Nitric Oxide Regulates Stomatal Development and Stress Responses by S-Nitrosylation-Mediated Inhibiting Phosphorylation of MPK6

In plants, stomata on the aerial epidermis play critical roles in various biological processes, including gas exchange, photosynthesis, transpiration, and immunity. Stomatal development is negatively and positively regulated by the mitogen-activated protein kinase (MAPK) cascade and nitric oxide (NO), respectively. However, the regulatory scheme of stomatal development by these signaling pathways remains elusive. Here, we show that NO-modulated stomatal development is genetically dependent on MPK3 and MPK6. Moreover, NO-mediated S-nitrosylation of MPK6 at Cys-201 inhibits its phosphorylation, resulting in the stabilization of SPCH, a master regulator of stomatal lineage initiation, thereby promoting stomatal development. Remarkably, a MPK6C201S mutation confers NO insensitivity during stomatal development and stress responses. We propose that NO positively regulates stomatal development and stress responses by the S-nitrosylation-mediated inhibition of the MPK6 activity, thus identifying a mechanism linking the coupled NO-MAPK signaling to specific biological outputs.