Integrated Transcriptomic and Physio-Biochemical Insights into Role of Melatonin in Alleviating Vanadium-Induced Phytotoxicity in <i>Brassica Napus</i> L.

Brassica napus L. (B. napus) is well known edible oil crop globally threatened by heavy metal pollution, particularly vanadium (V), which severely damaged plant growth, biomass accumulation, photosynthetic efficiency, and antioxidant defense by inducing oxidative stress. Melatonin (MT), a multifunctional phytohormone, has emerged as a potent regulator of plant responses to abiotic stress; however, its role in mitigating V-induced phytotoxicity remains largely unexplored in B. napus. In this study, we evaluated the physio-biochemical and transcriptomic responses of B. napus seedlings under V stress (100 mg L⁻¹) with or without MT supplementation (100 µM). MT treatment significantly reduced V uptake (~66.79%) and alleviated V-induced phenotypic and physiological damages, restoring biomass, water content, and photosynthetic pigment accumulation. Notably, MT enhanced chlorophyll fluorescence, gas exchange parameters, and ROS-scavenging by up-regulating antioxidant enzymes (SOD, POD, CAT, APX) and their corresponding genes. Transcriptomic analysis identified 1767 deferentially expressed genes (DEGs), with MT reversing V-down-regulated gene expression and promoting stress-responsive pathways. Weighted Gene Co-expression Network Analysis (WGCNA) revealed four key modules (turquoise, blue, brown, yellow), where gene expression was predominantly up-regulated in MT+V treatments. GO and KEGG enrichment highlighted MT-induced activation of pathways related to abiotic stress responses, including phenylpropanoid biosynthesis, flavonoid metabolism, and ROS detoxification. Notably, genes involved in flavonoid (PAL, FLS, CYP73A), lignin (PODs, CAD, COMT), and sinapine biosynthesis were significantly up-regulated by MT, correlating with reduced H₂O₂ accumulation and enhanced stress resilience. This study provides novel insights into the molecular and physiological mechanisms by which MT mitigates V toxicity in B. napus, underscoring its potential application in enhancing heavy metal stress tolerance in crops through sustainable bio-stimulant strategies.