The Arabidopsis 14-3-3 proteins positively regulate brassinosteroid signaling by stabilizing BRASSINOSTEROID INSENSITIVE1 at the plasma membrane

Brassinosteroid (BR) hormones regulate various physiological and developmental processes in plants. BR signaling is primarily influenced by the abundance of the BR receptor BR INSENSITIVE1 (BRI1) at the plasma membrane, 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 14-3-3 proteins, which are conserved regulatory proteins in eukaryotes, directly interact with BRI1 at Threonine 872 (T872) within its juxtamembrane domain. In addition, Serine 858 (S858) in the juxtamembrane domain of BRI1 positively affects the phosphorylation status of T872, thereby contributing to the binding of 14-3-3 proteins to BRI1. Consequently, without altering the kinase activity of BRI1 or its interaction with the co-receptor BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1), but by inhibiting BRI1 ubiquitination, the binding of 14-3-3 proteins increased the abundance of BRI1 at the plasma membrane 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 reveled a previously undescribed function of 14-3-3 proteins in regulating BRI1 stability and, in turn, positively influencing BR signaling.