Pre-treatment of roots with melatonin triggers a key tolerance mechanism underlying cadmium stress in cotton seedlings

Soil contamination with cadmium (Cd) negatively affects crop productivity and human health. Melatonin, a multifunctional plant/animal origin molecule maintains plant growth under abiotic stressors. However, the mitigating role of melatonin especially where the roots are main site for pre-treatment in Cd stress tolerance and its related process has not been explored yet. This study explored physiochemical and molecular mechanisms of melatonin induced Cd stress tolerance and detoxification in cotton seedlings. Cotton seedlings, with or without melatonin (15 µM) pre-treatment, were subjected to Cd (100 µM) stress in a hydroponic medium. Results showed that cotton seedlings under Cd stress significanlty inhibited phyto-hormonal, nutrient uptake and leaf photosynthetic capacity which was strongly associated with the Cd induced injury of root morphological traits. Melatonin pre-treated plants under Cd stress enhanced root length, root volume, root density, root diameter, root surface area, root tips and root forks number comapred with Cd stress. Better root growth of melatonin pre-treated seedlings under Cd stress further enhanced uptake of nitrogen (N) phosphorus (P) and potassium (K) by 45 %, 21 %, and 26 %, respectively, over Cd treatment alone. Melatonin pre-treated plants had 36 %, 27 % and 20 % higher salicylic acid, indole-3-acetic acid and gibberellic acid contents, respectively with a reduced level of abscisic acid under Cd stress. Melatonin pre-treated plants increased antioxidant enzyme activities, ascorbate, and glutathione contents which further enhanced leaf maximum quantum yield of PSII, quantum yield, electron transport rate and photochemical quenching under Cd stress. DEGs involved in chloroplasts and mitochondria regulated carbohydrate and energy metabolism (ripA and IDH1), cellular components and biological processes of melatonin treated plants under Cd stress.Melatonin pre-treated seedlings upregulated transcription factors (AP2-EREBP,NAC,bHLH, WRKY, and Trihelix) under Cd stress. Melatonin enhanced Cd stress tolerance by reducing Cd transport from root to leaves with higher antioxidant enzyme activities and gene expression. In conclusion, this study provides novel insights into the melatonin-mediated Cd stress response and ensures crop production in Cd-contaminated soils.