Melatonin improves nutrient uptake and confers Cd tolerance in mung bean via Cd detoxification and redox regulation

Cadmium (Cd) contamination in agricultural soils severely threatens crop productivity and food safety. Mung bean (Vigna radiata), a globally important legume, is highly susceptible to Cd toxicity, which disrupts nutrient homeostasis, photosynthesis, and redox balance. Melatonin (Mel), a pleiotropic signaling molecule, has emerged as a potential mitigator of abiotic stresses, but its mechanisms of Cd tolerance in mung bean remain poorly understood. Here, we demonstrate that Mel alleviates Cd toxicity by: (1) facilitating internal Cd sequestration by promoting its deposition in the leaf cell wall and vacuolar compartmentalization, a process mediated by the upregulation of key transporter genes PCR2 (Plant Cadmium Resistance 2) and HMA (Heavy Metal ATPase); (2) restricting Cd entry into the plant by downregulating the expression of root metal influx transporters, namely ZIP1 (Zinc-regulated, Iron-regulated Transporter-like Protein 1) and YSL3 (Yellow Stripe-Like 3); and (3) counteracting Cd-induced nutritional deficits achieved through the activation of plasma membrane H⁺-ATPase and the enhancement of the nitrate reductase (NR)/glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. Concurrently, Mel treatment robustly improved redox homeostasis by significantly boosting the activities of major antioxidant enzymes (SOD; CAT; APX) and stimulating the biosynthesis of phytochelatins for Cd chelation. Genome-wide transcriptomic profiling further corroborated these physiological observations, revealing Mel's targeted regulation of a comprehensive gene network implicated in metal trafficking, cell wall fortification, and reactive oxygen species (ROS) scavenging. Overall, this work provides novel insights into the mechanistic basis of Mel-induced Cd tolerance and underscores its potential as an effective agro-biotechnological strategy for safeguarding crop plants in metal-contaminated environments.