Spatial redistribution of key transcriptional regulators in brassinosteroid signaling.

Brassinosteroids (BRs) are steroid phytohormones required for plant growth and development. The perception of BRs at the plasma membrane initiates intracellular signaling and induces dephosphorylation of two key transcription factors, BZR1 and BZR2/BES1. Phosphorylation of these factors is modulated by the GSK3 kinase BIN2 and phosphatase BSU1 and, in turn, controls DNA binding, protein stabilization, or/and nuclear translocation of BZR1 and BZR2/BES1. However, cytosolic signaling events and the biological roles of phosphorylation in BR signaling are still largely unknown. Recently, we demonstrated that BZR1 itself acts as a cytosolic signaling mediator and regulates expression of BR-responsive genes via phosphorylation-mediated nucleocytoplasmic shuttling. BIN2-mediated phosphorylation mediates nuclear export of BZR1 via interaction with a 14-3-3 protein, while BR activated phosphatases induce nuclear import of BZR1. The temporal and spatial expression of BIN2 appears to be essential in BR signaling. In this addendum, we summarize new findings in BR signaling and discuss the possibility that light and brassinosteroid signals intersect at BIN2 expression.