The Arabidopsis 14-3-3 proteins stabilize BRASSINOSTEROID INSENSITIVE1 at the plasma membrane

Brassinosteroid (BR) hormones regulate various physiological and developmental processes in plants. BR signaling is primarily influenced by the plasma membrane abundance of the BR receptor BR INSENSITIVE1 (BRI1), a process regulated by ubiquitination, endocytosis, and protein degradation. Despite extensive research, only a few negative regulators of BRI1 internalization and ubiquitination have been identified. In this study, we show that the conserved eukaryotic regulatory proteins 14-3-3 directly interact with BRI1 at Threonine 872 (T872) within its juxtamembrane domain. Furthermore, phosphorylation at Serine 858 (S858) in BRI1’s juxtamembrane domain enhances T872 phosphorylation, facilitating 14-3-3 protein binding. Consequently, by inhibiting BRI1 ubiquitination without affecting its kinase activity or BAK1 interaction, 14-3-3 binding increased BRI1 plasma membrane abundance and enhanced BR signaling. Both non-epsilon and epsilon isoforms of 14-3-3 proteins contribute to the regulation of BRI1 and, consequently, to plant responsiveness to BRs. Our results revealed a previously undescribed function of 14-3-3 proteins in regulating BRI1 stability.