Title : Synergistic biopriming with 28-homobrassinolide and Piriformospora indica enhances cadmium stress tolerance
Abstract:
This study investigates a synergistic biopriming strategy combining seed priming with 28-homobrassinolide (28-HBL) and root colonization by the endophytic fungus Piriformospora indica to alleviate cadmium (Cd) toxicity in Brassica juncea. Seedlings exposed to 50 µM Cd under controlled laboratory conditions exhibited substantial accumulation of reactive oxygen species (O-2, H2O2, and •OH), leading to oxidative stress and significant impairment of plant growth. The integrated biopriming approach provided protection through two coordinated physiological and molecular mechanisms. The first mechanism involved restoration of cellular redox homeostasis, supported by both biochemical assays and transcriptional regulation of antioxidant machinery. This was evidenced by enhanced activities and expression of key enzymatic antioxidants including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APOX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR). In parallel, increased accumulation of non-enzymatic antioxidants such as glutathione, phenolics, and flavonoids further strengthened the cellular defense system. Elevated levels of osmolytes, including proline and glycine betaine, contributed to osmotic adjustment and stabilization of cellular structures under stress conditions. The second protective mechanism involved efficient cadmium detoxification through enhanced metal sequestration and chelation. Transcriptomic analysis revealed modulation of NRAMP and HMA transporter family genes in both roots and shoots, indicating regulation of Cd uptake and its translocation within plant tissues. This was accompanied by enhanced phytochelation responses, facilitating intracellular metal binding and detoxification. Quantification using atomic absorption spectroscopy confirmed reduced Cd accumulation in bioprimed seedlings. Furthermore, the synergistic interaction between P. indica and 28-HBL was validated through upregulation of brassinosteroid signaling genes (BRI1, BAK1, BES1, and BZR1), highlighting hormone-mediated regulation of stress tolerance pathways. Overall, the combined treatment was more effective than individual applications, demonstrating coordinated regulation of antioxidant defense, metal transport, and detoxification processes. These findings underscore the potential of hormone–microbe integration as a sustainable strategy for enhancing plant tolerance to heavy metal stress in crop systems.
