml resistance, gene. Mutations to loss of He toxin expression are recessive, and correlated with loss of virulence, in contrast to gene-for-gene interactions in which mutations to virulence are recessive. A major accomplishment was reported in 1992, with the isolation by tagging of the Hml gene (Johal and Briggs, 1992) . Plausible arguments have been made for how gene-for-gene interactions could evolve from toxin-dependent virulence. For example, plant genes whose products were the target of the toxin might mutate to confer even greater sensitivity to the toxin, leading to HR, and the conversion of a sensitivity gene to a resistance gene. However, this does not seem to be the mode of action of Hml , whose gene product inactivates He toxin. Pathogen avirulence genes are still poorly understood. Several bacterial Avr genes encode hydrophilic proteins with no homology to other classes of protein, while others carry repeating units whose number can be modified to change the range of plants on which they exhibit avirulence (Keen, 1992; Long and Staskawicz, 1993) . Additional bacterial genes {hrp genes) are required for bacterial Avr genes to induce HR, and also for pathogenicity (Keen, 1992; Long and Staskawicz, 1993) . It is not clear why pathogens make products that enable the plant to detect them. It is widely believed that certain easily discarded Avr genes contribute to but are not required for pathogenicity, whereas other Avr genes are less dispensable (Keen, 1992; Long and Staskawicz, 1993) . The characterization of one fungal avirulence gene has also been reported; the Avr9 gene of Cladosporium fulvum, which confers avirulence on C. fulvum races that attempt to attack tomato varieties that carry the Cf-9 gene, encodes a secreted cysteine-rich peptide with a final processed size of 28 amino acids but its role in compatible interactions is not clear (De Wit, 1992) . The technology for gene isolation based primarily on genetic criteria has improved dramatically in recent years, and many workers are currently attempting to clone a variety of R genes. Targets include (amongst others) rust resistance genes in maize, Antirrhinum and flax (by transposon tagging) ; downy mildew resistance genes in lettuce and Arabidopsis (by map based cloning and T-DNA tagging) ; Cladosporium fulvum ( Cf) resistance genes in tomato (by tagging, map based cloning and affinity labelling with avirulence gene products) ; virus resistance genes in tomato and tobacco (by map based cloning and tagging) ; nematode resistance genes in tomato (by map based cloning) ; and genes for resistance to bacterial pathogens in Arabidopsis and tomato (by map based cloning) . 5 The map based cloning of the tomato Pto gene that confers "gene-for-gene" resistance to the bacterial speck pathogen Pseudomonas syringae pv tomato (Pst) has been reported (Martin et al , 1993) . A YAC (yeast artificial chromosome) clone was identified that carried restriction 0 fragment length polymorphism (RFLP) markers that were very tightly linked to the gene. This YAC was used to isolate homologous cDNA clones. Two of these cDNAs were fused to a strong promoter, and after transformation of a disease sensitive tomato variety, one of these gene fusions was 5 shown to confer resistance to Pst strains that carry the corresponding avirulence gene, AvrPto. These two cDNAs show homology to each other. Indeed, the Pto cDNA probe reveals a small gene family of at least six members, 5 of which can be found on the YAC from which Pto was isolated, and which 0 thus comprise exactly the kind of local multigene family inferred from genetic analysis of other R gene loci. Pto is atypical in encoding a small serine/threonine protein kinase which interacts directly with the avirulence determinant of Pst strains known as AvrPto (Tan et al . , 25 1996, and Scofield et al . , 1996) . Since the isolation of the Pto gene a number of other resistance genes have been isolated. The isolation of the tobacco mosaic resistance gene N from tobacco was reported by Whitham et al (1994) . The isolation of the Arabidopsis thaliana gene for resistance to Pseudomonas syringae RPS2 was reported by Bent et al (1994) and by Mindrinos et al (1994) . These genes probably encode cytoplasmic proteins that carry a P-loop and a leucine-rich repeat. The ligands with which they interact are uncharacterised and it is not known what other plant proteins they interact with to accomplish the defence response. Our own laboratory has reported the isolation of the tomato Cf-9 which confers resistance against the fungus Cladosporium fulvum . This is a subject of a previous patent application (PCT/GB94/02812 published as WO 95/18230) and has been reported in Jones et al (1994) . Cf-9 and Avr9 sequences, and sequences of the encoded polypeptides are given in WO95/18320 and Jones et al (1994) . The Cf-2 gene is the subject of PCT/GB96/00785 filed 1 April 1996 by John Innes Centre Innovations Limited. The Cf-4 gene is the subject of GB 9509575.8 filed 11 May 1995, to be used as priority for an International application by the same company. Sequences for Cf-9, Cf-2 and Cf-4 are disclosed in WO 95/31564. We have now cloned Cf-5. W093/11241 reports the sequence of a gene encoding a polygalacturonase inhibitor protein (PGIP) that has some homology with Cf-9 and, as we have now discovered, Cf-5 (the subject of the present invention) . Cf-9, Cf-5 and others ( Cf-4 , -2 etc.) are termed by those skilled in the art "pathogen resistance genes" or "disease resistance genes" . PGIP-encoding genes are not pathogen resistance genes. A pathogen resistance gene (R) enables a plant to detect the presence of a pathogen expressing a corresponding avirulence gene (Avr) . When the pathogen is detected, a defence response such as the hypersensitive response (HR) is activated. By such means a plant may deprive the pathogen of living cells by localised cell death at sites of attempted pathogen ingress. On the other hand, the PGIP gene of W093/11241 (for example) is a gene of the kind that is induced in the plant defence response resulting from detection of a pathogen by an R gene. Thus, a pathogen resistance gene may be envisaged as encoding a receptor to a pathogen-derived and Avr dependent molecule. In this way it may be likened to the RADAR of a plant for detection of a pathogen, whereas PGIP is involved in the defence the plant mounts to the pathogen once detected and is not a pathogen resistance gene. Expression of a pathogen resistance gene in a plant causes activation of a defence response in the plant . This may be upon contact of the plant with a pathogen or a corresponding elicitor molecule, though the possibility of causing activation by over-expression of the resistance gene in the absence of elicitor has been reported. The defence response may be activated locally, e.g. at a site of contact of the plant with pathogen or elicitor molecule, or systemically. Activation of a defence response in a plant expressing a pathogen resistance gene may be caused upon contact of the plant with an appropriate, corresponding elicitor molecule, e.g. as produced by a Cladosporium fulvum avr gene as discussed. The elicitor may be contained in an extract of a pathogen such as Cladosporium fulvum, or may be wholly or partially purified and may be wholly or partially synthetic. An elicitor molecule may be said to "correspond" if it is a suitable ligand for the R gene product to elicit activation of a defence response. The "Cf-x"/"Avrx" terminology is standard in the art. The Cf resistance genes and corresponding fungal avirulence genes (Avr) were originally defined genetically as interacting pairs of genes whose measurable activities fall into mutually exclusive interacting pairs. Avr9 elicits a necrotic response on Cf-9 containing tomatoes but no response on Cf-5 containing tomatoes, the moeity recognised by Cf-5 being different from that recognised by Cf-9. Expression of Cf-5 function in a plant may be determined by investigating compatibility of various C. fulvum races. A race of C. fulvum that carries functional copies of all known Avr genes (race 0) will grow (compatible) only on a tomato which lacks all the Cf genes. It will not grow (incompatible) on a plant carrying any functional Cf gene. If the C. fulvLim race lacks a functional Avr5 gene (race 5) it will be able to grow not only on a plant lacking any Cf genes but also a plant carrying the Cf-5 gene. A race also lacking a functional Avr4 gene (race 4,5) will also be able to grow on a plant carrying the Cf-4 gene. A race only lacking a functional Avr4 gene (race 4) will not be able to grow on a plant carrying Cf-5. Similarly, a C. fulvum race 2 (lacking a functional Avr2 gene) will not be able to grow on a plant carrying a Cf -5 gene. Neither a race 4 nor a race 4, 5 will be able to grow on a plant carrying any of the other Cf genes . Various races are commonly available in the art, e.g. from the Research Institute for Plant Protection (IPO-DLO) , PO Box 9060, 6700 GW Wageningen, The Netherlands. A race 4 is available