The ArabidopsisCERK1-associated kinase PBL27 connects chitin perception to MAPK activation

Perception of microbe-associated molecular patterns by host cell surface pattern recognition receptors (PRRs) triggers the intracellular activation of mitogen-activated protein kinase (MAPK) cascades. However, it is not known how PRRs transmit immune signals to MAPK cascades in plants. Here, we identify a complete phospho-signaling transduction pathway from PRR-mediated pathogen recognition to MAPK activation in plants. We found that the receptor-like cytoplasmic kinase PBL27 connects the chitin receptor complex CERK1-LYK5 and a MAPK cascade. PBL27 interacts with both CERK1 and the MAPK kinase kinase MAPKKK5 at the plasma membrane. Knockout mutants of MAPKKK5 compromise chitin-induced MAPK activation and disease resistance to Alternaria brassicicola. PBL27 phosphorylates MAPKKK5 in vitro, which is enhanced by phosphorylation of PBL27 by CERK1. The chitin perception induces disassociation between PBL27 and MAPKKK5 in vivo. Furthermore, genetic evidence suggests that phosphorylation of MAPKKK5 by PBL27 is essential for chitin-induced MAPK activation in plants. These data indicate that PBL27 is the MAPKKK kinase that provides the missing link between the cell surface chitin receptor and the intracellular MAPK cascade in plants. Synopsis Chitin receptor CERK1 transmits immune signals to the intracellular MAPK cascade in plants. This occurs via phosphorylation of MAPKKK5 by the CERK1-associated kinase PBL27, providing a missing link between pathogen perception and signaling output. CERK1-associated kinase PBL27 interacts with MAPKKK5 at the plasma membrane. Chitin perception induces disassociation of PBL27 and MAPKKK5. PBL27 functions as a MAPKKK kinase. Phosphorylation of MAPKKK5 by PBL27 is enhanced upon phosphorylation of PBL27 by CERK1. Phosphorylation of MAPKKK5 by PBL27 is required for chitin-induced MAPK activation in planta.