Abstract
Cadmium (Cd) is one of the most injurious heavy metals, affecting plant growth and development. Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995, and it is since known to act as a multifunctional molecule to alleviate abiotic and biotic stresses, especially Cd stress. Endogenously triggered or exogenously applied melatonin re-establishes the redox homeostasis by the improvement of the antioxidant defense system. It can also affect the Cd transportation and sequestration by regulating the transcripts of genes related to the major metal transport system, as well as the increase in glutathione (GSH) and phytochelatins (PCs). Melatonin activates several downstream signals, such as nitric oxide (NO), hydrogen peroxide (H2O2), and salicylic acid (SA), which are required for plant Cd tolerance. Similar to the physiological functions of NO, hydrogen sulfide (H2S) is also involved in the abiotic stress-related processes in plants. Moreover, exogenous melatonin induces H2S generation in plants under salinity or heat stress. However, the involvement of H2S action in melatonin-induced Cd tolerance is still largely unknown. In this review, we summarize the progresses in various physiological and molecular mechanisms regulated by melatonin in plants under Cd stress. The complex interactions between melatonin and H2S in acquisition of Cd stress tolerance are also discussed.
Highlights
Heavy metal pollution is the most widespread contamination resulting from anthropogenic activities in the world [1]
Recent studies have strongly indicated that melatonin, a multifunctional molecule, regulates Cd tolerance in plants
To further promote related research in plant Cd tolerance, this review summarizes the regulatory roles and mechanisms of melatonin in response to Cd stress
Summary
Heavy metal pollution is the most widespread contamination resulting from anthropogenic activities in the world [1]. Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995, and it is since known to act as a pleiotropic molecule to participate in multiple physiological processes, such as plant growth and development, and protection against abiotic and biotic stresses [15,16]. Numerous studies have focused on the protective role of melatonin against Cd stress in plants [17]. Exogenous application of H2S donors regulated plant growth, and conferred tolerance to salinity, heavy metal, heat, and drought stress among others [27,31,32]. Over the past several years, numerous studies focusing on the role of melatonin in alleviating Cd stress have been steadily increasing in plants. For a better understanding of this topic, we propose and discuss the future studies on the complex interactions between melatonin and H2S during Cd stress
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