under accession number IPO10379 and a race 2,4 available under Accession number IPO50379. We have now isolated a tomato gene, Cf-5, which confer resistance against the fungus Cladosporium fulvum and we have sequenced the DNA and deduced the amino acid sequence. The DNA sequence of the tomato Cf-5 gene is shown in Figure 1 and the deduced amino acid sequence shown in Figure 2. Two variant sequences are provided in these figures. According to one aspect, the present invention provides a nucleic acid isolate encoding a pathogen resistance gene, the gene being characterized in that it comprises nucleic acid encoding the amino acid sequence shown in Figure 2A or Figure 2B, or a fragment thereof. The nucleic acid isolate may comprise DNA, and may comprise the sequence shown in Figure IA or Figure IB, or a sufficient part to encode the desired polypeptide (eg. from the initiating methionine codon to the first in frame downstream stop codon) . A suitable fragment is nucleotides 654-3558 of the sequence of Figure IA or Figure IB. A further aspect of the invention provides a nucleic acid isolate encoding a pathogen resistance gene, or a fragment thereof, obtainable by screening a nucleic acid library with a probe comprising nucleotides of Figure IA or Figure IB or a fragment, derivative, mutant, variant or allele thereof, and isolating DNA which encodes a polypeptide able to confer pathogen resistance to a plant, such as resistance to Cladosporium fulvum (eg . expressing AvrS) . The plant may be tomato. Suitable techniques are well known in the art . Nucleic acid according to the present invention may encode the amino acid sequence shown in Figure 2A or Figure 2B or a mutant, variant, derivative or allele of the sequence provided. Preferred mutants, variants, derivatives and alleles are those which retain a functional characteristic of the protein encoded by the wild-type gene, especially the ability to confer pathogen resistance and most especially the ability to confer resistance against a pathogen expressing the Avr5 elicitor molecule. A fragment of the sequence, or an altered version thereof, may be employed which is able to confer recognition of Avr5, such as a fragment including the region LRR5 to LRR18, for example within the context of a synthetic construct comprising other domains of one or more resistance genes, including one or more domains required for induction of a resistance response upon recognition of Avr5. Changes to a sequence, to produce a mutant, variant or derivative, may be by one or more of addition, insertion, deletion or substitution of one or more nucleotides in the nucleic acid, leading to the addition, insertion, deletion or subsitution of one or more amino acids. Of course, changes to the nucleic acid which make no difference to the encoded amino acid sequence are included. A mutant, allele, variant or derivative amino acid sequence in accordance with the present invention may include within the sequence shown in Figure 2A or Figure 2B, a single amino acid change with respect to the sequence shown in Figure 2A or Figure 2B, e.g. at residue 36, or 2, 3, 4, 5, 6, 7, 8, or 9 changes, about 10, 15, 20, 30, 40 or 50 changes, or greater than about 50, 60, 70, 80 or 90 changes. In addition to one or more changes within the amino acid sequence shown in Figure 2A or Figure 2B, a mutant, allele, variant or derivative amino acid sequence may include additional amino acids at the C-terminus and/or N-terminus . A sequence related to a sequence specifically disclosed herein shares homology with that sequence. Homology may be at the nucleotide sequence and/or amino acid sequence level. Preferably, the nucleic acid and/or amino acid sequence shares homology with the coding sequence or the sequence encoded by the nucleotide sequence of Figure IA or IB, preferably at least about 50%, or 60%, or 70%, or 80% homology, most preferably at least about 90%, 95%, 96%, 97%, 98% or 99% homology. As is well-understood, homology at the amino acid level is generally in terms of amino acid similarity or identity. Similarity allows for "conservative variation", i.e. substitution of one hydrophobic residue such as isoleucine, valine, leucine or methionine for another, or the substitution of one polar residue for another, such as arginine for lysine, glutamic for aspartic acid, or glutamine for asparagine. Similarity may be as defined and determined by the TBLASTN program, of Altschul et al . (1990) J. Mol . Biol . 215: 403-10, which is in standard use in the art, or, and this may be preferred, the standard program BestFit, which is part of the Wisconsin Package, Version 8, September 1994, (Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA, Wisconsin 53711) . BestFit makes an optimal alignment of the best segment of similarity between two sequences. Optimal alignments are found by inserting gaps to maximize the number of matches using the local homology algorithm of Smith and Waterman Homology may be over the full-length of the relevant sequence shown herein, or may more preferably be over a contiguous sequence of about 20, 25, 30, 40, 50 or more amino acids or codons, compared with the relevant amino acid sequence or nucleotide sequence as the case may be. Also provided by an aspect of the present invention is nucleic acid comprising a sequence of nucleotides complementary to a nucleotide sequence hybridisable with any encoding sequence provided herein. Another way of looking at this would be for nucleic acid according to this aspect to be hybridisable with a nucleotide sequence complementary to any encoding sequence provided herein. Of course, DNA is generally double-stranded and blotting techniques such as Southern hybridisation are often performed following separation of the strands without a distinction being drawn between which of the strands is hybridising. Preferably the hybridisable nucleic acid or its complement encode a polypeptide able to confer pathogen resistance on a host, i.e., includes a pathogen resistance gene. Preferred conditions for hybridisation are familiar to those skilled in the art, but are generally stringent enough for there to be positive hybridisation between the sequences of interest to the exlucsion of other sequences. Nucleic acid according to the present invention, for instance mutants, variants, derivatives and alleles of the specific sequences disclosed herein, may be distinguished from Cf-9, Cf-2 and/or Cf -4 by one or more of the following, or other features directly derivable from comparison of the Cf-5 amino acid sequence with the respective other sequence: - eliciting a defence response, in a plant expressing the nucleic acid, upon contact of the plant with Avr5 elicitor molecule, e.g. as provided by a Cladosporium fulvum race expressing Avr5 (available in the art) ; eliciting a defence response, in a plant expressing the nucleic acid, upon contact of the plant with the C. fulvum race 4 deposited at and available from the Research Institute for Plant Protection (IPO-DLO) , PO Box 9060, 6700 GW Wageningen, The Netherlands, under accession number IPO10379, or an extract thereof, and eliciting a defence response in the plant upon its contact with the C. fulvum race 2,4 deposited at and available from the same institute under Accession number IPO50379, or an extract thereof; not eliciting a defence response, in a plant expressing the nucleic acid, upon contact of the plant with Avr9 elicitor molecule, e.g. as provided by a Cladosporium fulvum race or other organism expressing Avr9 (de Wit, 1992) , the amino acid and encoding nucleic acid sequences of chimaeric forms of which are given for example in WO95/18230 as SEQ ID NO 3 and in W095/31564 as SEQ ID NO 4; not eliciting a defence response, in a plant expressing the nucleic acid, upon contact of the plant with Avr4 elicitor molecule, e.g. as provided by a Cladosporium fulvum race or other organism expressing Avr4, the amino acid and encoding nucleic acid sequences of which are given in W095/31564 SEQ ID NO. 13; not eliciting a defence response, in a plant expressing the nucleic acid, upon contact of the plant with Avr2 elicitor molecule, e.g. as provided by a Cladosporium fulvum race or other organism expressing Avr2. comprising the number of leucine rich repeats (LRR's) identifiable from the sequence information provided herein for Cf-5, 32. A nucleic acid isolate according to the invention may encode a pathogen resistance gene whose expression in a plant can cause activation of a defence response in the plant, comprising a sequence of nucleotides encoding a polypeptide comprising the sequence of amino acids shown in Figure 2A or Figure 2B. The activation may be upon contact of the plant with a pathogen or corresponding elicitor molecule. The sequence of nucleotides may comprise an encoding sequence shown in Figure IA or Figure IB. The nucleic acid may include a sequence of nucleotides comprising an allele, derivative, variant or mutant, by way of addition, insertion, deletion or substitution of one or more nucleotides, of an encoding