Pre treatment of melatonin rescues cotton seedlings from cadmium toxicity by regulating key physio-biochemical and molecular pathways

Melatonin, a plant/animal origin hormone, regulates plant response to abiotic stresses by protecting them from oxidative damage. This study identified physiochemical and molecular mechanism of melatonin-induced cad-mium (Cd) stress tolerance and detoxification in cotton seedlings. Cotton seedlings, with or without melatonin (15 mu M) pretreatment, were subjected to Cd (100 mu M) stress in a hydroponic medium for eight days. We found that higher cellular Cd accumulation in leaf tissues significantly inhibited the growth and physiology of cotton seedlings. In contrast, melatonin-treated seedlings maintained leaf photosynthetic capacity, producing relatively higher fresh (17.4%) and dry (19.3%) weights than non-melatonin-treated plants under Cd-contaminated en-vironments. The improved growth and leaf functioning were strongly linked with the melatonin-induced repression of Cd transporter genes (LOC107894197, LOC107955631, LOC107899273) in roots. Thus, melatonin induced downregulation of the Cd transporter genes further inhibited Cd ion transport towards leaf tissues. This suggests that the differentially expressed transporter genes (DEG) are key drivers of the melatonin-mediated regulation of Cd transportation and sequestration in cotton. Melatonin also protected cotton seedlings from Cd-induced oxidative injury by reducing tissues malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels and increasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) enzymes. Transcriptomic analysis revealed that melatonin activated mitogen-activated protein kinase (MAPK) signaling pathways to simulate stomatal adjustment and photosynthesis in Cd-stressed leaves. Further, melatonin protects intercellular organs, particularly ribosomes, from Cd-induced oxidative damage by promoting ribosomal biosynthesis and improving translational efficiency. The findings elucidated the molecular basis of melatonin-mediated Cd stress tolerance in plants and provided a key for the effective strategy of Cd accumulation in cotton.

Automated summary

Melatonin, a hormone found in plants and animals, plays a role in protecting plants from oxidative damage caused by abiotic stress. This study examined the physiological and molecular mechanisms of melatonin-induced cadmium (Cd) stress tolerance and detoxification in cotton seedlings. The presence of melatonin in seedlings subjected to Cd stress helped maintain leaf photosynthetic capacity and improved growth and leaf functioning. The downregulation of Cd transporter genes in roots further inhibited the transport of Cd ions to leaf tissues. Melatonin also protected cotton seedlings from Cd-induced oxidative injury and activated signaling pathways related to stomatal adjustment and photosynthesis. These findings provide insights into the molecular basis of melatonin-mediated Cd stress tolerance and its potential application in crop improvement.