Influence Of Sulfur Onarsenic Translocation In Soil-rice System And Possible Mechanisms Under Arsenic Stress

Arsenic （As） is a class one carcinogenic metalloid. In South and Southeast Asia whereAs-contaminated soil of mining and irrigation with As-contaminated groundwater hascaused higher As accumulation in crops. Rice is an important source of inorganic Asfor populations dependent on a rice diet, as a consequence of the highe efficiency ofrice in accumulating As. It is therefore an urgent need to prevent As transport from thepolluted soil to plant root, and to reduce the concentration of As in the crops. Theexploitation of chemisty technology and rice cultivars that can accumulate less As inthe grain are the promising technologies for reducing As threat to human health. Inthis study, sulfur fertilizer was used as a tool to reduce As tranlocation andtransformation in soil and rice tissues. The pot experiments were carried out to studythe influence of sulfur spceciation and dosage on As transport and transformation inthe soil-rice system under arsenat （As（V）） and dimethyarsinic acid （DMA） stress.Furthermore the transcriptomic mechanisms of As metabolism and accumulation wereinvestiganted though solution and pot experiments. Main results are as follows:1. The influence of sulfate supply on As availability of rizosphere was higher than thatof Non-rizosphere. Arsenic availability in soil deceased with the enhancement ofsulfate application. The soluable As in water was also decreased by sulfate supply.The influence of sulfur on As concentration in soil solution were complex whichmight be due to the complicated redox around rice root.Root As concentration and shoot As concentration were equivalent in the sulfateand DMA treatment, which meaned higher translocation efficiency of As from root toshoot. In the lower DMA treatment with S and HS, As concentrations in root andshoot were less than the LS treatment. In the higher DMA treatment, Asconcentrations in rice plant were higher in HS than that in LS.2. In the background As soil with the less sulfate treatment, shoot biomass of rice wasincreased; As concentration in shoot and root was higher than the control without sulfate application. It might be caused by the hormesis of the interaction of sulfate andAs. In the10mg/kg As treatment, As concentration in root, stem, leaves and brownrice were higher than that in the control at maturity stage. More dosage of sulfate wassupplied, less As concentration was detected in husk. No significant difference wasfound between the two sulfate treatments. The results showed that sufficient Sapplication decreased the possibility of As transportation to husk and grains.3. Sulfate deficiency decreased As accumulation in rice root, but increased thetranslocation of As from root to shoot. Sulfate deficiency reduced maximum quantumyield （Fv/Fm）, minimum fluorescence （Fo） and electron transport rate （ETR） of adark-adapted leaf. Compared with low sulphate treatments, significant increases wereobserved in the parameters of rapid light curves （RLCs）, rETRmaxand Ikofphotosystem I （PSI） and photosystem II （PSII） of rice grown in the high sulphatetreatments. Therefore, an adequately high sulphate supply may result in less Astranslocation from root to shoot, and protecting the reaction pathways of PSI and PSIIof rice seedlings grown in higher As contaminated medium.4. The As species that was detected including As（III）, As（V） and DMA andmonomethylarsonous acid （MMA） have not been detectd in rice grains harvestedfrom the background As soil. There were82.09%-87.89%inorganic As in the controlgrains. In the sulfate treatments with arsenate addition, the percent of DMA wasincreased in brown rice. Sulfur application reduced As concentration of rice grain.Therefore, sulfur might decrease As accumulation and toxity in rice grain. The genesinvolved in As transport and metabolism were mainly down-regulated both in Astreatment and As with sulfur treatment. Several As-responsive genes encodingphosphate transporter, aquaporin and citrate transporter were increased in expressionby As treatment. In the20As+120S treatment,4genes encoding ABC transporters（OsABCB6, OsABCG42₂, OsABCG5and OsABCI7₂） were up-regulated and3genes encoding ABC transporters （OsABCA6, OsABCG12and OsABCG36） weredown-regulated comparing with control and the20As treatment. It is noticed toinvestigate the effect of these genes on As accumulation in rice grain in the nearfuture.These results indicated that a limited ability of sulfate to restrict As uptake andtranslocation in the DMA treatments based on a solution culture experiment.