ABSTRACT
Melatonin is a small multifunctional signaling molecule that is predominant in both animals and plants. However, it was first identified in animals, and later in plants. Despite the ubiquitous distribution of melatonin in both animals and plants, its biosynthesis process is completely different in them. Two pathways share two common aspects between them, which include the similar precursor tryptophan and a common intermediate serotonin. Melatonin has been found to promote several physiological responses in plants, such as inducing seed germination, growth and development of different plants parts, improving photosynthetic activity, restricting chlorophyll degradation, delaying the process of senescence, acting as a bio-stimulant and an excellent antioxidant, and showing defensive action against the attacking pathogen and providing resistance from them. Biotic stress leads to profound alterations in the physiology, biochemistry, and metabolism in plants, resulting in a total damage to the agricultural products across the world. In addition to several pathways in plants for the resistance against stress by themselves, application of melatonin triggers the effectiveness of resistance manifold. This chapter specifically focuses on how melatonin alleviates the deleterious effects of ROS in plants and exhibits strong antibacterial, antifungal, as well as antiviral action against a vast variety of pathogens in different crops. However, the specific mechanism of how exactly the melatonin receptors, CAND2/PMTR1, can overcome the pathogen attack in plants is still ambiguous. Therefore, intensive research is required in the future to find the answer to that.
