PLANT-DISEASE RESISTANCE GENES - UNRAVELING HOW THEY WORK

Resistance (R) genes confer on a plant the ability to defend itself fo llowing microbial attack. Each R gene exhibits an extreme specificity of action and is only effective against a microbe that has the corresp onding functional avirulence (Avr) gene. This article reviews the stra tegies and experimental approaches deployed to understand the molecula r events underlying the specificity of action of various tomato Cf res istance genes that results in incompatibility to the fungal pathogen C ladosporium fulvum. Topics covered include the clustering of Cf genes, the biology of Cf-dependent incompatibility, the map-based and transp oson tagging approaches used to clone the Cf-2 and Cf-9 genes, respect ively, identification by mutagenesis of other plant loci required for full Cf-9 mediated resistance, the expression of a functional Avr9 gen e in planta and its lethal consequences to Cf-9 containing plants, the physiological and molecular host responses to C. fulvum and AVR elici tor challenges and some genetic approaches to ascertain the crucial co mponents of the defense response.