Melatonin Mitigates Cadmium Toxicity by Promoting Root Architecture and Mineral Homeostasis of Tomato Genotypes

Crop production has been extensively affected globally due to heavy metal pollution. Cadmium (Cd) toxicity is a major abiotic factor that reduces crop yield via reducing plant growth, nutrient losses, and root morphological changes. Melatonin (ME) is a well-studied strong antioxidant and nontoxic biomolecule, enhancing plants’ tolerance to environmental stresses. In this study, we examined the response of tomato genotypes to Cd toxicity and the potential functions of ME in improving Cd stress tolerance. Roots of three tomato genotypes (Fenli, Badli2, and Shidun 197) were pretreated with ME (100 µM) for 3 days, followed by application of Cd (35 μM) for one week. The treatments were as follows: (1) CK (control); (2) Cd treatment; and (3) ME + Cd treatment. Cd was applied in the form of CdCl2. The results revealed that seedlings with ME pretreatment (100 µM for 3 days) exposed to Cd (35 µM for 7 days) had better plant growth and root activity. ME supplementation significantly enhanced root architecture and boosted mineral nutrient uptake. Moreover, pretreatment of ME reduced leaf Cd levels by decreasing root-to-shoot transport of Cd. In conclusion, ME application modifies the root growth pattern as well as increases mineral nutrient homeostasis in tomato genotypes under Cd stress. Our results showed that ME reduced Cd-induced phytotoxicity in all tomato genotypes by enhancing biomass production, root system architecture, and macro and micro nutrient uptake by decreasing Cd uptake in all tomato genotypes.