The Ralstonia solanacearum effector RipAV targets plant U-box proteins and induces proteasomal-dependent degradation of BIK1.

Plants have developed a complex immune system to detect and respond to invading pathogens. A critical aspect of this defense relies on regulatory mechanisms that control the activation of immune responses, ensuring these are efficient yet do not compromise overall plant performance. Ralstonia solanacearum is a soil-borne bacterial pathogen that causes bacterial wilt disease in many plant species. Its virulence depends on the secretion of type III effector proteins that suppress plant immune responses. In this study, we investigated the function of one such effector, RipAV, in Arabidopsis thaliana, focusing on its role in modulating pattern-triggered immunity. We show that RipAV targets members of the plant-specific ubiquitin ligase (PUB) family and calcium-dependent protein kinase 28 (CPK28), which has been shown to phosphorylate a set of PUBs to enhance their activity and regulate the stability of the key immune regulator BIK1. RipAV association enhances the CPK28-mediated phosphorylation of PUBs, inducing the proteasome-mediated degradation of BIK1 and the suppression of immunity. Importantly, we found that RipAV is required for maintaining a low accumulation of BIK1 during a R. solanacearum infection. These findings provide new insights into the sophisticated strategies employed by pathogens to subvert plant immunity.