Rice pathogen Xanthomonas oryzae pv. oryzae employs inducible hrp-dependent XopF type III effector protein for its growth, pathogenicity and for suppression of PTI response to induce blight disease

Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is a major disease of rice worldwide. Xoo secretes effector proteins (T3Es) directly into the rice cell through a hrp-encoded specialized type III secretion apparatus to induce blight. We examined the function of XopF, one of the conserved effectors in Xoo, using a null mutant developed through a PCR-based homologous recombination strategy. We studied inducible, hrp-dependent expression pattern of xopF. We confirmed that XopF is translocated in rice cytosol through T3SS using adenylate cyclase activity assay. XopF regulate the in planta Xoo growth and suppress PAMP-triggered immune (PTI) response in rice. Xoo wild but not mutant, Xoo a dagger xopF produced intense blight lesions upon inoculation using leaf clipping method. Further, Xoo a dagger xopF showed significant reduction in planta colonization relative to the wild strain. The relative expression analysis of PTI marker genes, PR10, OsWRKY13, OsRLCK16, and OsFLS2 indicated that these genes were up-regulated 1.5 to 5 fold upon challenge-inoculation with Xoo a dagger xopF indicating the role of XopF in suppressing PTI in rice. Xoo a dagger xopF mutant induced more callose deposition in infected rice leaves. XopF::EYFP fusion gene product was localized to the plasma membrane when transiently expressed in Nicotiana benthamiana as well as in living onion epidermal cells. Collectively, the present study shows that XopF repress basal PTI response in plants, and thus favours Xoo growth and pathogenicity in rice.