Abstract
Heavy metal toxicity is one of the most devastating abiotic stresses. Heavy metals cause serious damage to plant growth and productivity, which is a major problem for sustainable agriculture. It adversely affects plant molecular physiology and biochemistry by generating osmotic stress, ionic imbalance, oxidative stress, membrane disorganization, cellular toxicity, and metabolic homeostasis. To improve and stimulate plant tolerance to heavy metal stress, the application of biostimulants can be an effective approach without threatening the ecosystem. Melatonin (N-acetyl-5-methoxytryptamine), a biostimulator, plant growth regulator, and antioxidant, promotes plant tolerance to heavy metal stress by improving redox and nutrient homeostasis, osmotic balance, and primary and secondary metabolism. It is important to perceive the complete and detailed regulatory mechanisms of exogenous and endogenous melatonin-mediated heavy metal-toxicity mitigation in plants to identify potential research gaps that should be addressed in the future. This review provides a novel insight to understand the multifunctional role of melatonin in reducing heavy metal stress and the underlying molecular mechanisms.
Highlights
Plant stresses can be classified into two categories, i.e., biotic and abiotic stress, both of which have a negative impact on plant growth, development, and yield
Application of MT in Cr-stressed Origanum majorana plants preserved higher levels of photosynthetic pigments and less reactive oxygen species (ROS) by stimulating the antioxidant machinery and osmotic balance, decreased lipid peroxidation, and improved cellular membrane integrity [77]. These results indicate that exogenous MT-mediated improvement of photosynthesis and oxidant scavenging systems enable plants to fight against Cr stress
Under non-stress condition, MT treatment significantly improved the levels of expression of different anthocyanin-biosynthesisrelated genes such as PAL, cinnamic acid 4-hydroxylase (C4H), chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 30 -hydroxylase (F30 H), dihydroflavonol 4-reductase (DFR), and leucoanthocyanidin dioxygenase (LDOX) in cabbage plants [96]
Summary
Plant stresses can be classified into two categories, i.e., biotic and abiotic stress, both of which have a negative impact on plant growth, development, and yield. Abiotic stresses are associated and manifested as osmotic stress, oxidative stress, ionic imbalance, and cell metabolism dyshomeostasis, all of which influence plant growth and productivity [3,4]. Most HMs cause continuous ROS production in the chloroplast, mitochondria, and peroxisomes, which can cause oxidative stress in plants and result in the unexpected consequence of HM toxicity [7,8]. Mitigation of HM stress by exogenous application of MT in plants is reviewed in this study, and in addition to this, we will explore the in-depth mechanisms of melatoninmediated HM stress tolerance in plants
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