A current model for brassinosteroid (BR) signaling in Arabidopsis. BRs are perceived by the extracellular domain of the BR receptor BRI1. BR binding activates BRI1 through homodimerization and heterodimerization with its partner BAK1 and releasing it from its inhibitory protein BKI1. The activated BRI1 then sequentially phosphorylates and activates two downstream kinases BSK1 and CDG1 and a Ser/Thr phosphatase BSU1 that will inactivate the BIN2 kinase, a negative regulator of BR signaling. BR signal also activates PP2A, a phosphatase that phosphorylates and activates the transcription factors BZR1 and BES1. The dephosphorylated BZR1 and BES1 will bind the BRRE or E-box motif of their target genes and regulate their expression. When BR signal is absent, BRI1 is bound by BKI1 and cannot interact with BAK1. BIN2 is also in its active form that will phosphorylate and inactivate BZR1 and BES1. The phosphorylated BZR1 and BES1 are retained in the cytoplasm by 14-3-3 proteins and can be degraded by the 26S proteasome.