A complete MAPK cascade, a calmodulin, and a protein phosphatase act downstream of CRK receptor kinases and regulate Arabidopsis innate immunity

Mitogen-activated protein kinase (MAPK) cascades are critical signal transduction modules in stress responses, but how their composition and mode of activation induces a stress response is poorly understood. We showed in Arabidopsis that CRK21, a cysteine-rich receptor-like protein kinase (CRK), phosphorylates MAPK kinase kinase 20 (MKKK20) and thus directly activates a novel MAPK cascade, consisting of MKKK20, the MAPK kinase MKK3, and the MAPK MPK6. Furthermore, the protein phosphatase PP2C76 and the calmodulin CaM7 were identified as negative and positive modulators of the cascade, respectively. Loss-of-function in components of the MAPK cascade or in CaM7 led to susceptibility to the bacterial pathogen Pseudomonas syringae and the fungal pathogen Botrytis cinerea. In contrast, loss-of-function of PP2C76 as well as transient overexpression of the genes in the MAPK cascade and CaM7 conferred resistance to the pathogens. Moreover, seven additional CRKs interacted with MKKK20 in vivo, and four of these were highly expressed after inoculation with P. syringae. In summary, our findings demonstrate that the novel CRK21-MKKK20-MKK3-MPK6 signaling pathway functions in immunity to fungal and bacterial pathogens and that CRKs may function in directly activating MKKKs. ### Competing Interest Statement The authors have declared no competing interest.