Effect of exogenous nitric oxide on antioxidative system and S-nitrosylation in leaves of Boehmeria nivea (L.) Gaud under cadmium stress

Cadmium (Cd)-induced growth inhibition is one of the primary factors limiting phytoremediation effect of Boehmeria nivea (L.) Gaud in contaminated soil. Sodium nitroprusside (SNP), a donor of nitric oxide (NO), has been evidenced to alleviate Cd toxicity in many plants. However, as an important mechanism of NO in orchestrating cellular functions, S-nitrosylation is still poorly understood in its relation with Cd tolerance of plants. In this study, higher exogenous NO levels were found to coincide with higher S-nitrosylation level expressed as content of S-nitrosothiols (SNO). The addition of low concentration (100 mu M) SNP increased the SNO content, and it simultaneously induced an alleviating effect against Cd toxicity by enhancing the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR) and reduced the accumulation of H2O2 as compared with Cd alone. Application of S-nitrosoglutathione reductase (GSNOR) inhibitors dodecanoic acid (DA) in 100 mu M SNP group brought in an extra elevation in S-nitrosylation level and further reinforced the effect of SNP. While the additions of 400 mu M SNP and 400 mu M SNP + 50 mu M DA further elevated the S-nitrosylation level, it markedly weakened the alleviating effect against Cd toxicity as compared with the addition of 100 mu M SNP. This phenomenon could be owing to excess consumption of glutathione (GSH) to form SNO under high S-nitrosylation level. Therefore, the present study indicates that S-nitrosylation is involved in the ameliorating effect of SNP against Cd toxicity. This involvement exhibited a concentration-dependent property.