Specific phosphorylation at Ser60 in N terminus of SlCAK1 propels bacterial cold shock protein-induced immunity in Solanaceae plants

Plants perceive microbe-derived molecular patterns to initiate the innate immune system. The csp22 peptide, derived from bacterial cold shock protein, is uniquely recognized by cold shock protein receptor (CORE) in Solanaceae plants, yet the signaling pathway remains largely obscure. In this study, we identify that tomato csp22-activated kinase 1 (SlCAK1), belonging to the receptor-like cytoplasmic kinase subgroup VII, is essential for csp22-induced immune responses and resistance to bacterial wilt disease. SlCAK1 is rapidly recruited to the csp22 receptor complex, which results in SlCAK1 phosphorylation upon csp22 treatment. Notably, csp22-induced phosphorylation specifically occurs at the Ser60 residue in the N terminus of SlCAK1, which is critical for SlCAK1 function in immunity. Interestingly, the conservation of Ser60 is unique to Solanaceae plants. Taken together, our findings reveal the pivotal role of SlCAK1 in transducing csp22-triggered immune signaling and provide a novel activation mechanism for receptor-like cytoplasmic kinases that involves phosphorylation in the N terminus.