The role of AvrXa7 of Xanthomonas oryzae pv. oryzae in resistance and disease.

AvrXa7 is dual acting member of the avrBs3 gene family, which encodes proteins targeted to plant cells by the Hrp type III secretion apparatus. AvrXa7, the product of avrXa7, elicits resistance to bacterial blight disease in rice harboring the gene Xa7 and is a virulence factor in strain PXO86 of X oryzae pv. oryzae. AvrXa7 has a C-terminal domain with transcriptional activation activity and three nuclear localization signal sequences, mutations in which caused the loss of both avirulence and virulence activities. AvrXa7 also was found to bind double-stranded DNA with a preference for dA/dT rich sequences. The results indicate that products of the avrBs3-related genes are virulence factors targeted to host cell nuclei and potentially interact with the host transcriptional machinery as part of their mode-of-action. The results also lend evidence to the suggestion that the host defensive recognition mechanisms are targeted to the virulence factors site-of-action. The dependency of the disease complex on the action of the avrBs3 family of genes will be discussed.