A Bacterial Effector Acquired via Horizontal Gene Transfer Suppresses Plant Immunity by Hijacking the Rubisco Small Subunit via Condensate Formation

The genus Xanthomonas demonstrates remarkable tissue specificity and employs distinct infection strategies, making it an ideal model system for elucidating the molecular mechanisms underlying pathogen adaptation. This specialization is primarily attributed to the lineage-specific diversification of variable type III secreted effectors (T3SEs) through horizontal gene transfer (HGT). However, comprehensive empirical evidence is required to fully validate this hypothesis. In this study, we identified a horizontally acquired T3SE, CitG, present in X . oryzae pv. oryzicola ( Xoc ) but absent in X . oryzae pv. oryzae ( Xoo ). Functionally, CitG is indispensable for the virulence of the nonvascular pathogen Xoc , and its ectopic expression in the vascular pathogen Xoo confers nonvascular behavior. CitG undergoes phase separation within rice cells, forming condensates that sequester a subset of plastid-localized Rubisco small subunit RbcS2 proteins. The relocation of OsRbcS2 disrupts Rubisco activity, attenuating reactive oxygen species (ROS) accumulation, thereby facilitating Xoc invasion. These findings reveal a novel mechanism whereby bacterial effectors exploit phase separation to manipulate host plastid functions, providing profound insights into the molecular basis of tissue specificity in plant-pathogen interactions.