Stomatal ontogenesis,patteing,and function are hallmarks of environmental plant adaptation,especially to conditions limiting plant growth,such as elevated temperatures and reduced water availability.The specification and distribution of a stomatal cell lineage and its terminal differentiation into guard cells require a master regulatory protein phosphorylation cascade involving the YODA mitogen-activated protein kinase kinase kinase.YODA signaling results in the activation of MITOGEN-ACTIVATED PROTEIN KINASEs (MPK3 and MPK6),which regulate transcription factors,including SPEECHLESS (SPCH).Here,we report that acute heat stress affects the phosphorylation and deactivation of SPCH and modulates stomatal density.By using complementary molecular,genetic,biochemical,and cell biology approaches,we provide solid evidence that HEAT SHOCK PROTEINS 90 (HSP90s) play a crucial role in transducing heat-stress response through the YODA cascade.Genetic studies revealed that YODA and HSP90.1 are epistatic,and they likely function linearly in the same developmental pathway regulating stomata formation.HSP90s interact with YODA,affect its cellular polarization,and modulate the phosphorylation of downstream targets,such as MPK6 and SPCH,under both normal and heat-stress conditions.Thus,HSP90-mediated specification and differentiation of the stomatal cell lineage couples stomatal development to environmental cues,providing an adaptive heat stress response mechanism in plants.