MAMP-triggered plant immunity mediated by the LysM-receptor kinase CERK1

Lysin motif (LysM) receptor-like proteins/kinases (LysM-RLPs/RLKs) are well known to play an important role in the induction of defense or symbiosis signaling through the recognition of carbohydrate ligands in plants. Chitin elicitor receptor kinase 1 (CERK1) is the receptor-like kinase (RLK) essential for chitin-induced defense signaling in Arabidopsis and rice. At/OsCERK1 is also known to be important for peptidoglycan (PGN)-induced defense signaling. To induce chitin and PGN responses, CERK1 forms a receptor complex with corresponding LysM-type receptors such as CEBiP, LYM1/3 or LYP4/6. Furthermore, OsCERK1 in rice also plays a key role in lipopolysaccharide (LPS) signaling, and AtCERK1 in Arabidopsis is involved in unbranched -glucan signaling. In any case, CERK1 plays a crucial role in the activation of defense signaling by these ligands and is an essential hub-RLK/co-receptor in these systems. After chitin perception, autophosphorylation of the CERK1 kinase domain is essential for inducing immune signaling, and some functionally important phosphorylation sites have been identified. In addition, several CERK1-interacting proteins and their contribution to the downstream signaling have been reported. In particular, phosphorylation of receptor-like cytoplasmic kinases (RLCKs) by CERK1 is important for the activation of chitin signaling. These findings have contributed to our understanding of the early steps of chitin signaling. This review focuses on current knowledge about CERK1-mediated receptor complex formation and subsequent intracellular signaling.