Melatonin Modulates Plant Tolerance to Heavy Metal Stress: Morphological Responses to Molecular Mechanisms

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.

Effects of Soil Application of Chitosan and Foliar Melatonin on Growth, Photosynthesis, and Heavy Metals Accumulation in Wheat Growing on Wastewater Polluted Soil

“…The synthesis of Chl-a and Chl-b in the plants is a critical indicator under HM stress. However, the synthesis of Chl-a and Chl-b in plant leaves is severely decreased when the plants grow in the HMs polluted soils [11,66]. For this reason, measuring Chl in plants can help to detect plant stress affected by HM exposure [67].…”

Carex rigescens caffeic acid O-methyltransferase gene CrCOMT confer melatonin-mediated drought tolerance in transgenic tobacco

“…Melatonin (N-acetyl-5-methoxytrytamine) was first identified in vascular plants in 1995, and since then, studies have reported multiple functions for plant growth and development ( Hattori et al, 1995 ; Arnao and Hernández-Ruiz, 2019 ). In addition, over the last two decades, the multifunctional role of melatonin in plants as a protective agent against different abiotic and biotic stressors has been fully accepted, and it is well known to improve tolerance to salt, drought, high temperatures, cold, and heavy metal stress ( Hoque et al, 2021 ; Sun et al, 2021 ). As a master regulator, melatonin improves plant tolerance to abiotic stress by regulating the expression of downstream genes, osmolyte accumulation, carbon assimilation, stomatal conductance, and photochemical efficiency of photosystems ( Sharma and Zheng, 2019 ; Moustafa-Farag et al, 2020 ).…”

Perspective of Melatonin-Mediated Stress Resilience and Cu Remediation Efficiency of Brassica juncea in Cu-Contaminated Soils

“…Numerous studies have explored the scavenging role of MEL in improving stress tolerance in plants. For instance, MEL induces tolerance in crops under heavy metals ( Hoque et al, 2021 ), drought ( Imran et al, 2021 ), high temperature ( Jahan et al, 2019 ), cold stress ( Turk et al, 2014 ), and salinity ( Farouk and Al-Huqail, 2022 ) conditions. During stress conditions, MEL enhances a wide range of adaptive responses such as improvement of photosynthesis and gas exchange attributes (chlorophyll content, stomatal conductance, photosynthetic rate, and transpiration rate), enrichment of nutrient levels, improvement of sugar metabolism and hormonal regulation, enhancement of secretion of organic acid anions and phenolic compounds, and improvement of ROS scavenging via improved antioxidant defense system, thus reducing lipid peroxidation and oxidative burst in plant cells ( Farouk and Al-Amri, 2019 ; Farouk and Al-Huqail, 2022 ).…”