ABSTRACT
Abiotic stress in plants seems to be a more common problem in agriculture these days. So it is imperative to investigate various compounds that could help plants to cope with the negative consequences of abiotic stress. Melatonin (MEL), a tryptophan derivative, plays an important role in reducing the detrimental effects of abiotic stress in plants and has become increasingly essential in recent years. MEL is a multifunctional signalling molecule found throughout a plant and is responsible for triggering a variety of physiological and chemical reactions in response to adverse environmental conditions. In plants, MEL biosynthesis occurs mostly in chloroplasts, and associated metabolic pathways have been thoroughly studied. MEL controls plant stress responses by limiting the accumulation of reactive oxygen and nitrogen species and by influencing stress response pathways indirectly. MEL interacts with a variety of signalling molecules, including reactive oxygen species (ROS) and nitric oxide (NO), and is involved in a wide range of physiological processes. It is well understood that NO causes S-nitrosylation and NO2-Tyr of important antioxidant-related proteins and that this is an important mechanism for maintaining the antioxidant capacity of the AsA/GSH cycle under nitro-oxidative conditions. Although plants can manufacture MEL, exogenous application of MEL to diverse crops can boost plant growth and development in response to abiotic and biotic challenges by regulating plant antioxidant machinery. Exogenously applied MEL, according to new research, can improve plant stress tolerance by influencing both enzymatic and non-enzymatic antioxidant defence systems. Exogenous MEL increases the activity of antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase, whereas, under stressful conditions, non-enzymatic antioxidants like ascorbate, reduced glutathione, carotenoids, tocopherols, and phenolics are also higher. As a result of the improved antioxidant system, there is less lipid peroxidation and more plasma membrane integrity under stress. These reactions, however, differ significantly from crop to crop and are dependent on the severity and type of stress. To elucidate the gene regulatory networks implicated in the relationship between melatonin, antioxidants, and abiotic stress, a larger number of agricultural field trials and comprehensive transcriptome analyses are needed.
