24-Epibrassinolide Regulates Functional Components of Nitric Oxide Signalling and Antioxidant Defense Pathways to Alleviate Salinity Stress in Brassica juncea L. cv. Varuna

The present study investigates a possible mechanism of 24-Epibrassinolide (EBL) mediated alleviation of salinity stress in Brassica juncea L. seedlings. EBL (10–6 M and 10–8 M) pretreated seedlings were grown up to ten-days either in alone and/or combination with defined levels of salinity treatments (100 mM and 200 mM NaCl) under laboratory growth conditions. Results showed that salinity stress significantly (P ≤ 0.05) increased the Na+ accumulation in a dose-dependent manner, which aggravated the nutrient(s) homeostasis and pigment contents by inducing oxidative damage through elevated hydrogen peroxide, malondialdehyde, and methyl glyoxal production. EBL mediated alleviation of salinity stress was achieved through decreased nuclear damage and cell death, coupled with increased membrane stability index and nutrients accumulation. The positive roles of EBL were also reflected through maintenance of AsA/DHA and GSH/GSSG redox ratios’ that were maximally enhanced by 78% and 28%, respectively under 200 mM NaCl. EBL mediated up-regulation of γ-glutamyl cysteine synthetase activity increased the glutathione pool, which boosted the ascorbate–glutathione cycle and methyl glyoxal detoxification system. The nitrate reductase activity, endogenous nitric oxide and S-nitrosothiols pools were maximally increased by 45, 32, and 33% respectively in 200 mM NaCl treatment, over the salinity alone counterpart. Taken together, our findings suggested that EBL pretreatment alleviates salinity stress by regulating nitric oxide homeostasis, osmolytes synthesis, antioxidant defense systems, and increasing the nutrients and pigments contents in mustard seedlings under salinity stress. Therefore, seed-priming with EBL can be considered as an effective approach for improving the plant productivity under salinity stress.