Effects And Mechanisms Of Rhizosphere Arsenate-reducing Bacteria And Nutrient Supply On The Efficiency Of As Removal By Pteris Vittata

Arsenic(As)is a ubiquitous toxic metalloid in the environment,and its health impacts have raised great concern.As-hyperaccumulator Pteris Vittata has great potential in As removal from contaminated soil and water.However,the phytoremediation efficiency of this fern is still limited by many factors,such as low As availability in the soil,and slow growth rate and low biomass of plants.Rhizosphere AsV-reducing bacteria and nutrient supply are two important factors in the regulation of phyto-remediation process by P.vittata.Rhizobacteria mediated As transformation play an important role in controlling As bioavailability to plants.Besides,nutrients,especially Nitrogen(N)and Phosphorous(P)strongly promote plant growth,and also have effects on plant As uptake and accumulation.Therefore,in order to improve As removal efficiency of P.vittata,it is necessary to understand the underlying effects and action mechanism of these two factors.Rhizosphere is the primary medium for P.vittata to contact with As.Rhizobacteria mediated As transformation significantly changes As species and bioavailability in soils.Studies showed that most culturable As-resistant bacteria in P.vittata rhizosphere are arsenate(AsV)reducers,which may play an important role in rhizosphere As transformation.However,studies on AsV-reducing rhizobacteria are limited,and the molecular reduction mechanisms need further investigation.Here,we studied AsV reduction behaviors and associated genes of a Bacillus strain PVR-YHB 1-1 which was recently isolated from P.vittata rhizosphere.Results showed that the strain could tolerate 20 mM AsV.When grown in 1 mM AsV,96%of AsV was reduced to As? in 48h,indicating that it has strong ability of AsV reduction and As? efflux.Based on draft genome sequencing and gene annotations,two ars operons,arsRacr3arsCDA and arsRKacr3arsC for As metabolisms were identified.Both operons might have attributed to efficient As reduction in strain PVR-YHB 1-1.As a kind of rhizobacteria,strain PVR-YHB 1-1 mediated AsV reduction are likely to improve As bioavailability to the roots of P.vittata,and consequently promoting plant As uptake and accumulation.Thus,AsV-reducer Bacillus sp.PVR-YHB 1-1 may have great potential application value in plant-microbial remediation.Nitrate(NO3-)and phosphate(Pi)influence plant growth and element uptake,which are closely associated with As accumulation in P.vittata tissues.However,whether NO3-influences P.vittata As uptake and how it possibly functions remain unclear.Here,we investigate effects of NO3-supply on As uptake and accumulation in P.vittata.Results showed that increased NO3-supply decreased As accumulation in P.vittata tissues.In order to investigate whether the inhibition of NO3-on AsV uptake of P.vittata is possibly due to its regulation on Pi uptake and Phts,we analyzed the transcriptional expression of PvPht1;3 in P.vittata roots responding to NO3-supply.Under AsV exposure,the expression of PvPht1;3 was only up-regulated by 5 mM NO3-but was down-regulated by both 0.05 mM and 15 mM NO3-,suggesting that PvPhtl;3might contribute to lower As accumulation under high NO3-supply,but there are other possible mechanisms leading to NO3--inhibited As uptake and accumulation.Our study suggested that P.vittata accumulate more As under low N supply,which could provide clues on optimizing fertilizer use to improve phytoremediation efficiency.