Effect Of Abscisic Acid (ABA)-catabolizing Bacteria On Phytoextraction Of Heavy Metal And Its Mechanism

Phytoremediation,an environmentally friendly,no secondary pollution and low-cost technology,has become the research frontier in the field of heavy metals?HMs?contaminated soil remediation.In recent years,the enhanced efficiency of HMs phytoextraction by plants became a hot spot for phytoremediation of HMs contaminated soil.In the present study,the model plants Arabidopsis thaliana wild-type?wt?,ABA-sensitive mutant?abi1/hab1/abi2?,ABA insensitive mutant?snrk2.2/2.3?,IRT1-null mutants?irt1-1 and irt1-2?and the metal hyperaccumulator?Sedum alfredii Hance?were used to investigate the effects and mechanism of ABA-catabolizing bacteria?Rhodococcus qingshengii?on the phytoextraction of HMs in contaminated soil,which might help to develop new strategy for improving phytoremediation efficiency in HMs contaminated soil.The main research includes three parts as follows:?1?Rhodococcus qingshengii,an abscisic acid?ABA?-catabolizing bacteria,clearly boosts levels of Cd,Zn,and Ni in wild-type Arabidopsis by 47,24,and 30%,respectively,but no increase in Cu was noted,when compared with non-inoculated Arabidopsis plants in contaminated growth substrate.Furthermore,when compared with wild-type plants,R.qingshengii-induced increases in Cd,Zn,and Ni concentrations were more pronounced in abi1/hab1/abi2?ABA-sensitive mutant?strains of Arabidopsis,whereas little effect was observed in snrk2.2/2.3?ABA insensitive mutant?.This demonstrated that metabolizing ABA might be indispensable for R.qingshengii to improve metal accumulation in plants.Bacterial inoculation significantly elevated the expression of Cd,Zn,and Ni-related transporters?IRT1,ZIP1,ZIP4,NRAMP1,NRAMP3,NRAMP4 and HMA3?,whereas the transcript levels of Cu transporters?COPT1 and COPT2?remained unchanged.This result may be a reasonable explanation for why the uptake of Cd,Zn,and Ni in plants was stimulated by bacterialinoculation,while no effect was observed on Cu levels.It is interesting to note that in the IRT1 knock out mutants,irt1-1 and irt1-2,the phenomenon of ABA-catabolizing bacteria inoculation promoting the uptake of Cd,Zn and Ni of plants disappeared.This result indicated that IRT1 plays a major role in improving phytoextraction of HMs in plants through ABA-catabolizing bacteria treatments.From our results,we clearly demonstrate that R.qingshengii can increase the accumulation of Cd,Zn,and Ni in plants via an ABA-mediated HM transporters-associated mechanism.Metabolizing ABA in the plants by ABA-catabolizing bacterial inoculation might be an alternative strategy to improve phytoremediation efficiency in HMs contaminated soil.?2?In the present study,higher concentrations and accumulations of Cd,Zn,Pb,and Ni were found in the shoots and roots of R.qingshengii-inoculated S.alfredii than the control_{bacteria-free}acteria-free under the multi-metal contaminated paddy or mined soils.Furthermore,compared with R.qingshengii-inoculated S.alfredii in the paddy soil,the increasing percentage of Cd,Zn,Pb and Ni concentrations in the bacterium-inoculated S.alfredii in the mined soil were more prominent than that grown in the paddy soil.However,in terms of the accumulations of the above metals in S.alfredii after inoculation,the increasing percentage of total phytoextraction by R.qingshengii in the two types of soils were relatively close.In addition,in the two types of contaminated soils,the bacterium inoculation also increased the relevant indicators for evaluating phytoextraction,e.g.translocation factors?TF?,bioconcentration factor?BCF?and phytoextraction rates?PR?.Furthermore,R.qingshengii up-regulated the expression of the transporters related to the uptake of heavy metals?Cd,Zn and Ni?,e.g.SaIRT1,SaZIP1,SaZIP2,SaZIP3,SaNramp1,SaNramp3,SaNramp6,SaHMA2 and SaHMA3.Taken together,the above results show that the R.qingshengii-improved phytoextraction for multi-metals may provide a promising strategy for remediation of multiple or combined HM-contaminated soils..?3?In order to improve the colonization ability of R.qingshengii in soil and strengthen the effect of R.qingshengii on phytoremediation of heavy metal contaminated soil,preliminary optimization of the process conditions for the preparation of high-efficiency microbial agents for R.qingshengii was further studied.The optimized conditions were as follows:the pH of the added water was 9;the ratio of feed to water was 1:2;the ratio of carrier was 80%wheatbran,15%sawdust,and 5%activated carbon;culture temperature 30°C;culture time 3 d;drying temperature 40°C and optimal inoculum was 40%.The results show that compared with the bacterial liquid,the developed R.qingshengii agents had longer colonization ability in the soils,and the application of R.qingshengii agents had a better strengthening effect of R.qingshengii on the phytoextraction of HMs?Cd,Zn,and Ni?from S.alfredii in the multi-metal contaminated soils.In the paddy soil,compared with R.qingshengii liquid inoculation,the concentrations of Cd,Zn,Pb and Ni were increased by 42%,60%,30%and35%in the shoots of R.qingshengii agents-inoculated S.alfredii,and increased by 41%,31%,29%and 35%in that of roots,respectively.In the mined soil,compared with the bacterial liquid,the R.qingshengii agents further increased the phytoextraction of Cd,Zn,Pb and Ni by plants,which increased the Cd,Zn,Pb and Ni concentrations by 76.1%,22.7%,37.2%and 38.5%?shoots?,and 47.7%,27.8%,40.8%and 36.4%?roots?,respectively.The above results show that the development of the high-efficiency microbial agent of R.qingshengii has more practical application and popularization in strengthening the actual HMs contaminated soils for phytoremediation.