Exogenous application of hydrogen sulfide reduces chromium toxicity in maize seedlings by suppressing NADPH oxidase activities and methylglyoxal accumulation

Chromium (Cr) represents an important source of metallic stress in plants. Working with maize (Zea mays) seedlings, we characterize the suppressive effects of exogenously applied NaHS (a hydrogen sulfide; [H2S] donor) on the toxic effects of Cr (VI). Heavy metal treatment reduced radicle and epicotyl lengths and fresh weights in seedlings at 6 and 9 days following germination. The negative Cr (200 mu M) effect was countered by application with NaHS (500 mu M) but this countering was reduced with the co-application of the H2S generation inhibitor hydroxylamine (HA) or the H2S scavenger hypotaurine (HT). The Cr-elicited H2O2 production was suppressed by NaHS and also by an inhibitor of the reactive oxygen species (ROS) generating NADPH oxidase (NOX). These effects were correlated with relative changes in carbomyl (-CO) and thiol (-SH) groups. Nitric oxide (NO) production increased by NaHS application with associated increase in S-nitrosoglutathione (GSNO) level, but low S-nitrosoglutathione reductase (GSNOR) activities indicating an elevated S-nitrosylation. Assessment of the role of the ascorbate-glutathione antioxidant cycle indicated that whilst ascorbate played at a best minor role, glutathione was more prominent. Methylglyoxal (MG) production was increased by Cr but reduced by NaHS through a mechanism which could be based on glutathione-S-transferase (GST) detoxification. Taken together data suggest that H2S acts to counter Cr effect in maize by NOX suppression, mostly likely by the well-characterised S-nitrosylation mechanism, as well as a reduction of MG accumulation.