Brassinosteroid signaling in plants

In animals and humans, steroid hormones (SHs) regulate gene transcription v ia the binding of nuclear receptors. In addition, rapid nongenomic effects of steroids occur and appear to be mediated by plasma-membrane receptors. P lants also use steroids as signaling molecules. These brassinosteroids (BRs ) show structural similarity to the SHs of vertebrates and insects. Plant m utants defective in brassinosteroid biosynthesis or perception exhibit dwar fism and reduced fertility, and reveal the need for BRs during growth and d evelopment. BR signaling in Arabidopsis thaliana and rice (Oryza sativa)-di cotyledonous and monocotyledonous models, respectively-is mediated by the r eceptor kinases BRI1 and OsBRI1. The extracellular domain of BRI1 perceives BRs and the signal is mediated via an intracellular kinase domain that aut ophosphorylates Ser and Thr residues and apparently has the potential to ph osphorylate other substrates. BRI1 transduces steroid signals across the pl asma membrane and mediates genomic effects.