14-3-3 proteins facilitate the activation of MAP kinase cascades by upstream immunity-related kinases

Activation of mitogen-activated protein kinase (MAP kinase) cascades is essential for plant immunity. Upon activation by surface-localized immune receptors, receptor-like cytoplasmic kinases (RLCKs) in the cytoplasm phosphorylate MAP kinase kinase kinases (MAPKKKs) to initiate MAP kinase activation. Surprisingly, we found that both the phosphorylation of Arabidopsis (Arabidopsis thaliana) MAPKKKs and the subsequent activation of MAP kinase cascades require the λ and κ isoforms of 14-3-3 proteins, which directly interact with multiple RLCKs and MAPKKKs. The N- and C-termini of MAPKKK5 interact intramolecularly to inhibit the access to the C terminus by RLCKs, whereas the 14-3-3 proteins relieve this inhibition and facilitate the interaction of RLCKs with the C-terminus of MAPKKK5. This enables the phosphorylation of MAPKK5 at Ser599 and Ser682, thus promoting MAP kinase activation and enhancing plant disease resistance. Our study reveals a role of 14-3-3 proteins as scaffolds and activators in the regulation of the RLCK-MAPKKK5 module and provides insight into the mechanism of plant immune signaling.