The Type III Effector XopLXcc in Xanthomonas campestris pv. campestris Targets the Proton Pump Interactor 1 and Suppresses Innate Immunity in Arabidopsis

<i>Xanthomonas campestris</i> pathovar <i>campestris</i> (<i>Xcc</i>) is a significant phytopathogen causing black rot disease in crucifers. <i>Xcc</i> injects a variety of type III effectors (T3Es) into the host cell to assist infection or propagation. A number of T3Es inhibit plant immunity, but the biochemical basis for a vast majority of them remains unknown. Previous research has revealed that the evolutionarily conserved XopL-family effector XopL<i>_Xcc</i> inhibits plant immunity, although the underlying mechanisms remain incompletely elucidated. In this study, we identified proton pump interactor (PPI1) as a specific virulence target of XopL<i>_Xcc</i> in <i>Arabidopsis</i>. Notably, the C-terminus of PPI1 and the Leucine-rich repeat (LRR) domains of XopL<i>_Xcc</i> are pivotal for facilitating this interaction. Our findings indicate that PPI1 plays a role in the immune response of <i>Arabidopsis</i> to <i>Xcc</i>. These results propose a model in which XopL<i>_Xcc</i> binds to PPI1, disrupting the early defense responses activated in <i>Arabidopsis</i> during <i>Xcc</i> infection and providing valuable insights into potential strategies for regulating plasma membrane (PM) H⁺-ATPase activity during infection. These novel insights enhance our understanding of the pathogenic mechanisms of T3Es and contribute to the development of effective strategies for controlling bacterial diseases.