1. Plant Biology Laboratory, and
2. Howard Hughes Medical Institute, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, USA

Correspondence to: Joanne Chory^1,2 Correspondence and requests for materials should be addressed to J.C. (Email: chory@salk.edu).

Abstract

Brassinosteroids (BRs) are steroid hormones that control many aspects of plant growth and development^{1, 2}. BRs bind to the plasma membrane receptor kinase BRI1, and act through a signalling pathway that involves a glycogen synthase kinase-3-like kinase (BIN2) and a serine/threonine phosphatase (BSU1)^{3, 4, 5}. Previous models proposed that BIN2 negatively regulates BR signalling by controlling the stability and subcellular localization of the related transcription factors BES1 and BZR1 by phosphorylation, in a manner reminiscent of the canonical Wnt signalling pathway of metazoans^{6, 7, 8, 9}. Here we present strong evidence for a different mode of regulation of BR signalling. We show that BES1 is localized constitutively to the nucleus, where its activity is modulated by nuclear-localized BIN2 kinase. BIN2-mediated phosphorylation of BES1 inhibits its DNA-binding activity on BR-responsive target promoters and its transcriptional activity through impaired multimerization. Our observations demonstrate that phosphorylation-dependent inhibition of DNA binding and trans-activation is the key primary mechanism of BES1 regulation.

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