OsCERK1-Mediated Chitin Perception and Immune Signaling Requires Receptor-like Cytoplasmic Kinase 185 to Activate an MAPK Cascade in Rice

Conserved pathogen-associated molecular patterns (PAMPs), such as chitin, are perceived by pattern recognition receptors (PRRs) located at the host cell surface and trigger rapid activation of mitogenactivated protein kinase (MAPK) cascades, which are required for plant resistance to pathogens. However, the direct links fromPAMPperception toMAPKactivation in plants remain largely unknown. In this study, we found that the PRR-associated receptor-like cytoplasmic kinase Oryza sativa RLCK185 transmits immune signaling from the PAMP receptor OsCERK1 to an MAPK signaling cascade through interaction with an MAPK kinase kinase, OsMAPKKK epsilon, which is the initial kinase of the MAPK cascade. OsRLCK185 interacts with and phosphorylates the C-terminal regulatory domain of OsMAPKKK epsilon. Coexpression of phosphomimetic OsRLCK185 and OsMAPKKK epsilon activates MAPK3/6 phosphorylation in Nicotiana benthamiana leaves. Moreover, OsMAPKKK epsilon interacts with and phosphorylates OsMKK4, a key MAPK kinase that transduces the chitin signal. Overexpression of OsMAPKKK epsilon increases chitin-induced MAPK3/6 activation, whereas OsMAPKKK epsilon knockdown compromises chitin-induced MAPK3/6 activation and resistance to rice blast fungus. Taken together, our results suggest the existence of a phospho-signaling pathway from cell surface chitin perception to intracellular activation of an MAPK cascade in rice.