Suppression of XopQ–XopX‐induced immune responses of rice by the type III effector XopG

Effectors that suppress effector‐triggered immunity (ETI) are an essential part of the arms race in the co‐evolution of bacterial pathogens and their host plants. Xanthomonas oryzae pv. oryzae uses multiple type III secretion system (T3SS) secreted effectors such as XopU, XopV, XopP, XopG, and AvrBs2 to suppress rice immune responses that are induced by the interaction of two other effectors, XopQ and XopX. Here we show that each of these five suppressors can interact individually with both XopQ and XopX. One of the suppressors, XopG, is a predicted metallopeptidase that appears to have been introduced into X. oryzae pv. oryzae by horizontal gene transfer. XopQ and XopX interact with each other in the nucleus while interaction with XopG sequesters them in the cytoplasm. The XopG E76A and XopG E85A mutants are defective in interaction with XopQ and XopX, and are also defective in suppression of XopQ–XopX‐mediated immune responses. Both mutations individually affect the virulence‐promoting ability of XopG. These results indicate that XopG is important for X. oryzae pv. oryzae virulence and provide insights into the mechanisms by which this protein suppresses ETI in rice.