Nuclear-Localized BZR1 Mediates Brassinosteroid-Induced Growth and Feedback Suppression of Brassinosteroid Biosynthesis

Abstract

Plant steroid hormones, brassinosteroids (BRs), are perceived by a cell surface receptor kinase, BRI1, but how BR binding leads to regulation of gene expression in the nucleus is unknown. Here we describe the identification of BZR1 as a nuclear component of the BR signal transduction pathway. A dominant mutation bzr1-1D suppresses BR-deficient and BR-insensitive (bri1) phenotypes and enhances feedback inhibition of BR biosynthesis. BZR1 protein accumulates in the nucleus of elongating cells of dark-grown hypocotyls and is stabilized by BR signaling and the bzr1-1D mutation. Our results demonstrate that BZR1 is a positive regulator of the BR signaling pathway that mediates both downstream BR responses and feedback regulation of BR biosynthesis.

The brassinosteroid signal transduction pathway

“…Same mutation in BZR1's closest homolog, BZR2/BES1, was later identified in a bri1 suppressor screen [44]. BZR1 and BES1/BZR2 show 88% sequence identity and their phosphorylation and accumulation are similarly regulated by BR and BIN2 [42,44,46]. While both bzr1-1D and bes1-D suppress bri1 and bin2 and are resistant to brassinazole, they show opposite cell elongation phenotypes when grown in the light.…”

“…While both bzr1-1D and bes1-D suppress bri1 and bin2 and are resistant to brassinazole, they show opposite cell elongation phenotypes when grown in the light. The bzr1-1D plants grown in the light show weak dwarfism with reduced hypocotyl and petiole lengths, while bes1-D has elongated organs [42,44]. The bzr1-1D plants contain reduced level of BR due to increased feedback inhibition of BR synthesis, and its weak dwarf phenotype can be rescued by BR treatment.…”

“…Downstream components of this cascade include the BIN2 kinase, the BSU1 phosphatase, and the transcription factors BZR1 [42,43] and its homolog BES1/BZR2 [44,45]. BZR1 and BES1/BZR2 are substrates of the BIN2 kinase [44,46] and BSU1 phosphatase [47], and phosphorylated BZR1 is degraded by the proteasome [46].…”

“…Phosphorylated BZR1 is stabilized by MG132, an inhibitor of the proteasome, suggesting that BIN2 phosphorylation leads to degradation of BZR1 by the proteasome. The large number of putative GSK3 phosphorylation sites in BZR1 and BZR2 sequence and the severe mobility shift caused by BIN2 phosphorylation suggests that BIN2 phosphorylates the substrates at multiple amino acid residues [42,46]. The positions of these phosphorylation sites and their effects on BZR1/BES1 accumulation and activity are yet to be determined experimentally.…”

“…A genetic screen for mutants resistant to brassinazole lead to the identification of the brassinazole resistant 1-1D (bzr1-1D) mutant, which turned out to suppress the bri1 and bin2 mutants [42]. BZR1 encodes a nuclear protein and a P234L mutation in bzr1-1D stabilizes the protein.…”