Characterization of AvrHah1, a novel AvrBs3-like effector from Xanthomonas gardneri with virulence and avirulence activity

Many phytopathogenic bacteria inject virulence effector proteins into plant cells. To identify novel virulence effectors of the bacterial plant pathogen Xanthomonas, a worldwide collection of pepper (Capsicum annuum) pathogenic Xanthomonas strains was studied. Xanthomonas gardneri strains produced in pepper enhanced watersoaking, a phenotype that is typical of a compatible interaction. Transfer of X. gardneri library clones into a Xanthomonas euvesicatoria recipient strain revealed that enhanced watersoaking was attributable to avrHah1 (avirulence (avr) gene homologous to avrBs3 and hax2, No. 1), a novel avrBs3-like gene. avrHah1 is a novel member of the avrBs3 family that encodes tandemly arranged repeat units of both 34 and 35 amino acid lengths. Although AvrHah1 is only distantly related to AvrBs3, it was shown to trigger a Bs3-dependent hypersensitive response (HR). When fused to a nuclear export signal, AvrHah1 is no longer capable of triggering a Bs3 HR, indicating that nuclear targeting of AvrHah1 is crucial to its recognition. Phylogenetic analysis revealed that, although AvrBs3 and AvrHah1 are only distantly related, they share blocks of high homology within potentially solvent-exposed repeat units. Thus, these data suggest that the recognition specificity of AvrBs3-like proteins is predominantly determined by solvent-exposed residues, rather than by overall homology or repeat unit length.