RACE-SPECIFICITY OF PLANT-RESISTANCE TO BACTERIAL SPOT DISEASE DETERMINED BY REPETITIVE MOTIFS IN A BACTERIAL AVIRULENCE PROTEIN

ELUCIDATION of the genetic and molecular basis of plant disease resistance is a major objective in the investigation of plant-microbial interactions. Xanthomonas campestris pathovar vesicatoria (Xcv), the causal agent of bacterial spot disease of pepper and tomato, has been developed as a model host-pathogen system to study the genetic interactions that specify the expression of plant disease resistance 1-6. Several plant resistance genes (Bs1, Bs2, Bs3) have been genetically characterized from pepper (Capsicum annuum) that determine resistance to particular races of the pathogen carrying specific avirulence genes 7-9. For example, pepper plants carrying the resistance locus Bs3 are resistant to Xcv strains expressing the avirulence gene avrBs3. Nucleotide sequence analysis of the avrBs3 gene revealed that the internal portion of the predicted protein product consists of a nearly identical 34 amino acid repeat unit, present in 17.5 copies 4. We report here that the repetitive region of the avrBs3 gene determines race-specificity and that deletions of repeat units generate new avirulence specificities and unmask undiscovered resistance genes in pepper and tomato.