Heavy metals tolerance and toxic effects on plants: A review

The accumulation and biomagnification of heavy metals in terrestrial ecosystems threaten plant productivity and ecological stability. Heavy-metal toxicity arises from metal-specific disruptions of cellular redox homeostasis, causing excessive reactive oxygen species (ROS) and damage to nucleic acids, proteins, and membranes. Plants counteract these effects through a multilayered tolerance network integrating antioxidant enzymes, metal-chelating systems, redox buffers, stress-inducible proteins, and hormonal signalling. This review synthesises recent advances showing that antioxidant enzymes act not only as ROS scavengers but also as regulators of redox signalling, shaping stress perception and acclimation. We emphasise the coordinated roles of phytochelatins, metallothioneins, and non-enzymatic antioxidants such as glutathione, ascorbate, and proline in metal detoxification and redox buffering. The significance of micronutrients and metal transporters in balancing essential metal homeostasis against toxic accumulation is discussed. Additionally, symbiotic interactions with arbuscular mycorrhizal fungi and siderophore activity are highlighted as external modulators that restrict metal bioavailability and enhance stress resilience. By integrating molecular, physiological, and ecological perspectives, we propose a unified framework in which redox regulation, metal chelation, and signaling cross-talk collectively determine plant tolerance. This mechanistic understanding provides a foundation for improving crop performance and phytoremediation efficiency in metal-contaminated environments.