Biopriming With 28-Homobrassinolide and Piriformospora indica Mitigates Cadmium Toxicity in Brassica juncea

Cadmium (Cd) is a toxic heavy metal that severely disrupts plant growth, induces oxidative stress and disruption of physiological processes in plants. In this study, we evaluated the potential of two biostimulants, 28-homobrassinolide (28-HBL), a brassinosteroid, and the root endophytic fungus Piriformospora indica, to alleviate Cd-induced stress in Brassica juncea. seedlings. Exposure to Cd significantly impaired seedling morphology and elevated reactive oxygen species (ROS) levels, while biopriming with 28-HBL and root inoculation with P. indica markedly improved morphology and stress resilience. Enzymatic antioxidants (SOD, CAT, APOX, DHAR, MDHAR) and non-enzymatic antioxidants (GSH, phenolics, flavonoids) effectively attenuated ROS levels, with the combined treatment showing the strongest impact. These biochemical enhancements were supported by transcriptional upregulation of antioxidant defense genes. In addition, osmolyte accumulation (proline, glycine betaine [GB]) supported cellular homeostasis under stress. Atomic absorption spectroscopy confirmed reduced Cd uptake in treated seedlings. Notably, P. indica colonization upregulated key brassinosteroid signaling genes (BRI1, BAK1, BES1, and BZR1), suggesting a hormone-mediated mechanism of stress mitigation. The findings highlight a promising synergy between 28-HBL and P. indica, offering an eco-friendly strategy to enhance plant tolerance to Cd stress.