Effect of Melatonin in Regulating Salt Stress Responses in Plants

Salt stress is one of the most significant constraints in global agricultural production by causing a significant yield loss, every year. Attempts have been made to mitigate the harmful effects of salt stress by using various bio-stimulants such as some biomolecules, plant extracts or soil-inhabiting microorganisms. One of such potential biomolecules is the melatonin that functions as potent growth regulator, bio-stimulant, antioxidant, and plant stress protectant. Melatonin is a multifunctional molecule found in both prokaryotes as well as eukaryotes and has received extensive validation for its ability to regulate plant stress tolerance, particularly its critical function in improving tolerance toward salt stress. The application of exogenous melatonin to the plant under salt stress has received a lot of attention from researchers as a stress ameliorative treatment. Various modes of application of exogenous melatonin have been tried and found effective in conferring a better tolerance toward salt stress. In order to boost antioxidant systems under salt stress, melatonin also plays crucial roles as an antioxidant and free radical scavenger. These processes enhance photosynthesis, maintain ion homeostasis, and trigger a number of downstream signals that regulate the production of other plant growth regulators. Attempts have also been made for increasing the endogenous level of melatonin by the over-expression of the genes involved in the biosynthetic pathway of melatonin, suggesting them as key gene candidates for enhancing the tolerance toward salt stress via the genetic transformation. Furthermore, melatonin also regulates some salt stress-responsive genes in plants. Overall, the present chapter presents melatonin as a potential bio-stimulant for mitigating the harmful effects of salt stress in plants, along with the regulation of melatonin accumulation. An attempt has also been made to discuss the underlaying mechanisms and challenges for improving salt stress in different plant species.