Melatonin: a key player in alleviating heavy metal stress in plants—current insights and future directions

Heavy metal (HM) contamination severely impacts global agricultural production. HMs toxicity effectively damaged the physiological functions such as imbalanced redox homeostasis, altered antioxidant enzyme activity, damage root system architecture, hindered photosynthetic apparatus, cellular toxicity, restricted mineral accumulation, and changed the metabolite production. Using phytohormones may be a successful strategy for enhancing and stimulating plant tolerance to HMs toxicity without affecting the environment. Melatonin (MT), a novel plant growth regulator, and powerful antioxidant molecule, enhances plant resilience to HMs stress by enhancing seedling growth, protecting the photosynthetic system, increasing nutritional status, balanced redox homeostasis, and restricting HMs accumulation from root to shoot. In addition, MT enhances the activity of antioxidant enzymes and triggers the ascorbate-glutathione (AsA-GSH) cycle, which helps remove excessive ROS. MT improves RuBisCO activity to improve photosynthesis and reduce the breakdown of chlorophyll. To identify future research needs, it is crucial to understand the comprehensive and intricate regulatory mechanisms of exogenous and endogenous MT-mediated reduction of heavy metal toxicity in plants. Melatonin has several functions, and this review sheds light on those functions and the molecular processes by which it alleviates HMs toxicity. More research is needed to fully understand how melatonin affects plant tolerance to heavy metals stress.