Physiological and Molecular Mechanisms of Melatonin-Induced Tolerance to Heavy Metal Stress

Rapid development of the industry has increased the risk of environmental pollution, especially heavy metal pollution. When the crop uptakes large amounts of heavy metals from the soil, plant growth and development are severely hampered. Recently, the use of plant derivatives and other measures to alleviate heavy metal stress in plants has become a hot research topic. Almost all plants in nature contain melatonin, a small molecule indoleamine, which plays an important role in plant health because it promotes plant growth and helps plants better adapt to environmental stress. The exogenous application of melatonin plays a very important role in promoting the resistance of plants to environmental stress. Studies showed that the level of melatonin in plants increased under heavy metal stress, which was mainly due to the increased expression of genes related to melatonin synthesis in plants stimulated by heavy metals. Compared with only heavy metal stress, the plant height, biomass accumulation, and root activity increased with exogenous melatonin treatment under heavy metal stress, which confirmed that melatonin could significantly improve plant tolerance to heavy metal stress. Here, the main functions of melatonin in plants under heavy metal stress are reviewed, which can provide a reference for related research in the future.