sequence shown in Figure IA or Figure IB For example, the sequence may comprise nucleotides 654-3558 of Figure IA or Figure IB. Nucleic acid according to the present invention may encode a pathogen resistance gene whose expression in a plant can cause activation of a defence response in the plant, comprising a sequence of nucleotides encoding a polypeptide, the polypeptide comprising an amino acid sequence which comprises an allele, derivative, variant or mutant, by way of addition, insertion, deletion or substitution of one or more amino acids, of the amino acid sequence shown in Figure 2A or Figure 2B. The nucleic acid, which may contain for example DNA encoding the amino acid sequence of Figure 2A or Figure 2B, as genomic or cDNA, may be in the form of a recombinant and preferably replicable vector, for example a plasmid, cosmid, phage or Agrobacterium binary vector. The nucleic acid may be under the control of an appropriate promoter or other regulatory elements for expression in a host cell such as a microbial, e.g. bacterial, or plant cell. In the case of genomic DNA, this may contain its own promoter or other regulatory elements and in the case of cDNA this may be under the control of an appropriate promoter or other regulatory elements for expression in the host cell. Those skilled in the art are well able to construct vectors and design protocols for recombinant gene expression. Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. For further details see, for example, Molecular Cloning: a Laboratory Manual : 2nd edition, Sambrook et al , 1989, Cold Spring Harbor Laboratory Press. Many known techniques and protocols for manipulation of nucleic acid, for example in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in Current Protocols in Molecular Biology, Second Edition, Ausubel et al. eds., John Wiley & Sons, 1992. The disclosures of Sambrook et al. and Ausubel et al. are incorporated herein by reference. Specific procedures and vectors previously used with wide success upon plants are described by Bevan (Nucl. Acids Res. 12, 8711-8721 (1984)) and Guerineau and Mullineaux (1993) (Plant transformation and expression vectors. In: Plant Molecular Biology Labfax (Croy RRD ed) Oxford, BIOS Scientific Publishers, pp 121-148) . Selectable genetic markers may be used consisting of chimaeric genes that confer selectable phenotypes such as resistance to antibiotics such as kanamycin, hygromycin, phosphinotricin, chlorsulfuron, methotrexate, gentamycin, spectinomycin, imidazolinones and glyphosate. Nucleic acid molecules and vectors according to the present invention may be provided isolated and/or purified from their natural environment, in substantially pure or homogeneous form, or free or substantially free of nucleic acid or genes of the species of interest or origin other than the sequence encoding a polypeptide with the required function. Nucleic acid according to the present invention may comprise cDNA, RNA, genomic DNA and may be wholly or partially synthetic. The term "isolate" encompasses all these possibilities. Where a DNA sequence is specified, e.g. with reference to a figure, unless context requires otherwise the RNA equivalent, with U substituted for T where it occurs, is encompassed. When introducing a chosen gene construct into a cell, certain considerations must be taken into account, well known to those skilled in the art. The nucleic acid to be inserted should be assembled within a construct which contains effective regulatory elements which will drive transcription. There must be available a method of transporting the construct into the cell. Once the construct is within the cell membrane, integration into the endogenous chromosomal material either will or will not occur. Finally, as far as plants are concerned the target cell type must be such that cells can be regenerated into whole plants. Plants transformed with the DNA segment containing the sequence may be produced by standard techniques which are already known for the genetic manipulation of plants. DNA can be transformed into plant cells using any suitable technology, such as a disarmed Ti-plasmid vector carried by Agrobacterium exploiting its natural gene transfer ability (EP-A-270355, EP-A-0116718, NAR 12(22) 8711 - 87215 1984) , particle or microprojectile bombardment (US 5100792, EP-A- 444882, EP-A-434616) microinjection (WO 92/09696, WO 94/00583, EP 331083, EP 175966, Green et al . (1987) Plant Tissue and Cell Cul ture , Academic Press) , electroporation (EP 290395, WO 8706614 Gelvin Debeyser - see attached) other forms of direct DNA uptake (DE 4005152, WO 9012096, US 4684611), liposome mediated DNA uptake (e.g. Freeman et al . Plant Cell Physiol . 29: 1353 (1984)) , or the vortexing method (e.g. Kindle, PNAS U. S.A . 87: 1228 (1990d) Physical methods for the transformation of plant cells are reviewed in Oard, 1991, Biotech. Adv. 9: 1-11. Agrobacterium transformation is widely used by those skilled in the art to transform dicotyledonous species. Recently, there has been substantial progress towards the routine production of stable, fertile transgenic plants in almost all economically relevant monocot plants (Toriyama, et al. (1988) Bio/Technology 6, 1072-1074; Zhang, et al . (1988) Plant Cell Rep. 7, 379-384; Zhang, et al . (1988) Theor Appl Genet 76, 835-840; Shimamoto, et al . (1989) Nature 338, 274-276; Datta, et al . (1990) Bio/Technology 8, 736-740; Christou, et al . (1991) Bio/Technology 9, 957-962; Peng, et al . (1991) International Rice Research Institute, Manila, Philippines 563-574; Cao, et al . (1992) Plant Cell Rep . 11, 585-591; Li, et al . (1993) Plant Cell Rep. 12, 250- 255; Rathore, et al . (1993) Plant Molecular Biology 21, 871- 884; Fromm, et al . (1990) Bio/Technology 8, 833-839; Gordon- Kamm, et al . (1990) Plant Cell 2, 603-618; D'Halluin, et al . (1992) Plant Cell 4, 1495-1505; Walters, et al . (1992) Plant Molecular Biology 18, 189-200; Koziel, et al . (1993) Biotechnology 11, 194-200; Vasil, I. K. (1994) Plant Molecular Biology 25, 925-937; Weeks, et al . (1993) Plant Physiology 102, 1077-1084; Somers, et al . (1992) Bio /Technology 10, 1589-1594; W092/14828) . In particular, Agrobacterium mediated transformation is now emerging also as an highly efficient alternative transformation method in monocots (Hiei et al . (1994) The Plant Journal 6, 271-282) . The generation of fertile transgenic plants has been achieved in the cereals rice, maize, wheat, oat, and barley (reviewed in Shimamoto, K. (1994) Current Opinion in Biotechnology 5, 158-162.; Vasil, et al . (1992) Bio /Technology 10, 667-674; Vain et al. , 1995, Biotechnology Advances 13 (4) : 653-671; Vasil, 1996, Nature Biotechnology 14 page 702) . Microprojectile bombardment, electroporation and direct DΝA uptake are preferred where Agrobacterium is inefficient or ineffective. Alternatively, a combination of different techniques may be employed to enhance the efficiency of the transformation process, eg bombardment with Agrobacterium coated microparticles (EP-A-486234) or microprojectile bombardment to induce wounding followed by co-cultivation with Agrobacterium (EP-A-486233) . Following transformation, a plant may be regenerated, e.g. from single cells, callus tissue or leaf discs, as is standard in the art. Almost any plant can be entirely regenerated from cells, tissues and organs of the plant. Available techniques are reviewd in Vasil et al. , Cell Cul ture and Somatic Cell Genetics of Plants, Vol I, II and III, Labora tory Procedures and Their Applications, Academic Press, 1984, and Weissbach and Weissbach, Methods for Plant Molecular Biology, Academic Press, 1989. The particular choice of a transformation technology will be determined by its efficiency to transform certain plant species as well as the experience and preference of the person practising the invention with a particular methodology of choice. It will be apparent to the skilled person that the particular choice of a transformation system to introduce nucleic acid into plant cells is not essential to or a limitation of the invention, nor is the choice of technique for plant regeneration. A Cf-5 gene and modified versions thereof (alleles, mutants, variants and derivatives thereof) , and other nucleic acid provided herein may be used to confer resistance in plants, in particular tomatoes, to a pathogen such as C. fulvum. This may include cloned DNA from Lycopersicon pimp inelli folium which has the same chromosomal location as the Cf-5 gene or any subcloned fragment thereof. For this purpose nucleic acid such as a vector as described herein may be used for the production of a transgenic plant . Such a plant may possess pathogen resistance conferred by the Cf-5 gene. The invention thus further encompasses a host cell transformed with such a vector, especially a plant or a microbial cell. Thus, a host cell, such as a plant cell, comprising nucleic acid according to the present invention is provided. Within the cell, the nucleic acid may be incorporated within the chromosome. Also according to