Melatonin Biology in Plants Under Biotic Stress: Physiological and Molecular Aspects

Melatonin is a multifunctional regulatory molecule and an important antioxidant. It is widely involved in various plant growth processes, such as circadian rhythm, growth and development, photosynthesis, anti-senescence, and so on. It is also known for enhancing the tolerance of plants to biotic stress. Melatonin can regulate the generation and reduction of reactive oxygen species, activate the plant antioxidant system, stimulate the accumulation of secondary metabolites, upregulate the expression of defense-related genes, and coordinate with other hormones (salicylates, jasmonates, ethylene, etc.) to confer resistance to biotic stressors of various kinds including fungi, bacteria, viruses, and insects. In this chapter, we review the functions of melatonin in plant immunity and the mechanism of melatonin-induced enhanced plant resistance to biotic stress from both physiological and molecular aspects.