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 resis tance 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 la rge number of R genes and their cognate Avr genes have now been analyzed at the molecular level. R gene loci are extremely polymorphic, particularly i n sequences encoding amino acids of the leucine-rich repeat motif, A major challenge is to determine how Avr perception by R proteins triggers the pla nt defense response. Mutational analysis has identified several genes requi red for the function of specific R proteins. Here we report the identificat ion of Rcr3, a tomato gene required specifically for Cf-2-mediated resistan ce. We propose that Avr products interact with host proteins to promote dis ease, and that R proteins "guard" these host components and initiate Avr-de pendent plant defense responses.