Brassinosteroids: modes of BR action and signal transduction

Abstract

In humans and other animals, steroid hormones regulate transcription via membrane-bound receptors. These receptors have an extracellular ligand-binding domain and an intracellular domain that are responsible for transducing the signal to the next member along the signaling pathway. Brassinosteroids (BRs) are structurally similar to the animal steroid hormones found in vertebrates and insects. Plants also use steroids as signaling molecules. BRs regulate the expression of numerous genes associated with plant development, and require the activity of a serine/threonine (Ser/ Thr) receptor kinase to realize these effects. Signaling in dicotyledonous (e.g.,Arabidopsis thaliana)and monocotyle-donous (e.g.,Oryza sativa) models, is mediated by the receptor kinases BRI1 and OsBRI1, respectively. The extracellular domain of BRI1 perceives BRs, and the signal is then mediated via an intracellular kinase domain that autophosphorylates Ser and Thr residues and potentially, other substrates. BRI1 transduces steroid signals across the plasma membrane and mediates genomic effects.