Melatonin‐Mediated Vacuolar Compartmentalization in <scp><i>Solanum nigrum</i></scp> L. Under Cadmium Stress

ABSTRACT Melatonin has been identified as a potential mitigator of cadmium (Cd) stress in plants, though its precise cellular mechanisms remain poorly understood, particularly regarding its role in detoxification processes within the protoplast and vacuole. This study examines the effects of exogenous melatonin on Cd concentration and vacuolar compartmentalization in the leaf cells of Solanum nigrum L., a recognized hyperaccumulator plant. Employing subcellular separation techniques, we quantified levels of total Cd, Cd 2+ , ascorbic acid–glutathione (AsA‐GSH), and reactive oxygen species (ROS) in the protoplasts and vacuoles of leaf cells. Results reveal that vacuolar compartmentalization serves as a dynamic mechanism, balancing strategic sequestration and continuous metabolic activity in the cytoplasm, with 72.19%–79.89% of the Cd retained within the vacuole. Melatonin enhanced the effective allocation of these resources, contributing to the preservation of cellular homeostasis. In particular, outside the vacuole, melatonin treatment maintained relatively higher levels of glutathione/oxidized glutathione (GSH/GSSG) and ascorbic acid/dehydroascorbate (AsA/DHA) under 50 μM Cd stress. At this concentration, the AsA/DHA ratio increased from 0.71 to 2.20, and the GSH/GSSG ratio increased from 2.20 to 4.28 outside the vacuole, in comparison to treatment with Cd alone. These findings enhance our understanding of melatonin‐mediated cellular tolerance mechanisms, offer insights for improving plant Cd stress tolerance, and could inform genetic strategies for developing more efficient hyperaccumulator plants.