Melatonin Alleviates Cadmium and Chromium-Induced Oxidative Stress in Muskmelon: Insights into Physiological and Molecular Mechanisms.

Cadmium (Cd) and chromium (Cr) contamination in agricultural soils is an escalating environmental issue mainly due to the rising levels of the Cd/Cr that are as a result of industrial pollution, wastewater irrigation, and over-application of fertilizers. Melatonin (ME) is a well-known potent antioxidant and versatile compound that enhances the resistance of plants to environmental factors. Here, we explored the potential of ME to reduce Cd/Cr stress in muskmelon plants through the regulation of cellular oxidative repair. A hydroponic study was conducted, in which muskmelon seedlings were first exposed to ME (0.2μML-1) for 5 days and later exposed to Cd (80μML-1) and Cr (100μML-1) stress for 7 days. Results indicated that ME pretreatment significantly improved plant fresh weight (47.16/30.47%) and dry weight (62.5/71.42%) of muskmelon plant under Cd/Cr stress, respectively, as compared to only Cd/Cr-treated plants. ME treatment also had a positive effect on photosynthesis and leaf gas exchange properties, which reflect the improvement of photosynthetic efficiency. Moreover, ME pretreatment enhanced leaves/root activity of antioxidant and glyoxalase enzymes, such as superoxide dismutase (64.99/61.99% and 51.91/52.57%), peroxidase 63.02/59.71% and 68.07/63.62%), catalase (59.66/63.56% and 54.83/56.08%), and ascorbate peroxidase (56.85/49.12% and 131.73/100.43%), Gly I (71.05/65.38% and 83.52/57.69%) and Gly II (58.82/61.22% and 55.29/58.16%), under Cd and Cr stress, respectively, which may play a role in ROS and MG removal and alleviation of oxidative stress caused by Cd/Cr. TEM analysis indicated that ME treatment was effective in the mitigation of Cd/Cr-induced destruction of the leaf ultrastructure whereas the use of qRT-PCR indicated that ME treatment altered the expression of the chlorophyll biosynthesis, oxidative stress, and antioxidant defense related genes. Furthermore, ME pretreatment showed a significant effect on Cd/Cr build-up in muskmelon plants. Overall, our results highlight that ME pretreatment is a potential, environmentally-friendly, and non-toxic approach to alleviate Cd/Cr toxicity in muskmelon plants.