1. Howard Hughes Medical Institute and Plant Biology Laboratory, The Salk Institute for Biological Studies, 10010 N. Torrey Pines Road, La Jolla, California 92037, USA
2. Plant Functions Lab, RIKEN (The Institute of Physical and Chemical Research), Wako-shi, Saitama 351-0198, Japan

Correspondence to: Joanne Chory¹ Correspondence and requests for materials should be addressed to J.C. (e-mail: Email: chory@salk.edu).

Abstract

Most multicellular organisms use steroids as signalling molecules for physiological and developmental regulation. Two different modes of steroid action have been described in animal systems: the well-studied gene regulation response mediated by nuclear receptors^{1, 2}, and the rapid non-genomic responses mediated by proposed membrane-bound receptors^{3, 4}. Plant genomes do not seem to encode members of the nuclear receptor superfamily⁵. However, a transmembrane receptor kinase, brassinosteroid-insensitive1 (BRI1), has been implicated in brassinosteroid responses^{6, 7}. Here we show that BRI1 functions as a receptor of brassinolide, the most active brassinosteroid. The number of brassinolide-binding sites and the degree of response to brassinolide depend on the level of BRI1 protein. The brassinolide-binding activity co-immunoprecipitates with BRI1, and requires a functional BRI1 extracellular domain. Moreover, treatment of Arabidopsis seedlings with brassinolide induces autophosphorylation of BRI1, which, together with our binding studies, shows that BRI1 is a receptor kinase that transduces steroid signals across the plasma membrane.