The Ralstonia solanacearum E3 Ligase Effector RipV1 Targets Plant U-Box Domain-Containing Receptor-Like Cytoplasmic Kinases That Negatively Regulate Immunity in Nicotiana benthamiana.

Plants detect microbe-associated molecular patterns from pathogens via plasma membrane-localised receptors, which activate multiple signalling cascades that lead to pattern-triggered immunity (PTI). Receptor-like cytoplasmic kinases (RLCKs) are central to plant immune signalling and common targets of pathogen effectors. Here, we conducted a yeast two-hybrid screen between tomato (Solanum lycopersicum) RLCKs and type III secreted effectors conserved in Ralstonia solanacearum Korean isolates. We identified members of the RLCK subfamily IXb, containing an integrated plant U-box (PUB) domain, as interactors of RipV1, an effector containing a novel E3 ligase domain (NEL). We demonstrated that SlRLCK-IXb-1 exhibits E3 ligase activity but lacks detectable kinase activity in vitro. RipV1 trans-ubiquitinates SlRLCK-IXb-1 in vitro and enhances its stability in planta. Virus-induced gene silencing of SlRLCK-IXb homologues in Nicotiana benthamiana revealed their role as negative regulators of early PTI signalling and of RipV1-induced cell death. These findings highlight relevant substrates for an E3 ligase effector and provide insights into the complex modulation of plant defence responses.