Gene-for-gene-mediated recognition of nuclear-targeted AvrBs3-like bacterial effector proteins

Plant disease resistance (R) genes mediate specific recognition of pathogens via perception of cognate avirulence (avr) gene products. The numerous highly similar AvrBs3-like proteins from the bacterial genus Xanthomonas provide together with their corresponding R proteins a unique biological resource to dissect the molecular basis of recognition specificity. A central question in this context is if R proteins that mediate recognition of structurally similar Avr proteins are themselves functionally similar or rather dissimilar. The recent isolation of rice xa5, rice Xa27 and tomato Bs4, R genes that collectively mediate recognition of avrBs3-like genes, provides a first clue to the molecular mechanisms that plants employ to detect AvrBs3-like proteins. Their initial characterization suggests that these R proteins are structurally and functionally surprisingly diverge. This review summarizes the current knowledge on R-protein-mediated recognition of AvrBs3-like proteins and provides working models on how recognition is achieved at the molecular level.