Nitric oxide ameliorates aluminium toxicity in Anabaena PCC 7120: Regulation of aluminium accumulation, exopolysaccharides secretion, photosynthesis and oxidative stress markers

Higher amount of aluminium (Al) is a general feature of acidic soils which limits crop productivity. Cyanobacteria are also found on acidic soils where they encounter Al toxicity which can affect paddy productivity directly (due to reduction in nitrogen fixation) and indirectly (due to reduction in their biomass). In the past decade, tremendous progress in establishing the role of nitric oxide (NO) in regulating/ameliorating metal stress including Al has been achieved in higher plants. Studies on role of NO in regulating metal stress in cyanobacteria, however to our knowledge, are still scant. Therefore, in the present study, we have investigated probable role of NO in the regulation of Al stress in the cyanobacterium Anabaena PCC 7120. Aluminium (50 mu M) declined growth (30%) of Anabaena PCC 7120 which was accompanied by the increase in intracellular accumulation of Al and decrease (19%) in exopolysaccharides (EPS), photosynthetic pigments and photosynthesis. However, exogenous addition of SNP reverses negative impacts of Al. Aluminium enhanced oxidative stress markers i.e. superoxide radical (SOR, 55%), hydrogen peroxide (H2O2, 49%), lipid peroxidation (as malondlaldehyde equivalents, 35%) and membrane damage (as evidenced by in vitro as well as in vivo analysis) and antioxidant defense system i.e. activities of superoxide dismutase (SOD), catalase (CAT) and glutathione-Stransferase (GST) and decreased the contents of cysteine, proline and non-protein thiols. But SNP declined considerably Al-induced levels of oxidative stress markers by stimulating antioxidant defense system. The results also showed that NG-nitro-t-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase) worsened negative impacts of Al while 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO, a scavenger of NO) reverses SNP-mediated amelioration of Al stress. These results clearly show that NO has a role in ameliorating Al stress in Anabaena PCC 7120 by regulating intracellular accumulation of Al, secretion of EPS, photosynthesis, oxidative stress markers and antioxidant defense system.