An introduction to brassinosteroids

Brassinosteroids (BRs) are plant steroidal hormones involved in several key physiological and biochemical processes of plants, i.e., vascular differentiation, seed germination, fertility, shoot and root growth, and flowering and also in response to environmental stresses. BRs were first identified in the pollen of Brassica napus in the early 1970s, and later during the same decade several members of the BR family were isolated from different plant species. BRs are synthesized most likely in the endoplasmic reticulum of plant cell and its synthesis is controlled by several transcription factors. External factors such as nutrient availability also influence BR biosynthesis and signaling. Moreover, BR signaling plays a crucial role in regulating diverse processes related to plant growth under normal and suboptimal growth environments. These compounds exist in free and conjugated form and their activity is highly dependent on the presence of hydroxyl group on side chain or steroid ring. Among various forms of BRs, 24-epibrassinolide (EBL) has been identified as the most active BR. During the past five decades, the BRs biosynthesis pathways have been well investigated with reverse and forward genetic techniques and nearly 60 compounds with a structure similar to that of BRs have been detected and isolated from various plant parts. These compounds were found in bryophytes (2 families), algae (6 families), pteridophytes (8 families), gymnosperms (4 families), angiosperms (35 families), and in some other plant-derivative products. This chapter discusses the initiation of research on BRs, their discovery, inhibitors, classification, and biodiversity. Moreover, it also highlights the mechanism of BR biosynthesis and signaling in plants.