Exogenous nitric oxide mediates alleviation of mercury toxicity by promoting auxin transport in roots or preventing oxidative stress in leaves of rice seedlings

Nitric oxide (NO), a multifunctional gaseous molecule, mediates a variety of responses to biotic and abiotic stresses. The effects of exogenous NO on rice (Oryza sativa cv. 'Zhonghua 11') growth under mercuric chloride (HgC12) stress were investigated. The results showed that 60 1..1M Hg significantly inhibited the root elongation of rice plantlets after seed germination. While 100 1..1M or 200 1..1M sodium nitroprusside (SNP, a donor of NO) could increase the root length by attenuating the effects of 2,3,5-triiodobenzoic acid (TIBA) and Hg, which indicated the role of NO in auxin transport-promoting in roots. On the other hand, SNP decreased the absorption and transportation of Hg in roots and shoots of rice seedlings at five-leaf stage. Moreover, the levels of superoxide radical (02) and hydrogen peroxide (H202) in leaves were also decreased significantly. However, the activities of antioxidant enzymes were not enhanced by SNP. Moreover, NO promoted the growth of rice plantlets under Hg stress even when superoxide dismutase (SOD, EC 1.15.1.1) or catalase (CAT, 1.11.1.6) activity was inhibited by diethyldithiocarbamate (DDC, an inhibitor of SOD) or 3-amino-1,2,4triazole (AT, an inhibitor of catalase), respectively. These results confirmed that NO could act as the direct quencher of O-2(-) and then prevent the oxidative damage caused by Hg ion in leaves.