the invention there is provided a plant cell having incorporated into its genome nucleic acid, particularly heterologous nucleic acid, as provided by the present invention (e.g. a functional resistance gene) , under operative control of a regulatory sequence for control of expression. The coding sequence may be operably linked to one or more regulatory sequences which may be heterologous or foreign to the gene, such as not naturally associated with the Cf-gene for its expression. The nucleic acid according to the invention may be placed under the control of an externally inducible gene promoter to place expression under the control of the user. A further aspect of the present invention provides a method of making such a plant cell involving introduction of nucleic acid or a suitable vector including the sequence of nucleotides into a plant cell and causing or allowing recombination between the vector and the plant cell genome to introduce the sequence of nucleotides into the genome. The invention extends to plant cells containing nucleic acid according to the invention as a result of introduction of the nucleic acid into an ancestor cell . The term "heterologous" may be used to indicate that the gene/sequence of nucleotides in question have been introduced into said cells of the plant or an ancestor thereof, using genetic engineering, ie by human intervention. A transgenic plant cell, i.e. transgenic for the nucleic acid in question, may be provided. The transgene may be on an extra-genomic vector or incorporated, preferably stably, into the genome. A heterologous gene may replace an endogenous equivalent gene, ie one which normally performs the same or a similar function, or the inserted sequence may be additional to the endogenous gene or other sequence. An advantage of introduction of a heterologous gene is the ability to place expression of a sequence under the control of a promoter of choice, in order to be able to influence expression according to preference. Furthermore, mutants, variants and derivatives of the wild-type gene, e.g. with higher or lower activity than wild-type, may be used in place of the endogenous gene. Nucleic acid heterologous, or exogenous or foreign, to a plant cell may be non-naturally occuring in cells of that type, variety or species. Thus, nucleic acid may include a coding sequence of or derived from a particular type of plant cell or species or variety of plant, placed within the context of a plant cell of a different type or species or variety of plant. A further possibility is for a nucleic acid sequence to be placed within a cell in which it or a homologue is found naturally, but wherein the nucleic acid sequence is linked and/or adjacent to nucleic acid which does not occur naturally within the cell, or cells of that type or species or variety of plant, such as operably linked to one or more regulatory sequences, such as a promoter sequence, for control of expression. A sequence within a plant or other host cell may be identifiably heterologous, exogenous or foreign. Plants which include a plant cell according to the invention are also provided, along with any part or propagule thereof, seed, selfed or hybrid progeny and descendants . A plant according to the present invention may be one which does not breed true in one or more properties. Plant varieties may be excluded, particularly registrable plant varieties according to Plant Breeders' Rights. It is noted that a plant need not be considered a "plant variety" simply because it contains stably within its genome a transgene, introduced into a cell of the plant or an ancestor thereof. In addition to a plant, the present invention provides any clone of such a plant, seed, selfed or hybrid progeny and descendants, and any part of any of these, such as cuttings, seed. The invention provides any plant propagule, that is any part which may be used in reproduction or propagation, sexual or asexual, including cuttings, seed and so on. Also encompassed by the invention is a plant which is a sexually or asexually propagated off-spring, clone or descendant of such a plant, or any part or propagule of said plant, off-spring, clone or descendant. The invention further provides a method of conferring pathogen resistance to a plant including expression of a heterologous nucleic acid sequence as discussed within cells of the plant. The invention further provides a method of comprising expression from nucleic acid encoding the amino acid sequence of Figure 2A or Figure 2B, or a mutant, variant, allele or derivative of the sequence, within cells of a plant (thereby producing the encoded polypeptide) , following an earlier step of introduction of the nucleic acid into a cell of the plant or an ancestor thereof . Such a method may confer pathogen resistance on the plant . This may be used in combination with the Avr5 gene according to any of the methods described in W091/15585 (Mogen) or, more preferably, PCT/GB95/01075 (published as WO 95/31564) , or any other gene involved in conferring pathogen resistance. The Cf-5, Cf-4 , Cf-2 and Cf-9 genes function in a similar manner in that they confer a resistance to tomato that prevents the growth of tomato leaf mould C. fulvum. They, however, by recognition of different Avr products and have subtle differences in the speed with which they stop growth of the pathogen and stimulate a resistance response (Hammond-Kosack and Jones 1994; Ashfield et al 1994) . These differences may be exploited to optimise applications disclosed herein. Mutants have been isolated termed "required for Cladosporium resistance" or "rcr" in which a genetic lesion completely abolishes the resistance reaction as determined by the Cf-2 gene. These mutants do not affect the resistance reaction as determined by the Cf-9 resistance gene. This indicates that the Cf-2 and Cf-9 work via significantly different mechanisms. As Cf-5 is related to Cf-2, it is likely that the Cf-5 gene also works via this mechanism which is different from that for Cf-9. A gene stably incorporated into the genome of a plant is passed from generation to generation to descendants of the plant, cells of which deeendants may express the encoded polypeptide and so may have enhanced pathogen resistance. Pathogen resistance may be determined by assessing compatibility of a pathogen (eg. Cladosporium fulvum) or using recombinant expression of a pathogen avirulence gene, such as Avr-5 or delivery of the Avr-5 gene product. Down-regulation of expression of a target gene may be achieved using anti-sense technology or "sense regulation" ("co-suppression") . In using anti-sense genes or partial gene sequences to down-regulate gene expression, a nucleotide sequence is placed under the control of a promoter in a "reverse orientation" such, that transcription yields RNA which is complementary to normal mRNA transcribed from the "sense" strand of the target gene. See, for example, Rothstein et al, 1987; Smith et al, (1988) Nature 334, 724-726; Zhang et al , (1992) The Plant Cell 4, 1575-1588, English et al . , (1996) The Plant Cell 8, 179-188. Antisense technology is also reviewed in Bourque, (1995), Plant Science 105, 125- 149, and Flavell, (1994) PNAS USA 91, 3490-3496. An alternative is to use a copy of all or part of the target gene inserted in sense, that is the same, orientation as the target gene, to achieve reduction in expression of the target gene by co-suppression. See, for example, van der Krol et al . , (1990) The Plant Cell 2 , 291-299; Napoli et al . , (1990) The Plant Cell 2 , 279-289; Zhang et al . , (1992) The Plant Cell 4, 1575-1588, and US-A-5,231, 020. The complete sequence corresponding to the coding sequence (in reverse orientation for anti-sense) need not be used. For example fragments of sufficient length may be used. It is a routine matter for the person skilled in the art to screen fragments of various sizes and from various parts of the coding sequence to optimise the level of anti- sense inhibition. It may be advantageous to include the initiating methionine ATG codon, and perhaps one or more nucleotides upstream of the initiating codon. A further possibility is to target a conserved sequence of a gene, e.g. a sequence that is characteristic of one or more genes, such as a regulatory sequence. The sequence employed may be 500 nucleotides or less, possibly about 400 nucleotides, about 300 nucleotides, about 200 nucleotides, or about 100 nucleotides. It may be possible to use oligonucleotides of much shorter lengths, 14-23 nucleotides, although longer fragments, and generally even longer than 500 nucleotides are preferable where possible. It may be preferable that there is complete sequence identity in the sequence used for down-regulation of expression of a target sequence, and the target sequence, though total complementarity or similarity of sequence is not essential. One or more nucleotides may differ in the sequence used from the target gene. Thus, a sequence employed in a down-regulation of gene expression in accordance with the present invention may be a wild-type sequence (e.g. gene) selected from those available, or a mutant, derivative, variant or allele, by way of insertion, addition, deletion or substitution of one or more nucleotides, of such a sequence. The sequence need not include an open reading frame or specify an RNA that would be translatable. It may be preferred for there to be sufficient homology for the respective anti-sense and sense RNA molecules to hybridise. There may be down regulation of gene expression even where there is about 5%, 10.