Arsenic And Antimony Uptake And Speciation Transformation In Rice

Arsenic(As)and antimony(Sb)are among the most toxic elements to plants and animals.As and Sb contamination in the environment occurs in many regions and depends on environmental factors.Their accumulation in food crops may pose a health risk to humans.Rice can take up and translocate more As and Sb than other cereal crops due to flooded paddy cultivation that leads to arsenite and antimonite mobilization.Recent progresses in understanding the mechanisms of As uptake and metabolism in rice plants are well-studied.However,the issue that AsⅢ was oxidized easily to AsⅤ in media during exposure,and the As speciation during the process are relatively ignored.Another issue was that As uptake and toxicity were rarely studied rarely under co-contamination such as Hg and Sb,which was ubiquitous in natural environment.Sb and As are chemical analogs,but the mechanisms of Sb uptake,speciation transformation and other metabolism processes are not fully elucidated so far.In this work,a series of hydroponic experiment was conducted to investigate As and Sb metabolism mechanisms in rice plants.The main results were as follows:(1)A hydroponic experiment was conducted to investigate their interactive effects on their uptake,translocation,speciation transformation and toxicity in rice seeding after exposed to As Ⅲ and Hg.Rice was much more effective in taking up Hg than As and sequestered both in the roots.As and Hg reached 339 and 433 mg/kg in the roots.But it was poor in translocation from roots to shoots and TF were 0.14-0.87 and 0.012-0.038 only.Arsenite(54 to 100%)and inorganic Hg(100%)were the predominant form in the plant based on HPLC-ICP-MS.Though Hg inhibited As uptake,translocation and transformation,it enhanced toxicity to rice comparing to single exposure.Root cell structural damage and organelles number reduction were observed based on TEM and malondialdehyde(MDA)in the roots and shoots increased with increasing As and Hg concentrations.(2)As speciation in nutrient solution,roots and shoots were monitored in short-time exposure.Arsenite was oxidized to arsenate little,whereas arsenate remained stable during 10 h exposure.In root cells arsenate is rapidly reduced to arsenite accounting for 52.6%after 1 h.Both arsenite and arsenate were detected in the efflux solution,but the amount of the AsⅢ was about 9 times higher than AsⅤ.As was effluxed from rice roots accounted 24.6-48.3%of total As.(3)A hydroponic experiment was conducted to investigate uptake,translocation,speciation,efflux and subcellular distribution of Sb in rice plants exposed to antimonite(SbⅢ)and antimonate(SbⅤ).More Sb was accumulated in iron plaque than the plant,with both the roots(～10-12 times)and Fe plaque(～28-54 times)sequestering more SbⅢ than SbⅤ.SbⅢ and Sbv uptake and efflux kinetics were fitted well using Michaelis-Menten function.Antimonate(56 to 98%)was the predominant form in rice plant with little methylated species being detected using HPLC-ICP-MS.Cell walls accumulated more Sb than organelles and cytosol,which was considered as the first barrier of Sb entering into cells.About 70%Sb of total efflux during 48 h efflux period.The study reveals a new aspect of Sb metabolism in rice plants.(4)Antimonate uptake was considerably inhibited by P-type ATPase enzyme inhibitor sodium vanadate and metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone(CCCP)in this study,which suggested that the absorption of SbⅤ in rice may be an active process.However,SbⅤ uptake was unaffected by Ag and glycerol,indicating that it did not share the aquaporin transport pathway.Antimonite uptake was considerably inhibited not only by CCCP and V,but also Ag,AsⅢ and glycerol.It indicated that SbⅢ shared the aquaporin transport pathway with As and maybe other active pathways as well.The CCCP,V,Ag and glycerol did not affect Sb efflux from roots when exposed to SbⅤ,but increased Sb efflux when exposed to SbⅢ.Iron plaque induced SbⅢ uptake by roots at low SbⅢ concentration,but inhibited uptake at high concentration.It induced SbⅢ efflux at 5 mg/L SbⅢexposure.Iron plaque reduced SbⅤ influx and efflux to rice.(5)The Sb accumulation,translocation,speciation transformation and iron plaque formation were different between rice cultivars.Two types of hybrid rice(cultivar Fengliangyou No.6(F),Guodao No.6(G))and two kinds of japonica(cultivar Nanjing No.45(N),Zhendao No.11(Z))were exposed to Sbv for 7 d.The results showed that concentration of Fe in iron plaque on the root surface extracted by DCB was cultivar-dependent.Concentration of Fe on the root was 2-4 times more Cultivar(N)than others.There was no significant difference between other three cultivars.There was a significant correlation between the concentrations of Sb and Fe on the root surface(R2=0.97).Sbv translocation ability of japonicas was higher than hybrid rice.Sb translocation factors from roots to shoots of Nanjing were 106%,78%and 33%respectively after 7 d exposure to 0,0.2 and 1 mg/L Sb.Sbv(more than 80%)was the predominant form of Sb in all rice cultivars and had some difference between four cultivars.