Brassinosteroids: Occurrence, Structure and Stress Protective Activities

Brassinosteroids (BRs) are a group of naturally occurring endogenous steroidal plant growth regulators (PGRs) which are hydroxylated derivatives of a 5α- cholestane ring structure. Both spectroscopy and X-ray based structural elucidation methods have revealed the structure of the first discovered BR, i.e. Brassinolide (BL) as (22R, 23R, 24S) - 2α,3α,22,23-tetrahydroxy-24-methyl-B-homo-7-oxa-5α- cholestan-6-one. Generally, the natural BRs are hydroxylated at the C-2, C-3, C-22 and C-23 positions. The most biologically active BRs differ from other BRs solely on the basis of either a 6-oxo functional group or a lactone structure at one of its rings. Presently, more than 60 types of BRs have been identified and isolated from a diverse range of plants. Recently, BRs have emerged as important PGRs that are a pre-requisite for growth and various developmental processes along with other PGRs namely auxins, gibberellins, cytokinins, ethylene and abscisic acid. Further, these BRs also provide protection to the plants under both biotic and abiotic stresses. Therefore, the present chapter highlights the structure, occurrence, mode of action, biosynthetic pathway, plant stress ameliorating properties of BRs and their interactions with other PGRs.

Keywords: Brassinosteroids, Occurrence, Oxidative stress, Plant stress protection, Structure, Steroidal PGRs.