%, 15% or 20% or more mismatch between the sequence used and the target gene. The invention further provides the use of fragments of a sequence according to the present invention, or a sequence complementary thereto as disclosed, in down-regulation of a target gene, such as a gene including a sequence shown in Figure IA or Figure IB, or encoding an amino acid sequence shown in Figure 2A or Figure 2B, or a mutant, allele, variant, derivative or homologue thereof, such as by anti- sense or sense suppression of expression (e.g. including at least transcription) . Methods of down-regulating gene expression are thus provided. Sequencing of the Cf-5 gene has shown that like the Cf- 9 gene it includes DNA sequence encoding leucine-rich repeat (LRR) regions. The Cf-5 and Cf- 9 genes contain all the same general features and as such form a new class of disease resistance genes separate from other disease resistance genes characterised to date. As discussed in WO 95/18230, and validated herein, the presence of LRRs may be characteristic of many pathogen resistance genes and the presence of LRRs may be used in identifying further pathogen resistance genes. Furthermore, there are some striking homologies between Cf -9 and Cf-5. These homologies may also be used to identify further resistance genes of this class, for example using oligonucleotides (e.g. a degenerate pool) designed on the basis, of sequences conserved (preferably at the amino acid level) between the Cf-9 and the Cf - 5 genes. The sequence information provided in Figure IA, IB, 2A and/or 2B is sufficient for the person skilled in the art to clone a Cf-5 gene or homologue in accordance with the present invention. A Cf-5 resistance gene or homologue in accordance with the present invention is obtainable by means of a method which includes providing a preparation of plant cell nucleic acid, e.g. tomato, (see e.g. Tigchelaar, 1984) providing a nucleic acid molecule having a nucleotide sequence shown in or complementary to a nucleotide sequence shown in Figure IA and/or Figure IB, preferably from within the coding sequence (i.e. coding for an amino acid sequence shown in Figure 2A or Figure 2B) , contacting nucleic acid in said preparation with said nucleic acid molecule under conditions for hybridisation of said nucleic acid molecule to any said gene or homologue in said preparation, and identifying said gene or homologue if present by its hybridisation with said nucleic acid molecule. Hybridisation is generally followed by identification of successful hybridisation and isolation of nucleic acid which has hybridised, which may involve one or more steps of PCR. Hybridisation of nucleic acid molecule to a Cf-5 gene or homologue may be determined or identified indirectly, e.g using a nucleic acid amplification reaction, particularly the polymerase chain reaction (PCR) . PCR requires the use of two primers to specifically amplify target nucleic acid, so preferably two nucleic acid molecules with sequences characteristic of Cf-5 are employed. If RACE is used (see below) only one such primer may be needed. Two primer which may be used for specific amplification of Cf-5 are OLIGO 2-5CF: GCTATCTTTGGGTATCAACTT and OLIGO 2- 5CR: AGATGACATCGACAAAATGTG (the sequences or complementary sequences of which form part of Figure IA and Figure IB) . Under appropriate conditions, such as an annealing temperature of around 55oC, such as 57oc or 58< * >C, and with a suitable extension time, such as around 1 minute, these primers may be used in a PCR or other nucleic acid amplification reaction to amplify a 879 nucleotide fragment of Cf-5 and a 1023 nucleotide fragment of Hcr2-5D, which fragments may be used as or in the design of probes for obtaining the respective complete coding sequence from a plant that carries the Cf-5 gene. The size difference between the two fragments of Cf-5 and Hcr2-5D (144 nucleotides) reflects the additional two LRR's in the homologue. On a Cf-5 containing plant two bands are obtained on an electrophoretic gel of PCR products obtained using PCR with the conditions noted above, one for the Cf-5 gene and one for the Hcr2-5D homologue. Bands for other homologues are not identified on the gel. PCR techniques for the amplification of nucleic acid are described in US Patent No. 4,683,195 and Saiki et al . Science 239: 487-491 (1988) . PCR includes steps of denaturation of template nucleic acid (if double-stranded) , annealing of primer to target, and polymerisation. The nucleic acid probed or used as template in the amplification reaction may be genomic DNA, cDNA or RNA. PCR may be used to amplify specific sequences from genomic DNA, specific RNA sequences and cDNA transcribed from mRNA. References for the general use of PCR techniques include Mullis et al, Cold Spring Harbor Symp. Quant. Biol., 51:263, (1987), Ehriich (ed) , PCR technology, Stockton Press, NY, 1989, Ehriich et al, Science, 252:1643-1650, (1991) , "PCR protocols; A Guide to Methods and Applications", Eds. Innis et al, Academic Press, New York, (1990) . Assessment of whether or not a PCR product corresponds to a homologue gene of interest may be conducted in various ways . A PCR band might contain a complex mix of products . Individual products may be cloned and each one individually screened. One possible mode of analysis is by transformation to assess function on introduction into a plant or plant cell of interest, though part of a gene amplified directly using the specific primers on the plant nucleic acid may not be sufficient for any function study. A method involving use of PCR in obtaining nucleic acid according to the present invention may include providing a preparation of plant nucleic acid, e.g. tomato, providing a pair of nucleic acid molecule primers useful in (i.e. suitable for) PCR, at least one said primer having a sequence shown in or complementary to a sequence shown in Figure IA or Figure IB, contacting nucleic acid in said preparation with said primers under conditions for performance of PCR, performing PCR and determining the presence or absence of an amplified PCR product. The presence of an amplified PCR product may indicate identification of a gene of interest or fragment thereof . An oligonucleotide for use in nucleic acid amplification may have about 10 or fewer codons (e.g. 6, 7 or 8) , i.e. be about 30 or fewer nucleotides in length (e.g. 18, 21 or 24) . Generally specific primers are upwards of 14 nucleotides in length. For optimum specificity, primers of 16-24 nucleotides in length may be preferred. Those skilled in the art are well versed in the design of primers for use processes such as PCR. Some preferred oligonucleotides have a sequence set out in Figure IA or Figure IB (or complementary to such a sequence) or a sequence which differs from any of the sequences shown (or the complement) by addition, substitution, insertion or deletion of one or more nucleotides, but preferably without abolition of ability to hybridise selectively with a Cf-5 sequence shown in Figure IA or Figure IB or a complementary sequence, that is wherein the degree of homology of the oligonucleotide or polynucleotide with one of the sequences given is sufficiently high. Two oligos specific for Cf-5 and which do not amplify other close homologues from a Cf5 plant are disclosed above. A gene or fragment thereof identified as being to which a said nucleic acid molecule hybridises, which may be an amplified PCR product, may be isolated and/or purified and may be subsequently investigated for ability to confer on a plant resistance to a pathogen. If the identified nucleic acid is a fragment of a gene, the fragment may be used (e.g. by probing and/or PCR) in subsequent cloning of the full- length gene, which may be a full-length coding sequence. Inserts may be prepared from partial cDNA clones and used to screen cDNA libraries. The full-length clones isolated may be subcloned into expression vectors and activity assayed by introduction into suitable host cells and/or sequenced. It may be necessary for one or more gene fragments to be ligated to generate a full-length coding sequence. One or more changes to the coding sequence, by way of addition, deletion, substitution and/or insertion of one or more nucleotides, which may or may not be reflected by one or more changes at the amino acid level, may be made to a nucleotide sequence so-obtained. Molecules found to be resistance genes may be used as such, i.e. to confer on a plant resistance to a pathogen. Nucleic acid obtained and obtainable using