Melatonin-Induced Plant Tolerance to Heavy Metal Stress

The prime goal of this chapter is to provide the latest information regarding the role of melatonin in alleviation of heavy metal in crop plants and to encourage botanists to further evaluate the tolerance mechanisms of melatonin in plants. Consequently, plant growth and production are enhanced by exogenous melatonin application under heavy metal stress, acting as a strong plant growth regulator and antioxidant. Exogenous melatonin increases photosynthetic pigment level by up regulating chlorophyll and enzyme synthesis while down regulating degrading genes of chlorophyll. Exogenous melatonin reduces the negative impacts of heavy metal toxicity on plant growth, development and photosynthesis. Exogenous melatonin reduces the toxic effects of heavy metals by up regulating a variety of protective genes that are in charge of increased antioxidant activity and metal chelating abilities. By accumulating osmolytes, increasing antioxidant enzyme activity, adjusting osmotic pressure, maintaining integrity of membranes, reducing ROS production and peroxidation of lipids and boosting the activity of both non-enzyme and enzyme-based antioxidants, the application of melatonin improves plant tolerance to heavy metals. Different nutrient homeostasis is also improved by melatonin. Plant secondary metabolites are produced when melatonin is applied exogenously. Despite the abovementioned progress, we still do not fully understand the mechanisms underlying melatonin-regulated heavy metal absorption, sequestration and transport. For sustainable utilization of those pathways and a better understanding of the actions of this molecule to ensure higher crop yield in heavy metal stress situations, additional studies are required.