Interaction of the Xanthomonas effectors XopQ and XopX results in induction of rice immune responses

Xanthomonas oryzae pv. oryzae uses several type III secretion system (T3SS) effectors, namely XopN, XopQ, XopX, and XopZ, to suppress rice immune responses that are induced following treatment with cell wall degrading enzymes. Here we show that the T3SS secreted effector XopX interacts with two of the eight rice 14-3-3 proteins. Mutants of XopX that are defective in 14-3-3 binding are also defective in suppression of immune responses, suggesting that interaction with 14-3-3 proteins is required for suppression of host innate immunity. However, Agrobacterium mediated delivery of both XopX and XopQ into rice cells results in induction of rice immune responses. These immune responses are not observed when either protein is individually delivered into rice cells. XopQ-XopX induced rice immune responses are not observed in a XopX mutant that is defective in 14-3-3 binding. Yeast two-hybrid and BiFC assays indicate that XopQ and XopX interact with each other. In a screen for Xanthomonas effectors which can suppress XopQ-XopX induced rice immune responses, five effectors were identified, namely XopU, XopV, XopP, XopG and AvrBs2, which were able to do so. These results suggest a complex interplay of Xanthomonas T3SS effectors in suppression of pathogen triggered immunity and effector triggered immunity to promote virulence on rice. Significance statementThis work studies the role of the type III effector XopX in the suppression and induction of rice immune responses, by differential interaction with the 14-3-3 proteins, or with the type III effector XopQ respectively. We have also identified a subset of type III effectors which can suppress this form of immune responses.