9 - Brassinosteroids

Abstract

Brassinosteroids (BRs) are a class of growth-promoting steroidal phytohormones. BRs control almost all aspects of plant growth and development, and also play significant role in plant adaptation to biotic and abiotic stresses. Their biosynthetic and signaling pathways have been well characterized by forward and reverse genetics. The entire synthetic pathway includes the general cycloartenol-to-campesterol sterol biosynthesis pathway and a specific campesterol-to-brassinolide (BL) biosynthetic pathway in Arabidopsis. Campesterol converts to BL with a campestanol-dependent or a campestanol-independent pathway. BRs are perceived by a plasma membrane localized receptor and co-receptor complex including BRI1 and BAK1. The activated BRI1/BAK1 complex inactivates BIN2, which is one of the GSK3-like protein kinases, and negatively regulates BR signaling, to promote the activity of two critical transcription factors, BES1 and BZR1, and BR-responsive gene expression. BR-regulated plant developmental processes include cell elongation, root hair initiation, stomatal development, cell division, and reproductive development. BRs also interact with many other hormonal and environmental cues to regulate plant growth and development. In this chapter, we focus on the BR biosynthetic pathway, the BR signaling pathway, BR-regulated plant growth and development, and the cross talk between BRs and other signaling pathways.