Abstract

Steroid hormones are signaling molecules important for normal growth, development and differentiation of multicellular organisms. Brassinosteroids (BRs) are a class of polyhydroxylated steroids that are necessary for plant development. Molecular genetic studies in Arabidopsis thaliana have led to the cloning and characterization of the BR receptor, BRI1, which is a transmembrane receptor serine/threonine kinase. The extracellular domain of BRI1, which is composed mainly of leucine‐rich repeats, can confer BR responsivity to heterologous cells and is required for BR binding. Although downstream components of BR action are mostly unknown, multiple genes whose expression are regulated by BRs have been identified and suggest mechanisms by which BRs affect cell elongation and division. BioEssays 23:1028–1036, 2001. © 2001 John Wiley & Sons, Inc.