Xanthomonas Type III Effector XopN Targets Scaffold Protein OsRACK1B to Suppress Rice Immunity.

Bacterial leaf streak (BLS), caused by Xanthomonas oryzae pv. oryzicola (Xoc), is a major bacterial disease that poses a substantial threat to rice production. Although Xoc deploys a repertoire of effector proteins to subvert host defences, the mechanisms by which these effectors modulate plant immune signalling remain largely unclear. Here, we demonstrate that the Xoc effector XopN acts as a critical suppressor of rice basal immunity. Using yeast two-hybrid screening, bimolecular fluorescence complementation, and pull-down assays, we identified the WD40-repeat scaffold protein OsRACK1B as a direct target of XopN. Functional analyses revealed that OsRACK1B overexpression enhanced rice resistance to Xoc, whereas loss-of-function mutants exhibited heightened susceptibility. This increased susceptibility was characterised by compromised immunity, including reduced callose deposition, diminished reactive oxygen species (ROS) accumulation, and attenuated expression of defence-related genes such as OsPBZ1 and OsPAL1. Furthermore, we show that OsRACK1B physically associates with OsRap2.6, an AP2/ERF-type transcription factor essential for rice innate immunity. Mechanistically, XopN competes with OsRap2.6 for binding to OsRACK1B, thereby disrupting the formation of the OsRACK1B-OsRap2.6 immune module. Our findings uncover a virulence strategy whereby XopN subverts host basal immunity by targeting a conserved scaffold-mediated complex, providing fundamental insights into the molecular basis of Xoc pathogenicity.