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.
