Paraquat toxicity is reduced by nitric oxide in rice leaves

The role of nitric oxide (NO) in plants has been the object of intensive research. In the present work, we evaluated the protective effect of NO against paraquat (PQ) toxicity of rice (Oryza sativa) leaves. PQ toxicity in rice leaves was determined by the decrease of protein content. PQ toxicity of rice leaves was reduced by free radical scavengers such as thiourea, sodium benzoate, ascorbic acid and sodium azide. NO-releasing compounds [N-tertbutyl-alpha-phenylnitrone (PBN), sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1), and ascorbic acid + NaNO2] were effective in reducing PQ toxicity in rice leaves. SIN-1 and ascorbic acid + NaNO2 prevented both the PQ-induced increase in content of malondialdehyde and the PQ-induced decrease in activities of active oxygen species-detoxifying enzymes. The protective effect of SIN-1 or ascorbic acid + NaNO2 on PQ toxicity, PQ-induced lipid peroxidation and PQ-decreased antioxidative enzyme activities was reversed by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a NO-specific scavenger, suggesting that the protective effect of NO-releasing compounds is attributed to NO released. Reduction of PQ toxicity by NO in rice leaves is most likely mediated through increase in antioxidative enzyme activities and decrease in lipid peroxidation.