Selenium ameliorates chromium toxicity through modifications in pigment system, antioxidative capacity, osmotic system, and metal chelators in Brassica juncea seedlings

The present study was designed to understand the protective role of Selenium (Se) in Brassica juncea seedlings cultivated under chromium (Cr) stress. The seedlings were raised in Petri dishes containing solutions of Cr (300 μM) and Se (2, 4 and 6 μM) in unary and binary combinations. The 15 days old seedlings were harvested and assessed for pigments, stomatal morphology and stomatal density, total antioxidative capacity, osmolytes and metal chelators. The results showed that the seedlings treated with Cr showed a significant decrease in the contents of chlorophyll (40.95%) and carotenoids (21.38%). Cr application also caused reduction in stomatal density (42.10%) and negatively affected the morphology of stomata. Se supplementation reduced the toxicity of Cr by increasing the pigment contents and improving the morphology of leaves. Relative gene expression of chlorophyllase reduced, while that of phytoene synthase and chalcone synthase enhanced with Se application which supported the above observations. Cr stress increased the levels of lipid- and water-soluble antioxidants by 68.25 and 32.35%, respectively; however, supplementation of Se further increased their contents. Proline, glycine betaine, trehalose, and osmolality increased by 155.19, 49.18, 83.52, and 120.92%, respectively, in seedlings subjected to Cr stress. Further increment in the above parameters was observed by the application of Se to Cr-stressed seedlings. Seedlings treated with 300 μM Cr showed an increase of 27.27% in the content of total thiols, 127.27% in the content of non-protein thiols, and 21.05% in that of protein-bound thiols, and supplementation of Se showed additional an increase by 28.57, 34.00, and 26.08%, respectively, over those in the Cr-treated seedlings. In conclusion, Cr hampered the normal functioning of the B. juncea seedlings and Se application mitigated the negative impact through modulation in the contents of various osmolytes, antioxidants, and other metabolites. Thus, Se application might help the seedlings to withstand stress by strengthening their antioxidant system, osmoregulation, and metal-chelating ability.