Involvement of thiol-based mechanisms in plant growth, development, and stress tolerance

Abstract

Thiol-based mechanisms of plant growth regulation and stress response rely on cellular redox potential, depending mostly on glutathione content (EGSSG/2GSH). Thiol (SH) groups play various roles in the cell, with their redox state affecting the activity and structure of many enzymes, receptors, and transcription factors. The oxidation of -SH to the sulfinic (R-S02H) and sulfonic (R-S03H) acid may cause an irreversible enzyme inactivation and intermolecular protein cross-linking. The reversible oxidation of protein cysteine residues with the rise of formation of radical thiol, sulfenic acid (R-SOH), and S-nitrosothiol (SNT) is often an intermediate step to the formation of a mixed disulfide. Therefore, the most probable modification of these single reactive sulfhydryls is S-thiolation, resulting in formation of the mixed disulfide with low-molecular-weight cellular thiols such as glutathione. This modification is metabolically labile as it is evidenced by rapid "dethiolation" by several reductive processes. Intensity of revocation of Cys residues to reduced state is strongly based on glutaredoxin (Grx) and thioredoxin (Trx) activities, which are part of antioxidative system, regulating thiol-disulfide homeostasis in plant cells. Thus, the dynamic modification of proteins by S-thiolation/dethiolation represents one of the more important adaptive functions by reprogramming metabolism and protecting protein synthesis against irreversible oxidation. Alternatively, it may serve a regulatory role analogous to other posttranslational modifications such as protein phosphorylation. Thus, it can modulate cellular life cycle processes (division, differentiation, programmed cell death), energy metabolism, protein folding and degradation, pathogen resistance, and many others.