Melatonin Receptor–Mediated Signaling and PGPR‐Induced Secondary Metabolic Defense Dome Confer Cadmium Tolerance in <i>Brassica juncea</i> : Transcriptomic and Docking Insights

Anthropogenic activities have increased cadmium (Cd) accumulation in agricultural soils, severely affecting crop productivity. This study investigated the role of melatonin (Mlt) and a Pseudomonas consortium (PGPR) in modulating secondary metabolism in Brassica juncea L. seedlings under Cd stress. Combined Mlt-PGPR treatment significantly reduced Cd accumulation and regulated the expression of genes involved in Cd transport and detoxification. Enhanced levels of osmolytes and organic acids were observed, along with upregulation of genes associated with osmoprotection and metal chelation. Antioxidant defense was experimentally assessed through enzymatic and non-enzymatic analyses, confirming improved redox homeostasis. FTIR spectroscopy supported treatment-induced biochemical alterations. Transcriptomic profiling revealed activation of melatonin biosynthesis genes and secondary metabolism-related pathways. A structural model of BjCand2, a putative melatonin receptor, and docking analysis suggested its involvement in signaling regulation. Overall, Mlt-PGPR application mitigates Cd toxicity through coordinated receptor-mediated signaling and metabolic reprogramming.