Exogenous Melatonin as a Protective Agent Against Heavy Metal Stress: Mechanisms of Tolerance and Growth Enhancement in Plants

Abstract

Heavy metal contamination in agricultural soils has emerged as a critical environmental constraint affecting plant growth, productivity, and food safety worldwide. Exogenous melatonin, a multifunctional signaling molecule, has gained increasing attention as a promising protective agent that enhances plant tolerance to heavy metal stress. This paper reviews the physiological, biochemical, and molecular mechanisms through which melatonin mitigates heavy metal toxicity in plants. Melatonin improves antioxidant defense systems, regulates metal uptake and transport, enhances photosynthetic efficiency, and stabilizes cellular homeostasis under stress conditions. Additionally, it modulates gene expression associated with stress signaling pathways and chelation processes, thereby reducing metal-induced oxidative damage. The application of exogenous melatonin also contributes to improved plant growth, biomass accumulation, and yield stability under contaminated environments. Overall, melatonin represents an eco-friendly and effective strategy for improving plant resilience against heavy metal stress and ensuring sustainable agricultural productivity.