Biochemical defence of plants: redox molecules and their role in the alleviation of heavy metal stress
Heavy metal contamination poses a major constraint to plant growth and agricultural productivity by disrupting cellular metabolism and inducing oxidative stress. Exposure to toxic metals results in excessive accumulation of reactive oxygen species (ROS), which damage biomolecules and destabilise cellular homeostasis. Plants mitigate this challenge through a coordinated network of redox-mediated defence mechanisms that includes metal chelation and sequestration, enzymatic antioxidant detoxification, and metabolic adjustment. Beyond these direct responses, ROS also function as key signalling intermediates that activate stress-responsive transcriptional pathways, modulate ion transport, and remodel cell-wall architecture. Crosstalk between redox cues and phytohormones such as abscisic acid, ethylene and jasmonic acid further integrates environmental perception with long-term acclimation. This review synthesises current understanding of the biochemical, structural, and regulatory roles of redox molecules in heavy-metal tolerance, and highlights emerging avenues in omics-driven discovery, genetic enhancement, and microbial strategies. Improved insight into redox-centred defence systems will support the development of resilient crop varieties and sustainable approaches for agriculture in metal-polluted environments.