Dual Role of BKI1 and 14-3-3 s in Brassinosteroid Signaling to Link Receptor with Transcription Factors

The plasma membrane-localized plant steroid hormone receptor, BRASSINOSTEROID INSENSITIVE 1 (BRI1), is quiescent in the absence of steroids, largely due to a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1). Here, we report that the steroid-induced, plasma membrane-dissociated and phosphorylated BKI1 also plays positive roles in BR signaling by interacting with a subset of 14-3-3 proteins. The cytosolic fraction of BKI1 carboxyl terminal region enhances BR signaling. Mutations of two serine residues in this region lead to reduced phosphorylation by the BRI1 kinase and constitutive plasma membrane localization. The 14-3-3 proteins can interact with the phosphorylated BKIl through a motif that contains the two phosphorylation sites to release inhibition of BRI1 by BKI1. Meanwhile, the cytosolic BKIl antagonizes the 14-3-3 s and enhances accumulation of BRI1 EMS SUPPRESSOR 1 (BES1)/BRASSINAZOLE RESISTANT 1 (BZR1) in the nucleus to regulate BR-responses.