Exogenous melatonin mitigates vanadium toxicity in Brassica napus L.: a transcriptomic perspective

Brassica napus L. is one of the world's most important edible oil crops, and its productivity is severely affected by abiotic stresses, including heavy metal contamination. Vanadium (V) toxicity disrupts plant growth, impairs photosynthesis, and induces excessive reactive oxygen species (ROS) accumulation. Melatonin (MT) is an emerging plant regulatory molecule known to enhance stress tolerance; however, its physiological and transcriptomic roles in mitigating V-induced toxicity in B. napus remain poorly understood.

In this study, B. napus seedlings were subjected to four treatments: control (CK), MT (100 µM), V (100 mg L⁻¹), and MT + V (MTV). V stress significantly inhibited seedling growth, biomass accumulation, chlorophyll content, chlorophyll fluorescence (Fv/Fm), and water status, while increasing oxidative damage and ROS accumulation. MT supplementation markedly alleviated V-induced phytotoxicity by improving growth performance, chlorophyll accumulation, antioxidant enzyme activities (POD, CAT, and APX), and ROS scavenging capacity. MT treatment also reduced overall V accumulation in seedlings under V stress conditions. Transcriptomic analysis identified numerous differentially expressed genes (DEGs) associated with stress responses, antioxidant regulation, photosynthesis, and phenylpropanoid biosynthesis. Weighted Gene Co-expression Network Analysis (WGCNA) identified four key co-expression modules (turquoise, blue, brown, and yellow) strongly associated with MT-mediated stress responses. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed significant enrichment of genes involved in phenylpropanoid biosynthesis, glutathione metabolism, antioxidant defense, and transporter-related pathways.

Our findings demonstrate that MT alleviates V-induced toxicity in B. napus through coordinated physiological and transcriptomic regulation involving antioxidant defense, photosynthetic protection, and stress-responsive metabolic pathways. This study provides new insight into the molecular mechanisms underlying MT-mediated heavy metal stress tolerance and highlights the potential application of MT for improving crop resilience under vanadium-contaminated conditions.