ABSTRACT
ABSTRACT Melatonin can act as a universal hydrophilic and hydrophobic antioxidant. Moreover, melatonin probably acts as a night signal, shows coordinating responses to diurnal and photoperiodic environmental cues, and acts as a plant growth regulator. A large portion of the melatonin in a plant is synthesized by the plant itself. All required enzymes for melatonin biosynthesis have been identied in plants. Moreover, soil microorganisms are rich in melatonin, so their decomposition releases melatonin into the soil and plants can absorb melatonin from the soil via roots. The main function of melatonin in higher plants is to act as an antistress agent against abiotic stressors, such as drought, salinity, low and high ambient temperatures, UV radiation, and toxic chemicals. Several investigators have reported that melatonin may possess a variety of functions in vascular plants, including regulation of mitosis, delaying ower induction, stimulation of hypocotyl, coleoptile, and root growth, stimulation of adventitious-and lateral root regeneration, protection against cold-induced apoptosis, lowering chlorophyll degradation, and enhanced plant tolerance to environmental stress. Additionally, melatonin is considered to be involved in the regulation of circadian rhythm. Compared to the classic antioxidants such as vitamin C, vitamin E, and glutathione, melatonin exhibits a more potent antioxidant capacity to reduce oxidative injury. This chapter includes the most updated results about the role of melatonin in the enhancement of plant tolerance to environmental stress.