a method as disclosed herein is provided in various aspects of the present invention. Nucleic acid isolated and/or purified from one or more cells of a plant, or a nucleic acid library derived from nucleic acid isolated and/or purified from cells (e.g. a cDNA library derived from mRNA isolated from the cells) , may be probed under conditions for selective hybridisation and/or subjected to a specific nucleic acid amplification reaction such as the polymerase chain reaction (PCR) , as discussed. Test nucleic acid may be provided from a cell as genomic DNA, cDNA or RNA, or a mixture of any of these, preferably as a library in a suitable vector. Prior to any PCR that is to be performed, the complexity of a nucleic acid library may be reduced by creating a cDNA library for example using RT-PCR or by using the phenol emulsion reassociation technique (Clarke et al. (1992) NAR 20, 1289- 1292) on a genomic library. A method may include hybridisation of one or more (e.g. two) probes or primers to target nucleic acid. Where the nucleic acid is double-stranded DNA, hybridisation will generally be preceded by denaturation to produce single- stranded DNA. The hybridisation may be as part of a PCR procedure, or as part of a probing procedure not involving PCR. An example procedure would be a combination of PCR and low stringency hybridisation. A screening procedure, chosen from the many available to those skilled in the art, is used to identify successful hybridisation events and isolated hybridised nucleic acid. Binding of a probe to target nucleic acid (e.g. DNA) may be measured using any of a variety of techniques at the disposal of those skilled in the art. For instance, probes may be radioactively, fluorescently or enzymatically labelled. Other methods not employing labelling of probe include examination of restriction fragment length polymorphisms, amplification using PCR, RN'ase cleavage and allele specific oligonucleotide probing. Probing may employ the standard Southern blotting technique. For instance DNA may be extracted from cells and digested with different restriction enzymes. Restriction fragments may then be separated by electrophoresis on an agarose gel, before denaturation and transfer to a nitrocellulose filter. Labelled probe may be hybridised to the DNA fragments on the filter and binding determined. DNA for probing may be prepared from RNA preparations from cells. Preliminary experiments may be performed by hybridising under low stringency conditions various probes to Southern blots of DNA digested with restriction enzymes. For probing, preferred conditions are those which are stringent enough for there to be a simple pattern with a small number of hybridisations identified as positive which can be investigated further. It is well known in the art to increase stringency of hybridisation gradually until only a few positive clones remain. Suitable conditions would be achieved when a large number of hybridising fragments were obtained while the background hybridisation was low. Using these conditions nucleic acid libraries, e.g. cDNA libraries representative of expressed sequences, may be searched. Those skilled in the art are well able to employ suitable conditions of the desired stringency for selective hybridisation, taking into account factors such as oligonucleotide length and base composition, temperature and so on. According to a further aspect, the present invention provides a method of identifying a plant pathogen resistance gene comprising use of an oligonucleotide which comprises a sequence or sequences that are conserved between pathogen resistance genes such as Cf -9 and Cf-5, or Cf-5 and Cf-4 and/or Cf -2 to search for new resistance genes. Thus, a method of obtaining nucleic acid comprising a pathogen resistance gene (encoding a polypeptide able to confer pathogen resistance) is provided, comprising hybridisation of an oligonucleotide (details of which are discussed herein) or a nucleic acid molecular comprising such an oligonucleotide to target/candidate nucleic acid. Target or candidate nucleic acid may, for example, comprise a genomic or cDNA library obtainable from an organism known to encode a pathogen resistance gene. Successful hybridisation may be identified and target/candidate nucleic acid isolated for further investigation and/or use, as disclosed. Preferred amino acid sequences suitable for use in the design of probes or PCR primers are sequences conserved (completely, substantially or partly) between polypeptides able to confer pathogen resistance such as those encoded by Cf-5 and Cf-9, and/or Cf-5 and Cf-2 and/or Cf-4 . On the basis of amino acid sequence information, oligonucleotide probes or primers may be designed, taking into account the degeneracy of the genetic code, and, where appropriate, codon usage of the organism from the candidate nucleic acid is derived. Assessment of whether or not a PCR product corresponds to a resistance genes may be conducted in various ways, as discussed, and a PCR band may contain a complex mix of products . Individual products may be cloned and each sreened for linkage to known disease resistance genes that are segregating in progeny that showed a polymorphism for this probe. Alternatively, the PCR product may be treated in a way that enables one to display the polymorphism on a denaturing polyacrylamide DNA sequencing gel with specific bands that are linked to the resistance gene being preselected prior to cloning. Once a candidate PCR band has been cloned and shown to be linked to a known resistance gene, it may be used to isolate clones which may be inspected for other features and homologies to Cf-9 , Cf-5 or other related gene. It may subsequently be analysed by transformation to assess its function on introduction into a disease sensitive variety of the plant of interest. Alternatively, the PCR band or sequences derived by analysing it may be used to assist plant breeders in monitoring the segregation of a useful resistance gene. These techniques are of general applicability to the identification of pathogen resistance genes in plants. Examples of the type of genes that can be identified in this way include Phytophthora resistance in potatoes, mildew resistance and rust resistance in cereals such as barley and maize, rust resistance in Antirrhinum and flax, downy mildew resistance in lettuce and Arabidopsis , virus resistance in potato, tomato and tobacco, nematode resistance in tomato, resistance to bacterial pathogens in Arabidopsis and tomato and Xanthomonas resistance in peppers . Once a pathogen resistance gene has been identified, it may be reintroduced into plant cells using techniques well known to those skilled in the art to produce transgenic plants . According to a further aspect, the present invention provides a DNA isolate encoding the protein product of a plant pathogen resistance gene which has been identified by use of the presence therein of LRRs or, in particular, by the technique defined above. According to a yet further aspect, the invention provides transgenic plants, in particular crop plants, which have been engineered to carry pathogen resistance genes which have been identified by the presence of LRRs or by nucleic acid hybridisation as disclosed, or are otherwise provided by an embodiment of the present invention. Examples of suitable plants include tobacco, cucurbits, carrot, vegetable brassica, lettuce, strawberry, oilseed brassica, sugar beet, wheat, barley, maize, rice, soyabeans, peas, sorghum, sunflower, tomato, potato, pepper, chrysanthemum, carnation, poplar, eucalyptus and pine. Modifications to these and further aspects and embodiments of the present invention will be apparent to those skilled in the art. All documents mentioned herein are incorporated by reference. The term "comprises" should generally be interpreted herein as meaning "includes", not "consisting of" . As already indicated, the present invention is based on the cloning and sequencing of the tomato Cf-5 gene and this experimental work is outlined below with reference to the following figures. Figure 1 shows the nucleotide sequence, including coding sequence, of cloned Cf-5 gene according to the present invention. The Figure IA and Figure IB sequences each represent a variant sequence of the other. Figure 2 shows the amino acid sequence of CF-5 protein according to the invention. The Figure 2A and Figure 2B sequences each represent a variant sequence of the other, being respectively encoded by the encoding sequences of Figures IA and IB. Figure 3 shows a representation (not to scale) of the general structure of the genomic regions including the regions from the susceptible locus of CfO, Cf2 and Cf5. The terminology "Hcr-x" is used as short-hand for "homologue of Cladosporium resistance gene x" . The Cf-5 sequence of Figure IB is known also as Hcr2 -5C. Figure 4 shows the levels of similarity and identity between Cf-2, Cf-5 and several homologues. Analysis was performed using the standard program BestFit, which is part of the Wisconsin Package, Version 