Genetic complexity of pathogen perception by plants: the example of Rcr3, a tomato gene required specifically by Cf-2.

Genetic analysis of plant-pathogen interactions has demonstrated that resistance to infection is often determined by the interaction of dominant plant resistance (R) genes and dominant pathogen-encoded avirulence (Avr) genes. It was postulated that R genes encode receptors for Avr determinants. A large number of R genes and their cognate Avr genes have now been analyzed at the molecular level. R gene loci are extremely polymorphic, particularly in sequences encoding amino acids of the leucine-rich repeat motif. A major challenge is to determine how Avr perception by R proteins triggers the plant defense response. Mutational analysis has identified several genes required for the function of specific R proteins. Here we report the identification of Rcr3, a tomato gene required specifically for Cf-2-mediated resistance. We propose that Avr products interact with host proteins to promote disease, and that R proteins "guard" these host components and initiate Avr-dependent plant defense responses.

A second gene at the tomato Cf-4 locus confers resistance to cladosporium fulvum through recognition of a novel avirulence determinant

The tomato Cf-4 and Cf-9 genes confer resistance to the leaf mould pathogen Cladosporium fulvum and map at a complex locus on the short arm of chromosome 1. It was previously shown that the gene encoding Cf-4, which recognizes the Avr4 avirulence determinant, is one of five tandemly duplicated homologous genes (Hcr9-4s) at this locus. Cf-4 was identified by molecular analysis of rare Cf-4/Cf-9 disease-sensitive recombinants and by complementation analysis. The analysis did not exclude the possibility that an additional gene(s) located distal to Cf-4 may also confer resistance to C. fulvum. We demonstrate that a number of Dissociation-tagged Cf-4 mutants, identified on the basis of their insensitivity to Avr4, are still resistant to infection by C. fulvum race 5. Molecular analysis of 16 Cf-4 mutants, most of which have small chromosomal deletions in this region, suggested the additional resistance specificity is encoded by Hcr9-4E. Hcr9-4E recognizes a novel C. fulvum avirulence determinant that we have designated Avr4E.

Genetic and molecular analysis of tomato Cf genes for resistance to Cladosporium fulvum.

In many plant-pathogen interactions resistance to disease is controlled by the interaction of plant-encoded resistance (R) genes and pathogen-encoded avirulence (Avr) genes. The interaction between tomato and the leaf mould pathogen Cladosporium fulvum is an ideal system to study the molecular basis of pathogen perception by plants. A total of four tomato genes for resistance to C. fulvum (Cf-2, Cf-4, Cf-5 and Cf-9) have been isolated from two genetically complex chromosomal loci. Their gene products recognize specific C. fulvum-encoded avirulence gene products (Avr2, Avr4, Avr5 and Avr9) by an unknown molecular mechanism. Cf genes encode extracellular membrane-anchored glycoproteins comprised predominantly of 24 amino acid leucine-rich repeats (LRRs). Cf genes from the same locus encode proteins which are more than 90% identical. Most of the amino-acid sequence differences correspond to the solvent-exposed residues within a beta-strand/beta-turn structural motif which is highly conserved in LRR proteins. Sequence variability within this motif is predicted to affect the specificity of ligand binding. Our analysis of Cf gene loci at the molecular level has shown they comprise tandemly duplicated homologous genes, and suggests a molecular mechanism for the generation of sequence diversity at these loci. Our analysis provides further insight into the molecular basis of pathogen perception by plants and the organization and evolution of R gene loci.

Novel disease resistance specificities result from sequence exchange between tandemly repeated genes at the Cf-4/9 locus of tomato.

Tomato Cf genes confer resistance to C. fulvum, reside in complex loci carrying multiple genes, and encode predicted membrane-bound proteins with extracytoplasmic leucine-rich repeats. At least two Cf-9 homologs confer novel C. fulvum resistance specificities. Comparison of 11 genes revealed 7 hypervariable amino acid positions in a motif of the leucine-rich repeats predicted to form a beta-strand/beta-turn in which the hypervariable residues are solvent exposed and potentially contribute to recognition specificity. Higher nonsynonymous than synonymous substitution rates in this region imply selection for sequence diversification. We propose that the level of polymorphism between intergenic regions determines the frequency of sequence exchange between the tandemly repeated genes. This permits sufficient exchange to generate sequence diversity but prevents sequence homogenization.