Reduced Arsenic And Cadmium Uptake By Vegetables And Wheat In The Presence Of Metal(Loid)-Resistant And Plant Growth-Promoting Bacteria And The Mechanisms Involved

With the rapid development of industry,excessive use of pesticides and fertilizer,heavy metal(loid)contamination in agricultural soils has become an environmental hazard,especially in China.China is the largest producer and consumer of wheat in the world.Excessive accumulation of Cd and As in wheat can decrease grain yields and cause potential threats to human health through food chain.Phytostabilization is suitable for remediation of moderately metal(loid)-polluted farmlands.Plant growth-promoting bacteria(PGPB)can not only promote plant growth and reduce metal(loid)uptake,but also immobilize metal(loid)through adsorption,precipitation and so on.PGPB-assisted phytostabilization is a practical strategy for the remediation of metal(loid)-contaminated soils and the safe production of crops.Two hundred and eighty-nine Cd and As resistant bacteria were isolated from metal(loid)-contaminated agricultural soils.Among them,11 isolates had the abilities to produce high levels of indole-3-acetic acid(IAA)or siderophore and reduce water-soluble As content in soil.The 11 strains were characterized for their effects on the growth,Cd and As contents of edible tissue of Chinese cabbage and radish grown in metal(loid)-contaminated soils.Inoculation with the strains significantly increased the edible tissue biomass of the vegetables compared to the controls.Furthermore,Pseudomonas entomophila Q3-11 significantly decreased Cd(41%)and As(25%)contents of the edible tissues of Chinese cabbage and Cupriavidus necator Q2-8 significantly reduced Cd(34%)and As(45%)contents in the roots of radish,compared to the controls.A pot experiment was carried out to study the influence of the inoculation with strains Q2-8 and Q3-11 on the plant growth,Cd and As uptake of the two wheat cultivars(YM 16 and JM 22)in overwintering and maturity stages.In overwintering period,inoculated with strains Q2-8 and Q3-11 significantly increased the biomass of YM 16(ranging from 20%to 32%)and JM 22(ranging from 28%to 43%)in high(2 mg kg-1 Cd+60 mg kg-1 As)Cd+As-contaminated soil.The two strains significantly reduced the root and shoot Cd(ranging from 21%to 35%)and As(ranging from 25%to 38%)contents of YM 16 in high Cd+As-polluted soil respectively,compared with the controls.In maturity stage,strains Q2-8 and Q3-11 significantly reduced grain Cd content(ranging from 21%to 30%)of YM16 in the low(1 mg kg-1 Cd+40 mg kg-1 As)and high Cd+As-contaminated soils.Besides,inoculation with strain Q2-8 could notably decrease grain As content of YM 16 by 27%and 29%in Cd+As-contaminated soils respectively.However,strains Q2-8 and Q3-11 did not significantly reduce the Cd and As contents in the shoot and grain of JM22 in overwintering and maturity stages respectively.Microbes mediate As biotransformation which can determine the fates,mobility and bioavailability of As in soil.Quantitative real time PCR(qPCR)was used to analyse the relative abundance of As(Ⅲ)oxidase gene aioA,As(Ⅴ)reductase gene arsC,and As(Ⅲ)S-adenosylmethionine methyltransferase gene arsM in the rhizosphere soils of YM 16 in overwintering and maturity stages.Inoculation with strains Q2-8 and Q3-11 significantly increased the abundance of arsM gene in high Cd+As-contaminated soils compared to the controls.Strains Q2-8 and Q3-11 inoculation might have the prospect for efficiently removing arsenic from contaminated soil by volatilization of As.Leptothrix spp.are commonly found to be Fe-and Mn-oxidizing bacteria.Inoculation with strains Q2-8 and Q3-11 significantly increased the abundance of Leptothrix spp.in high Cd+As-polluted rhizosphere soils of YM 16 in overwintering and maturity stages,compared to the controls.The results suggested that strains Q2-8 and Q3-11 decreased available Cd and As contents in soil by the formation of Fe and Mn oxides which can absorb Cd and As.Hydroponic experiments were performed to screen bacteria which could reduce Cd and As contents in the shoot of the wheat under different concentrations of Cd or As stress.Inoculate with strain Q2-8 significantly increased the biomass and reduced the Cd and As contents in the shoot of YM 16 under Cd(5 μM),As(10 μM)and Cd(5 μM)+As(10 μM)stress,respectively.Meanwhile,strain Q2-8 inoculation increased the proportions of Cd and As in the cell walls of YM16.PGPB can reduce the heavy metal(loid)uptake by plants,but little is known about the details and the molecular mechanisms.Here,iTRAQ labeling,bioinformatics analysis and qPCR were used to analyze the proteome in the roots of the wheat seedling in response to Cd(5 μM)+As(10 μM)As stress in the presence of strain Q2-8.In this study,229 significant differential proteins were identified and categorized in energy,stress and defense,protein turnover,metabolism,signal transduction,sulfur compound metabolic,transporters,cell wall organization and biosynthesis,cell growth/division,cell structure,transcription and unclassified.Pyruvate kinase,isocitrate dehydrogenase and sucrose synthase involved in energy production,glutathione S-transferases which could catalyze the conjugation of GSH with metal(loid)s and expansin,UDP-glucose 6-dehydrogenase and UDP-arabinopyranose mutase 1 involved in cell wall synthesis were up-regulated in the presence of strain Q2-8 under Cd+As stress.The results indicated that inculation with strain Q2-8 could protect wheat against Cd and As toxicity and fix Cd and As on the cell walls of the roots to reduce Cd and As translocation into the shoots of the wheat.