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

ABSTRACT 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.