Genetic mapping and functional analysis of the tomato Bs4 locus governing recognition of the Xanthomonas campestris pv. vesicatoria AvrBs4 protein

Xanthomonas campestris pv, vesicatoria is the causal agent of bacterial spo t disease on pepper (Capsicum spp,) and tomato (Lycopersicon spp,), Analysi s of 17 different Lycopersicon accessions with avrBs4-expressing X, campest ris pv. vesicatoria strains identified 15 resistant and two susceptible tom ato genotypes, Genetic analysis revealed that AvrBs4 recognition in tomato is governed by a single locus, designated Bs4 (bacterial spot resistance lo cus no. 4). Amplified fragment length polymorphism and bulked DNA templates from resistant and susceptible plants were used to define a 2.6-cM interva l containing the Bs4 locus. A standard tomato mapping population was employ ed to localize Bs4-linked markers on the short arm of chromosome 5. Investi gation of X. campestris pv, vesicatoria hrp mutant strains revealed that Av rBs4 secretion and avirulence activity are hrp dependent. Agrobacterium-bas ed delivery of the avrBs4 gene into tomato triggered a plant response that phenotypically resembled the hypersensitive response induced by avrBs1-expr essing X, campestris pv. vesicatoria strains, suggesting symplastic percept ion of the avirulence protein. Mutations in the avrBs4 C-terminal nuclear l ocalization signals (NLSs) showed that NLSs are dispensable for Bs4-mediate d recognition. Our data suggest that tomato Bs4 and pepper Bs3 employ diffe rent recognition modes for detection of the highly homologous X, campestris pv, vesicatoria avirulence proteins AvrBs4 and AvrBs3.