Interaction of 24-epibrassinolide and salicylic acid regulates pigment contents, antioxidative defense responses, and gene expression in Brassica juncea L. seedlings under Pb stress

Lead (Pb) is considered one the most hazardous pollutant, and its accumulation in soil and plants is of prime concern. To understand the role of plant hormones in combating heavy metal stress, the present study was planned to assess the interactive effects of 24-epibrassinolide (EBL) (10(-7) M) and salicylic acid (SA) (1 mM) in regulating growth, pigment contents, antioxidative defense response, and gene expression in Brassica juncea L. seedlings exposed to different concentrations of Pb metal (0.25, 0.50, and 0.75 mM). Reduction in root and shoot lengths, chlorophyll and carotenoid content, and non-enzymatic antioxidants like glutathione, ascorbic acid, and tocopherol in response to Pb toxicity was observed. The enzymatic antioxidants such as guaiacol peroxidase (POD), ascorbate peroxidase (APOX), glutathione peroxidase (GR), dehydroascorbate reductase (DHAR), monodehydroascorbate redductase (MDHAR), glutathione-S-transferease (GST), and glutathione peroxidase (GPOX) were lowered in response to Pb treatments. Other antioxidative enzymes including superoxide dismutase (SOD), catalase (CAT), and polyphenol oxidase (PPO) enhanced under metal stress. Co-application of EBL + SA to 0.75 mM Pb-treated seedlings resulted in improvement of root and shoot lengths, chlorophyll, and carotenoid contents. Similarly, glutathione, ascorbic acid, and tocopherol contents were also elevated. Enzymatic antioxidants were also significantly enhanced in response to pre-sowing combined treatment of both hormones. Gene expression analysis suggested elevation in expression of CAT, POD, GR, DHAR, and GST genes by application of EBL. Our results reveal that Pb metal toxicity caused adverse impact on B. juncea L. seedlings, but pre-soaking treatment with EBL and SA individually and in combination help seedlings to counter the ill effects of Pb by improving growth, contents of pigment, and modulation of antioxidative defense system. The combined application of EBL and SA was found to be more effective in ameliorating Pb stress as compared to their individual treatments.