Facing Heavy Metal Stress, What Are the Positive Responses of Melatonin in Plants: A Review

With the growth of the population and the development of modern industry and the economy, the problem of heavy metal pollution in cultivated soil has become increasingly prominent. Moreover, heavy metal poses a serious threat to plant growth due to its characteristics of difficult degradation, high mobility, easy enrichment, and potential toxicity and has become a social topic. Melatonin is a new type of plant hormone widely present in animals, plants, fungi, and bacteria, and its biological role has begun investigated in the last dozen years. Facing heavy metal stress, melatonin can play a pleiotropic role in the physiological processes of plants, such as stress resistance and growth regulation, mitigate the damage caused by stress on plants, and provide a new research idea for alleviating heavy metal stress in plants. From the aspects of the plant phenotype, physiology, element absorption, and molecular structure, this paper, therefore, mainly reviews the effects of melatonin on plants subjected to heavy metal stress and the mechanism of melatonin alleviating heavy metal stress and then puts forward future research directions. This information may be of great significance to the normal growth of crops under heavy metal stress and will provide an important theoretical basis for the genetic improvement of crop resistance in the future.