Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP kinases to SPEECHLESS

Cell fate in eukaryotes is controlled by mitogen-activated protein kinases (MAPKs) that translate external cues into cellular responses. In plants, two MAPKs—MPK3 and MPK6—regulate diverse processes of development, environmental response and immunity. However, the mechanism that bridges these shared signalling components with a specific target remains unresolved. Focusing on the development of stomata—epidermal valves that are essential for gas exchange and transpiration—here, we report that the basic helix-loop-helix protein SCREAM functions as a scaffold that recruits MPK3/6 to downregulate SPEECHLESS, a transcription factor that initiates stomatal cell lineages. SCREAM directly binds to MPK3/6 through an evolutionarily conserved, yet unconventional, bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 with a resolution of 2.75 Å showed bipartite binding of SCREAM to MPK6 that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism that directly links extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell fate, and highlight a unique substrate-binding mode adopted by plant MAPKs. Arabidopsis MPK3 and MPK6 are involved in many biological processes. How do they achieve their signalling specificity? Structural, biochemical and genetic approaches show that the bHLH protein SCREAM acts as a scaffold to recruit downstream substrates during stomatal development.