Investigation of protective role of nitric oxide (NO) in the mitigation of mercury toxicity in maize (Zea mays L.) by physiological, biochemical and molecular approaches

In this research, it was aimed to investigate the effects of nitric oxide (NO), which is an important signal compound in plant antioxidant system by physiological, biochemical and molecular approaches in the improvement of mercury-induced toxicity in maize (Zea mays L., cv. Arifiye-2). For this aim, effect of NO and Hg applications on fresh-dry weight, plant height, relative water content, amount of reactive oxygen species (ROS), lipid peroxidation (LPO) level, activity and isoenzyme profile of antioxidant enzymes, and the contents of non-enzymatic antioxidants, total protein, proline, photosynthetic pigments and the change in gene expression of antioxidant enzymes were evaluated separately and together. It was used 100 M of mercury chloride (HgCl2) as Hg source, and 0.1 M of sodium nitroprusside (SNP) as NO source, according to preliminary experiments realized. Eighth day, the SNP concentration was supplied at root environments of the plants grown in hydroponic medium, then 24 hour after, the plants were exposed to 100 μM HgCl2 stress for 3 days and the samples of root and leaf of the harvested plants were used as research material. Maize roots accumulated more Hg than leaves and as a result of this effects of the toxicity appeared more prominently in roots. In roots and leaves, Hg application alone significantly decreased activity of antioxidant enzymes (superoxide dismutase, catalase, peroxidase and glutathione reductase), level of nonenzymatic metabolites (glutathione and ascorbate), amount of essential elements, and the contents of soluble protein, chlorophyll (in leaf) and proline while increased the levels of ROS and LPO (as MDA). SNP applied to the seedlings before from Hg stress decreased ROS and LPO levels that were increased by the Hg application in the both tissues, in contrast, increased the activities of antioxidant enzymes and the amount of nonenzymatic antioxidants. This finding was supported by the work of identifying isoenzymes profiling of the antioxidant enzymes. In addition, SNP application before from Hg stress caused improvements in physiological parameters, and increased the levels of protein, proline, and chlorophyll as compared to control. It also significantly affected the expression rates of 5 genes encoding antioxidant enzymes. In result, it has been determined that NO applied before exposure to mercury stress in maize plant provides an important contribution to increase plant tolerance by regulating the response mechanisms at physiological, biochemical and gene level.