8, September 1994, (Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA, Wisconsin 53711) . BestFit makes an optimal alignment of the best segment of similarity between two sequences. Optimal alignments are found by inserting gaps to maximize the number of matches using the local homology algorithm of Smith and Waterman. Figure5 shows the nucleic acid sequence of the clone Hcr2-5B gene. Figure 6 shwos the amino acid sequence of the HCR2-5B protein. Figure 7 shows the nucleic acid sequence of the cloned Hcr2-5D gene. Figure 8 shows the amino acid sequence of the HCR2-5D protein. Figure 9 shows a schematic representation of the proteins encoded by diferent Cf and Her genes of the CfO, Cf2 and Cf5 loci of chromosome 6 of the tomato. Horizontal boxes represent individual LRRs, grey boxes indicate A-type repeats and black boxes indicated B-tpye repeats. Vertical boxes indicate the transmembrane domain. the number of LRRs in each encoded protein is shown beneath. All proteins are aligned to Cf2. CLONING OF THE CF-5 GENE The Cf-5 resistance gene has been mapped to a similar chromosomal location to Cf-2 on tomato chromosome 6 (Dickinson et al. , 1993; Jones et al. 1993) . To more precisely determine the position of the Cf-5 gene, tomato crosses were set up to look for recombination between the Cf-2 and Cf-5 resistance genes. A plant that was heterozygous for both Cf-2 and Cf-5 was crossed to a C. fulvum-sensitive tomato line. Approximately 12,000 resulting FI progeny were screened for resistance to C. fulvum, and a single sensitive plant was identified. DNA from this plant was analysed with the molecular markers including Cf-2 and other markers which map closely to the Cf-2 gene. The fact that only a single recombinant between Cf-2 and Cf-5 was obtained strongly suggested that these two resistance genes are indeed allelic or very closely linked. The molecular analysis of this plant also suggested that the Cf-5 gene may even be identified by the Cf-2 gene probe. Isola tion of binary cosmid vector clones that carry a genomic Cf-5 gene To determine whether DNA identified by the molecular Cf-2 gene probe include the Cf-5 gene, DNA sequences were isolated from a plant that carried the Cf-5 gene and transformed into Cf-0 tomato plants. A genomic DNA library was constructed from a stock that carried both the Cf±4 gene on chromosome 1, and the Cfλ5 gene on chromosome 6. The library was constructed in a binary cosmid cloning vector pCLD04541, obtained from Dr C. Dean, John Innes Centre, Colney Lane, Norwich (see also Bent et al 1994) . The library was randomly distributed into 144 pools containing about 1500 clones per pool, cells were grown from each pool and from 10 ml of cells, 9 ml were used for bulk plasmid DNA extractions, and 1 ml was used after addition of 0.2 ml of glycerol, to prepare a frozen stock. Plasmid DNA from the pools was isolated by alkaline lysis (Birnboim and Doly, 1979) , and DNA samples were analyzed by hybridisation in "slot blots" and Southern blots with the molecular Cf-2 gene probe. Several pools proved positive in this assay. For each pool, approximately 10,000 colonies were plated out and inspected for Cf-2 gene homology by colony hybridisation with a radioactive Cf-2 gene probe, and from each pool, single clones were isolated that carried such homology. These techniques are all well known to those skilled in the art. These clones have been further characterized by Southern blot hybridisation using a Cf-2 gene probe, and by restriction enzyme mapping. These cosmids revealed four regions that hybridised to the Cf-2 gene probe. Subclones, each containing only a single region of homology, were generated in the same cosmid vector. These cosmids (Cosmid 8, KB2, KB3 and KB4) were subsequently used in plant transformation experiments, selecting for plant cells transformed to kanamycin resistance, using techniques well known to those skilled in the art. Transgenic tomato plants were produced (Fillati et al 1987; Horsch e al 1985) with at least one of each of cosmids. Assessment of cosmid function in transgenic tomato . The function of a putative cloned Cf-5 gene has been assessed in transformed tomato by testing transformants for resistance to Avr5 carrying C. fulvum. Most transgenic plants containing cosmid KB2 were resistant to C. fulvum carrying Avr5. All transgenic plants containing the other cosmids were sensitive to C. fulvum. These data indicate that the genomic DNA which carries the Cf-5 gene is that piece which corresponds to the region labeled KB2. Thus the Cf-5 gene is the region identified by the Cf-2 gene probe and labelled KB2. Of the cosmids which failed to confer resistance to C. fulvum carrying Avr5, cosmid 8 was shown to carry Hcr2-5B, KB3 was shown to carry Hcr2-5D and KB4 carries a region with only a small amount of similarity to any of the Her genes and which lacks any significant open reading frame. Progeny of transgenic plants carrying KB2 were again tested for resistance to C. fulvum carrying Avr5 and were shown to segregate 3:1 resistant plants to susceptible plants. This is consistent with the gene contained in cosmid KB2 (Cf-5) being heritable in a Mendelian fashion. Progeny from plants carrying the other cosmids (Cosmids 8, KB3 and KB4) were all susceptible to C. fulvum carrying Avr5. DNA seqruence analysis of the Cf-5 gene The DΝA sequence of the 4 kb ClaI to BamHl region encompassing all of the Cf-2 hybridising region of KB2 has been determined, and is shown in Figure 1. The central core sequence carries a single major open reading frame which upon conceptual translation has revealed an interesting motif (the leucine rich repeat, or LRR) that can by hypothesised to be diagnostic of other resistance genes, as previously hypothesised for the Cf-9 gene. The open reading frame initiates with the translation start codon (ATG) at position 654 and finishes with the translation termination codon TAG at position 3558 with an intervening sequence that encodes a 968 amino acid polypeptide. The amino acid sequence of the encoded polypeptide is shown in Figure 2. The DNA sequences of the regions encompassing all of the Cf-2 hybridising regions contained within Cosmid 8 and KB3 have been determined. Each sequence has a single major open reading frame which upon conceptual translation reveals encoded proteins with motifs characteristic of the Cf-5 resistance gene. The amino acid sequence encoded by the Cf- 2 hybridising region of KB3 (also referred to as Hcr2-5D) shows very high homology to the CF-5 protein (Figure 4) . Specifically, Hcr2-5D contains 2 additional leucine rich repeats between amino acids 298 and 299 of CF-5 and a single amino acid corresponding to position 329 in CF-5 from a serine to an alanine. Homologues of resistance genes Cf-2 and Cf-5, including Hcr2-5B and Hcr2-5D for which sequence information is provided herein, may be capable of and useful for conferring resistance when expressed in a plant, by a mechanism which does not involve recognition of Avr2 or Avr5 respectively. REFERENCES 1. Ashfield T, et al . , (1994) Mol . Plant .Mic. Int . 7:645- 657. 2. Balint-Kurti P, et al . , Theor.App.Genet . (1994) 88 pp 691-700. 3. Bent, A.F., et al . , (1994) Science 265:1856. 4. Birnboim, H.C. and Doly, J. (1979) Nucl. Acids. Res. 7:1513-1520. 5. Chang C, et al . , (1992) The Plant Cell 4:1263- 1271. 6. De Wit, PJGM (1992) Ann.Rev. Phytopathol . 30:391-418. 7. Dickinson M, et al . , (1993) Mol .Plant Mic. Int. 6:341-347. 8. Dixon M S, et al . (1995) . High resolution mapping of the physical location of the Cf-2 gene. Mol.Plant.Mic. Int. 8:200-206. 9. Dixon M S, et al . , (1996) Cell. 84:451-459. 10. Fillatti JJ, et al . (1987) . Bio/technol. 5:726- 730. 11. Flor HH (1971) . Ann.Rev. Phytopathol . 9:275-296. 12. Gabriel DW, et al . (1990) . Ann. Rev. Phytopathol. 28:365-391. 13. Hammond-Kosack KE, et al . (1994) Mol. Plant .Mic. Int . 7: 58-70. 14. Hohn B. et al . (1980) . Gene 11:291-298. 15. Horsch RB, et al . (1985) . Science (Wash.) . 227:1229-1231. 16. Johal GS, et al . (1992) . Science (Wash.) . 258:985-987. 17. Jones DA, et al . (1993) . Mol.Plant Mic. Int. 6:348-357. 18. Jones JDG, et al , (1992) . Transgen. Res. 1:285- 297. 19. Jones DA, et al . ,1994) . Science (Wash. ) 266:789-793. 20. Keen NT (1992) . Ann.Rev.Gen. 24:447-463. 21. Long SR (1993) . Cell 73:921-935. 22. Martin GB, et al . , (1993) . Science 262:1432-1436. 23. Mindrinos M, et al . , (1994) . Cell 78:1089-1055. 24. Olszewski NE, et al . (1988) . Nucl. Acids. Res 16:10765-10782. 25, Stein JC, et al . (1991) . Proc.Natl.Acad.Sci.USA 88:8816-8820. 26. Thomas CM, et al (1994) . Mol. & Gen.Genet. 242:573-585. 27 Valon C, et al (1993) . PI.Molec.Biol. 23:415- 421. 28 van den Elzen P, et al . (1985) . Plant. Mol. Biol. 5:149-154. 29, Walker JC (1993) . Plant Journal 3:451-456. 30 Whitham S, et al . (1994) . Cell 78:1011-1115. 31 Scofield , et al . (1996) Science 274:2063-2065. 32. Tang, et al . (1996) Science 274: 2060-2063. 33. Tigchelaar, EC (1984) Report of the Tomato Genetic Co-operative vol. 34, pp55-57.

