Exogenous Nitric Oxide Confers Tolerance to Cr(VI) in Maize (Zea mays L.) Seedlings by Modulating Endogenous Oxido-Nitrosative Events

The use of exogenous compounds such as ‘gasotransmitter’ molecules is a well-established agronomic strategy to improve the crop tolerance to environmental stresses. In this current work, when Cr (200 µM) was combined with the nitric oxide (NO) generator sodium nitroprusside SNP, 500 µM) there was a suppression of metal-induced alterations in embryo growth. Exogenous NO produced by SNP reduced the accumulation of toxic hydrogen peroxide and methylglyoxal and stress-linked proline in Cr-treated seedlings. Chromium increased thiol and S-nitrosothiol levels but this was restored to control levels by SNP, in spite of a competing NO reaction leading to increase of S-nitrosoglutathione content. However, added complexity was indicated by addition of arginine analogue N (ω)-nitro-L-arginine methyl ester (_L-NAME, 500 µM) in the germinating medium to suppress endogenous NO production. This suppressed endogenous NO production but superoxide dismutase (SOD) was suppressed not enhanced in Cr + _L-NAME treatments. In addition, Cr + _L-NAME significantly decreased the content of spermidine and spermine in epicotyls as compared to Cr treatment alone. Similarly, exposure to Cr + SNP decreased spermidine and spermine levels in both radicles and epicotyls. This is important as polyamines have been suggested as route for NO production. Thus, our observations suggest that exogenous NO mitigates Cr-induced damage and confers seedling tolerance to Cr through suppression of NADPH oxidase activity and increased GSNO contents. This may act to prevent an excess of methylglyoxal and hydrogen peroxide. However, the reduction in polyamine mediated cellular NO generation could also promote increased viability under Cr stress.