Origin of Brassinosteroids and Their Role in Oxidative Stress in Plants

Abstract

Brassinosteroids (BRs) are a class of plant polyhydroxysteroids that have been recognized as a kind of phytohormones and play essential roles in plant development. BRs occur at low concentrations in lower and higher plants. Natural 70 BRs identified so far have a common 5α-cholestan skeleton, and their structural variations come from the kind and orientation of oxygenated functions in rings A and B. As regards the B-ring oxidation, BRs are divided into the following types: 7-oxalactone, 6-oxo, 6-deoxo and 6-hydroxy. These steroids can be also classified as C₂₇, C₂₈ or C₂₉ BRs depending on the alkyl substitution on the C-24 in the side chain. In addition to free BRs, sugar and fatty acid conjugates have been also identified in plants. Plant growth and developmental processes as well as environmental responses require the action and cross talk of BRs and reactive oxygen species (ROS). ROS can partake in signalling, although these events will be modulated by the complement of antioxidants in, or even around, the cell. ROS can interact with other signal molecules, including BRs in regulation of these physiological responses. BRs can modify the synthesis of antioxidants and the activity of basic antioxidant enzymes, and some of these enzymes are also implicated in catabolism of plant hormone. However, it is still unclear whether endogenous BRs directly or indirectly modulate the responses of plants to oxidative stress. The recent progress made in understanding the response of BRs in plants under oxidative stress.