24-Epibrassinolide effectively alleviates UV-B stress-induced damage in the cyanobacterium Anabaena sp. PCC 7120 by employing nitric oxide: Improved PS II photochemistry, antioxidant system, and growth.

The alleviation of UV-B stress-induced damage by 24-epibrassinolide (EBL) in the cyanobacterium Anabaena sp. PCC 7120 and the role of nitric oxide (NO) signaling were investigated in this study. UV-B exposure caused a significant (P < 0.05) decline by 31% in the growth of test organism by negatively impacting photosynthetic pigments, whole cell O2 evolution, and PS II photochemistry [decrease in the values of Fv/F0, PIABS, and quantum efficiencies (Phi_Po, Phi_Eo, and Psi_0) and increase in specific energy fluxes (ABS, TR0, DI0, and ET0) per active RCs]. Stress enhanced the levels of oxidative stress biomarkers (superoxide radicals, hydrogen peroxide, and malondialdehyde equivalents contents), as evidenced by in-vivo and in-vitro analyses, and increased respiratory O2 consumption. The supplementation of EBL (0.5 nM) alone, NO (sodium nitroprusside as NO donor; 10 μM), and EBL + NO significantly (P < 0.05) alleviated the decline in growth with a reduction remaining 19%, 14%, and 8%, respectively, under UV-B stress. The exogenous supplementation of EBL and NO boosted the enzymatic antioxidant system (superoxide dismutase, peroxidase, catalase, and glutathione-S-transferase activity). However, the addition of NO scavenger (PTIO; 20 μM)] and its biosynthetic inhibitor (L-NAME; 100 μM) reversed the effect, thereby suggesting that EBL alleviates UV-B toxicity by involving NO in the N2-fixing cyanobacterium Anabaena sp. PCC 7120.