RECEPTOR-LIKE KINASE 902 Associates with and Phosphorylates BRASSINOSTEROID-SIGNALING KINASE1 to Regulate Plant Immunity.

Plants employ receptor-like kinases (RLKs) and receptor-like proteins for rapid recognition of invading pathogens, and RLKs then transmit signals to receptor-like cytoplasmic kinases (RLCKs) to activate immune responses. RLKs are under fine regulation mediated by subcellular trafficking, which contributes to proper activation of plant immunity. In this study, we show that Arabidopsis thaliana RECEPTOR-LIKE KINASE 902 (RLK902) plays important roles in resistance to the bacterial pathogen Pseudomonas syringae, but not to the fungal powdery mildew pathogen Golovinomyces cichoracearum. RLK902 localizes at the plasma membrane and associates with ENHANCED DISEASE RESISTANCE 4 (EDR4), a protein involved in clathrin-mediated trafficking pathways. EDR4 and CLATHRIN HEAVY CHAIN 2 (CHC2) regulate the subcellular trafficking and accumulation of RLK902 protein. Furthermore, we found that RLK902 directly associates with the RLCK BRASSINOSTEROID-SIGNALING KINASE1 (BSK1), a key component of plant immunity, but not with other members of the FLAGELLIN SENSING 2 immune complex. RLK902 phosphorylates BSK1, and its Ser-230 is a key phosphorylation site critical for RLK902-mediated defense signaling. Taken together, our data indicate that EDR4 regulates plant immunity by modulating the subcellular trafficking and protein accumulation of RLK902, and that RLK902 transmits immune signals by phosphorylating BSK1.