Fig. 4. Brassinosteroid signalling pathway. (A) In low brassinosteroid (BR) concentration, the BIN2 kinase rapidly phosphorylates the brassinosteroid-dependent transcriptional regulators, BES1 and BZR1, leading to their subsequent ubiquitination and degradation via the 26S proteasome. (B) When BR is perceived by the membrane-localised BRI1-BAK1 heterodimer, BIN2 is inhibited by an indirect, unknown mechanism. This leads to the accumulation of dephosphorylated BES1 and BZR1 in the nucleus, where they regulate the transcription of BR-regulated genes, either by transcriptional activation or inhibition. NM, nuclear membrane; +P, phosphorylation; PM, plasma membrane; +ub, ubiquitination.