Effects And Physiological Mechanisms Of Nitric Oxide On Alleviating Cadmium Toxicity In Rice Seedlings

Cadmium (Cd) is one of the most deleterious elements that pollute environment. Cd can be absorbed and accumulated easily by plants. Excess Cd typically causes direct or indirect inhibition of physiological processes, such as respiration, transpiration, photosynthesis, oxidative stress, cell elongation, nitrogen metabolism and mineral nutrition, resulting in growth retardation, leaf chlorosis and low biomass in plants. Cd can be absorbed and accumulated by rice without obvious deleterious symptoms, which gives rise to a threat on human health by food chain. Therefore, high accumulation of Cd in rice is a potential risk for both animals and human. As a crucial gaseous signaling molecule in plants, Nitric oxide (NO) plays a significant role in modulating several physiological and biochemical functions. In recent decades, an increasing number of articles have reported the effect of exogenous NO in alleviating heavy metal toxicity in different plants, although the physiological processes and mechanisms of NO in alleviating heavy metal toxicity are still far from clear. Therefore, it is necessary and meaningful to investigate the mechanisms of NO in alleviating Cd toxicity in rice, it will also provide significant theory basis to control the accumulation of Cd in rice seeds.Japonica rice Xiushui63was used in the studies, a hydroponic experiment was conducted to investigate the effect of exogenous sodium nitroprusside (SNP), a NO donor, on alleviating the toxicity of Cd to rice. In order to elucidate the mechanisms of mitigate effect of NO application on Cd toxicity, growth parameters, reactive oxygen species, photosynthesis, mineral nutrition, ultrastruture and proteomic were researched. The main results were summarized as follows:(1) Low concentrations of SNP (≤0.2mM) could alleviate the toxicity of Cd to rice, high concentrations of SNP deteriorated the toxicity of Cd to rice plants. A hydroponic experiment was conducted to investigate the effects of exogenous sodium nitroprusside (SNP), a NO donor, on growth in rice plants treated with0.1mM Cd and0.2mM Cd. The results concluded that the application of0.005mM SNP significantly alleviated0.1mM Cd-induced inhibition in rice seedlings growth, as plant height, root length, shoot weight and root weight. The application of0.1mM SNP significantly alleviated0.2mM Cd stress in rice growth. There is SNP concentration effect on alleviating Cd stress in rice plants.(2) Exogenous NO significantly reduced the Cd-induced lipid peroxidation, hydrogen peroxide (H2O2) content and ascorbic acid (ASA) content in rice plants. The addition of NO reversed (only partially) the Cd-induced increase in activities of antioxidant enzymes-superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR). The addition of NO with Cd increased catalase (CAT) activity and glutathione (GSH) content. The exogenous application of NO could be advantageous against Cd toxicity, and could confer tolerance to heavy metal stress in rice plants.(3) Exogenous NO significantly increased the chlorophyll content of rice leaves under Cd stress. NO significantly inhibited the decreases of net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), maximum fluorescence (Fm), PSII intrinsic photochemical efficiency (Fv/Fm), quantum yield of PSII (OPSII), and also inhibited the increase of intercellular CO2concentration (Ci). The results indicated that NO could modulate photosynthesis in plant, reduce energy dissipation and increase plant resistance by enhancing photosynthesis.(4) Exogenous NO made chloroplasts become better or in relatively normal shape with parallel pattern of lamellae. Addition of NO with Cd, the number of vacuoles was increased, but with much smaller size, compared with Cd alone treatment. Moreover, more EDG were deposited in vacuoles.(5) Exogenous NO decreased the Cd content in rice leaves, increased Cd content in root roots. NO significantly increased iron (Fe), zinc (Zn), manganese (Mn), copper (Cu) contents in rice shoot and potassium (K), magnesium (Mg), zinc (Zn), copper (Cu) contents in rice root; significantly decreased calcium (Ca) contents in rice roots induced by Cd stress. The results indicated that NO could maintain the mineral nutrition balance to mitigate Cd stress in rice plants.(6) In rice leaves and roots,41proteins were identified under SNP treatment. Phosphoglycerate kinase, Glyceraldehyde-3-phosphate dehydrogenase B, ATP synthase, NADP dependent malic enzyme, and aminomethyl transferase in rice leaves were down-regulated expressed, while monodehydroascorbate reductase, phosphoglycerate mutase and ferredoxin-nitrite reductase were up-regulated expressed. In rice roots,5-methyltetrahydropteroyltriglutamate-homocysteine methyltransferase, heat shock protein, ATP synthase, and sucrose-UDP glucosyltransferase1were up-regulated expressed, Fructokinase II, isoflavone reductase and cysteine conjugate betalyase were down-regulated expressed. The results indicated that these proteins which participated in photosynthesis, carbohydrate metabolism, nitrogen metabolism, oxidative phosphorylation, oxidative stress responses, signal transductions and cell division.