Brassinosteroids (BRs) play important roles in plant growth and development. BRs modulate the phosphorylation status of two crucial transcription factors, BRI1 EMS SUPPRESSOR1 (BES1) and BRASSINAZOLE RESISTANT1 (BZR1). Here we show that BES1 functions as a nucleocytoplasmic signal transmitter, and that its subcellular localization modulates the output intensity of the BR signal. BRASSINOSTEROID INSENSITIVE2 (BIN2) and other group II GLYCOGEN SYNTHASE KINASE 3 (GSK3)-like kinases phosphorylate BES1 and induce its nuclear export by regulating its binding affinity with 14-3-3 proteins. We identified twelve putative phosphorylation residues in BES1. Two of these residues, Ser 171 and Thr 175, are critical for interaction with 14-3-3 proteins. The other putative phosphorylation sites in the N-terminal region are required for the BIN2-mediated nuclear export of BES1. Mutations of these motifs result in increased nuclear accumulation of BES1 and enhanced BR responses in transgenic plants. Taken together, our results indicate that the spatial redistribution of BES1 is important for regulation of the BR signaling output.

Keywords: BES1, BIN2, Brassinosteroid, nucleocytoplasmic shuttling, phosphorylation