Brassinosteroid regulates stomatal development by GSK3-mediated inhibition of a MAPK pathway

Plants must coordinately regulate biochemistry and anatomy to optimize photosynthesis and water use efficiency. The formation of stomata, epidermal pores facilitating gas exchange, is highly coordinated with other aspects of photosynthetic development. However, the signaling pathways controlling stomata development are not fully understood(1,2), although Mitogen Activated Protein Kinase (MAPK) signaling is known to play key roles. Here we demonstrate that brassinosteroid (BR) regulates stomatal development by activating the MAPKKK, YODA. Genetic analyses indicate that receptor kinase-mediated BR signaling inhibits development through the GSK3-like kinase BIN2, and BIN2 acts upstream of YODA but downstream of the known ERECTA family of stomatal receptor kinases. Complementary in vitro and in vivo assays show that BIN2 phosphorylates YODA to inhibit YODA phosphorylation of its substrate MKK4, and activities of downstream MAPKs are reduced in BR-deficient mutants but increased by treatment with either BR or GSK3-kinase inhibitor. Our results indicate that BR inhibits stomatal development by alleviating GSK3-mediated inhibition of the MAPK module, providing two key links; that of a plant MAPKKK to its upstream regulators and BR to a specific developmental output.