During brassinosteroid (BR) signaling in Arabidopsis, BSU1 (bri1 SUPPRESSOR1) phosphatase and BIN2 (BRASSINOSTEROID INSENSITIVE2) kinase regulate the signal intensity by determining the phosphorylation status of the transcription factors BZR1 (BRASSINAZOLE RESISTANT1) and BES1 (bri1 EMS SUPPRESSOR1). Here, we report that BIN2 and BSU1 are nucleocytoplasmic regulators that modulate the subcellular localization of BES1, with differential activities between the nucleus and the cytosol. In our experiments, the nuclear BIN2 induced phosphorylation and nuclear export of BES1 more efficiently than cytosolic BIN2. The cytoplasmic BSU1 mediated the dephosphorylation and nuclear translocation of BES1 more efficiently than the nuclear one. BSU1 com-promised the dwarf phenotype of bri1-5, a weak allele of BRI1 (BR-INSENSITIVE 1) receptor kinase, more effectively when localized in the cytosol than in the nucleus in transgenic plants. In conclusion, the predomi-nance of cytosolic BSU1 and nuclear BIN2 might be re-quired for the efficient subcellular localization of BES1 in BR signaling.

Keywords: BES1, BIN2 kinase, Brassinosteroid, BSU1 phosphatase, nucleocytoplasmic shuttling