Dampened nuclear localization of SnRK1 by brassinosteroid signaling inhibits stomatal development in Arabidopsis.

The balance between stem cell division and differentiation is crucial for flexible organ development. In Arabidopsis leaves, the fate of meristemoids, which exhibit stem cell characteristics, is tightly regulated by multiple intrinsic developmental signals and environmental factors. KIN10, the catalytic subunit of the sucrose non-fermenting 1-related protein kinase 1 (SnRK1) complex, has been shown to preferentially localize in the nucleus of meristemoids, where it phosphorylates and stabilizes the SPEECHLESS transcription factor, thereby promoting stomatal development. However, the regulatory mechanism governing the nuclear localization of KIN10 in meristemoids remains unclear. Here, we demonstrate that brassinosteroid (BR) inhibits KIN10's nuclear localization by modulating KINβ2 through BR-INSENSITIVE2 (BIN2)-mediated phosphorylation. In meristemoids, KIN10 is predominantly nuclear, while KINβ2 is mainly cytosolic. Interfering with the nuclear localization of KIN10 or enhancing the membrane association of KINβ2 impairs stomatal development and leads to excessive epidermal cell proliferation. Cell biology and biochemical analyses reveal that BR signaling could inhibit KIN10 nuclear localization by enhancing KINβ2 membrane association, while BIN2 interacts with and phosphorylates KINβ2 to reduce its membrane association and its interaction with KIN10. Taken together, these findings suggest that the precise regulation of the subcellular localization of the SnRK1 complex, at least in part by BR signaling, is critical for meristemoid differentiation and stomatal development.