Modulation of antioxidant machinery in α-tocopherol-enriched transgenic Brassica juncea plants tolerant to abiotic stress conditions

The antioxidant machinery in plants consists of several components with unique or overlapping functions that combat the deleterious production of reactive oxygen species (ROS) induced by stress conditions. Tocopherols are a group of powerful antioxidants having additional roles in signaling and gene expression, with alpha-tocopherol being the most potent form. In the present study, we used wild-type (WT) and alpha-tocopherol-enriched transgenic (TR) Brassica juncea plants grown under salt, heavy metal, and osmotic stress to compare their relative tolerance to these stresses and to assess the effects of increased alpha-tocopherol content on the other antioxidative enzymes and molecules. The oxidative damage caused by induced stress was lower in TR plants compared to WT plants as assessed by their higher relative water content and lower electrolyte leakage, malondialdehyde content as well as H2O2 accumulation. Lesser superoxide and H2O2 accumulation was also observed by histochemical staining in TR seedlings exposed to stress. Though no significant differences were evident under normal growth conditions, TR plants showed higher activities and transcript levels of antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase than WT plants under similar stress conditions. A decrease in ascorbate and glutathione content with marginally higher reductive ratios of these compounds was also observed in TR plants under the stress conditions. Our findings implicate the role of higher alpha-tocopherol levels in conferring better tolerance against salt, heavy metal, and osmotic stresses and also establish the existence of interplay between this lipid-soluble antioxidant and other water-soluble components of plant antioxidant defense.