Claims: CLAIMS : 1. An isolated polynucleotide which is able to confer pathogen resistance on a plant on expression in the plant and which has a nucleotide sequence encoding a polypeptide which includes an amino acid sequence selected from: (i) the amino acid sequence shown in Figure 2A; (ii) the amino acid sequence shown in Figure 2B; (iii) an amino acid sequence mutant, allele, variant, derivative or homologue of the amino acid sequence shown in Figure 2A, by way of addition, substitution, deletion and/or insertion of one or more amino acids, which has at least 95% identity with the amino acid sequence shown in Figure 2A; (iv) an amino acid sequence mutant, allele, variant, derivative or homologue of the amino acid sequence shown in Figure 2B, by way of addition, substitution, deletion and/or insertion of one or more amino acids, which has at least 95% identity with the amino acid sequence shown in Figure 2B; (v) an amino acid sequence mutant, allele, variant, derivative or homologue of the amino acid sequence shown in Figure 2A, by way of addition, substitution, deletion and/or insertion of one or more amino acids, wherein the polynucleotide on expression in a plant elicits a defence response upon contact of the plant with Avr5 elicitor molecule; (iv) an amino acid sequence mutant, allele, variant, derivative or homologue of the amino acid sequence shown in Figure 2B, by way of addition, substitution, deletion and/or insertion of one or more amino acids, wherein the polynucleotide on expression in a plant elicits a defence response upon contact of the plant with Avr5 elicitor molecule. 2. A polynucleotide according to claim 1 which has a nucleotide sequence encoding a polypeptide which includes the amino acid sequence shown in Figure 2A. 3. A polynucleotide according to claim 2 wherein the coding sequence is the coding sequence sequence shown in Figure IA. 4. A polynucleotide according to claim 2 wherein the coding sequence is a mutant, allele, variant or derivative of the coding sequence shown in Figure IA, by way of addition, deletion, substitution and/or insertion of one or more nucleotides. 5. A polynucleotide according to claim 1 which has a nucleotide sequence encoding a polypeptide which includes the amino acid sequence shown in Figure 2B. 6. A polynucleotide according to claim 5 wherein the coding sequence is the coding sequence sequence shown in Figure IB. 7. A polynucleotide according to claim 5 wherein the coding sequence is a mutant, allele, variant or derivative of the coding sequence shown in Figure IB, by way of addition, deletion, substitution and/or insertion of one or more nucleotides. 8. A polynucleotide according to any preceding claim operably linked to a regulatory sequence for expression of said coding sequence. 9. An isolated polynucleotide which has a nucleotide sequence complementary to at least 300 nucleotides of the sequence of any of claims 1 to 7. 10. A polynucleotide according to claim 9 operably linked to a regulatory sequence for transcription of said nucleotide sequence complementary to a said coding sequence. 11. A polynucleotide according to claim 10 wherein the regulatory sequence includes an inducible promoter. 12. A nucleic acid vector suitable for transformation of a host cell and including a polynucleotide according to any preceding claim. 13. A nucleic acid vector according to claim 12 wherein said host cell is a microbial cell . 14. A nucleic acid vector according to claim 12 wherein said host cell is a plant cell. 15. A host cell containing a heterologous polynucleotide or nucleic acid vector according to any preceding claim. 16. A cell according to claim 15 which is microbial. 17. A cell according to claim 15 which is a plant cell. 18. A cell according to claim 17 having said heterologous polynucleotide incorporated within its genome. 19. A cell according to claim 18 having more than one said polynucleotide per haploid genome. 20. A cell according to any of claims 17 to 19 which is comprised in a plant. 21. A plant including a cell according to any of claims 17 to 19 . 22. A plant which is a sexually or asexually propagated off-spring, clone or descendant of a plant according to claim 21, or any part or propagule of said plant, off-spring, clone or descendant. 23. A part or propagule of a plant according to claim 21. 24. A plant according to claim 21 which does not breed true. 25. A method of producing a plant, the method including incorporating a heterologous polynucleotide according to any of claims 1 to 8 into a plant cell and regenerating a plant from said plant cell. 26. A method of producing a plant, the method including incorporating a heterologous polynucleotide according to any of claims 9 to 11 into a plant cell and regenerating a plant from said plant cell. 27. A method according to claim 25 or claim 26 including sexually or asexually propagating or growing off-spring or a descendant of said plant. 28. A method of conferring pathogen resistance on a plant, the method comprising causing or allowing expression of a product encoded by a heterologous polynucleotide according to any of claims 1 to 8 within cells of the plant. 29. Use of nucleic acid according to any of claims 1 to 8 in the production of a transgenic plant. 30. Use of nucleic acid according to any of claims 9 to 11 in the production of a transgenic plant.

Inventors: DIXON MARK STEWART, HATZIXANTHIS KOSTAS, JONES DAVID ALLEN, JONES JONATHAN DALLAS GEORGE, DIXON, MARK, STEWART, HATZIXANTHIS, KOSTAS, JONES, DAVID, ALLEN, JONES, JONATHAN, DALLAS, GEORGE
Applicants: DIXON MARK STEWART, HATZIXANTHIS KOSTAS, INNES JOHN CENTRE INNOV LTD, JONES DAVID ALLEN, JONES JONATHAN DALLAS GEORGE, DIXON, MARK, STEWART, HATZIXANTHIS, KOSTAS, JOHN INNES CENTRE INNOVATIONS LIMITED, JONES, DAVID, ALLEN, JONES, JONATHAN, DALLAS, GEORGE
Application year: 1997
Main classification: C12N15
IPC codes: C07K14, C12N15