ABSTRACT
Abiotic stresses exert severe constraints on crop production, and heavy metal toxicity is one of the major and most ruinous abiotic stresses, as it affects the agricultural sustainability. Plant physiology and biochemistry are impaired by heavy metal toxicity, as they generate metabolic alterations, cellular toxicity, oxidative stress, membrane disorganization, ionic imbalance and osmotic stress. Application of bio-stimulants could be an eco-friendly approach to enhance and stimulate plant tolerance to heavy metal stress. Higher levels of heavy metal tolerance could be promoted by a specific biostimulator, antioxidant and plant growth regulator known as melatonin (N-acetyl 5-methoxytryptamine), which highly improves primary and secondary metabolism, osmotic balance and redox and nutrient homeostasis. Melatonin is an indoleamine type of compound widely found in organisms; it works as an endogenous scavenger of free radicals exerting a strong antioxidant effect. Melatonin provides defenses against biotic and abiotic stresses, as a signaling molecule and plant growth regulator. Therefore, insights are needed into detailed mechanisms of melatonin activity as a plant growth regulator and heavy metal toxicity mitigator, while at the same time commitment must be devoted to address potential research gaps for future betterment. This chapter provides a detailed molecular mechanism supporting the multifunctional role of melatonin in mitigating heavy metal stress in plants.
