Melatonin Confers Plant Cadmium Tolerance: An Update

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.

Crosstalk between Melatonin and Reactive Oxygen Species in Plant Abiotic Stress Responses: An Update

“…As a master regulator, melatonin plays an important role in plant tolerance to abiotic stresses, such as salinity, cold, heat, drought, and heavy metals [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. Our previous review systematically summarized the melatonin biosynthesis and catabolism in plants [ 19 ]. We also showed that Cd stress strongly induced melatonin accumulation via regulating the expression of genes encoding tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), N -acetylserotonin methyltransferase (ASMT), caffeic acid O-methyltransferase (COMT), and serotonin N -acetyltransferase (SNAT, also called arylakylamine N -acetyltransferase (AANAT)) [ 21 ].…”

“…Afterwards, melatonin enhances the tolerance via up- or down-regulating downstream regulating elements within the physiological environments of various plants ( Figure 1 , Table 1 ). The increased melatonin decreases O 2 •– and H 2 O 2 accumulation via the enhanced antioxidant enzyme activities and antioxidant levels [ 9 , 10 , 18 , 19 ]. Some examples of the various roles of melatonin in the regulation of redox homeostasis in plants under abiotic stresses are illustrated in Table 1 .…”

“…To date, several papers have confirmed the crosstalk between melatonin and signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H 2 S), and hydrogen gas (H 2 ); therefore, the present paper does not elucidate it in detail [ 9 , 10 , 18 , 19 , 33 , 34 , 35 ]. Here, we systematically review the updated literature on the crosstalk between melatonin and ROS in plants upon abiotic stresses, highlight the role of RBOHs, and give perspectives for future research.…”

“…and metabolites (including ASC, GSH, flavonoid, anthocyanins, etc.) [ 11 , 12 , 17 , 18 , 19 , 20 ]. Several studies have revealed that melatonin regulates the primary and secondary metabolism via mediating the master factors of metabolic processes [ 10 , 19 , 20 , 21 , 22 ].…”

Hydrogen sulfide attenuates intracellular oxidative stress via repressing glycolate oxidase activities in Arabidopsis thaliana

“…Other than its toxicity, H 2 S has been gradually accepted as a key messenger involved in a vast number of physiologically important processes in photosynthetic organisms, such as stress resistance, autophagy, stomatal movement [4][5][6][7]. The plant itself can generate H 2 S in different subcellular sites including cytosol, chloroplasts, mitochondria, and nuclei [8][9][10][11][12][13].…”