Regulating Effect Of Cadmium-tolerant Growth-promoting Bacteria On Cadmium Uptake And Accumulation In Wheat

Cadmium（Cd）is one of the most common harmful heavy metal elements.In farmland,the spot exceeding standard rate of Cd is the highest among the heavy metals,up to 7.0%.Cd with high mobility is easy to be uptake by the roots of crops,including wheat,and then transports into the grains,which serious threats to human health through the food chain.Rhizosphere microorganisms can improve the utilization efficiency of soil nutrient elements,and effectively reduce soil Cd activity,thus inhibiting Cd absorption and accumulation in crops to ensure food production security.In this study,Cd-inhibiting growth promoting bacteria were screened and identified through wheat seedling experiment,and the Cd²⁺adsorption characteristics and plant growth-promoting activity of the strains were detected.Through pot experiment,the effects of Burkholderia sp.Y4 and Pseudomonas sp.W112 on Cd accumulation in wheat,physicochemical properties of rhizosphere soil,soil Cd chemical morphology,characteristics of Cd uptake and accumulation in wheat and rhizosphere microbial community were analyze,in order to clarify the mechanism of Cd absorption and transport regulation by the strains,as well as provide technical support for the remediation and safe utilization of Cd contaminated wheat fields.（1）Through wheat seedling experiment,the effects of identified growth-promoting bacteria on growth and Cd accumulation in wheat were studied.After analysis and identification,the strains W112,W135,W221,W225 and Y4 were Pseudomonas sp.W112,Bacillus sp.W135,Enterobacter sp.W221,Delftia sp.W225 and Burkholderia sp.Y4.It was found that all the five strains could without hemolytic toxicity,and decreased the content of Cd in fermentation liquor and chelate liquor.Under the application of strains W112 and Y4,the Cd contents in fermentation liquor decreased from 10 mg/kg to 6.1 and 5.8 mg/kg,respectively,and those in chelate solution decreased to 0.9 and0.8 mg/kg,respectively.The results showed that the applications of strains W112,W135,W221,W225 and Y4 promoted the growth of wheat seedlings under Cd stress,decreased the Cd content of roots and buds,and increased the contents of mineral elements.Among the five strains tested,strains W112 and Y4 had better Cd inhibition effects on wheat seedlings.The Cd content of wheat buds under the treatment of W112 and Y4 decreased from 15.2 mg/kg to 2.6 and 1.5 mg/kg,and the Cd content of wheat roots decreased from 54.3 mg/kg to 8.5 and 17.0 mg/kg,respectively.（2）Pot experiments were conducted to study the regulation effects of Burkholderia sp.Y4 on Cd uptake and accumulation in wheat and microbial community in rhizosphere soil.The results showed that the root application of strain Y4 at elongation stage significantly decreased the Cd content of wheat roots and grains by 7.7%and 30.3%,respectively.The transfer factor of Cd from basal node to internode was decreased by 79.3%under the treatment of strain Y4,and the Cd content in wheat internode was decreased by 51.9%.The study on the occurrence form of Cd in wheat showed that the treatment of strain Y4 increased the Cd content and proportion of residual Cd in root and basal segment,while decreased the Cd content of inorganic and water-soluble in roots.The results of Cd subcellular distribution showed that the Cd content in root cell wall and basal cytosol increased by 21.2%and 98.2%,respectively,indicating that the treatment of strain Y4 improved the fixation ability of root cell wall and basal cytosol.Meanwhile,the p H value of rhizosphere soil increased by 8.3%after the application of strain Y4.The contents of exchangeable Cd,carbonate-bound Cd and Fe-Mn oxide bound Cd significantly decreased by 44.4%,21.7%and 15.9%,respectively,while the proportion of residual Cd increased by 53.6%.The contents of nitrate nitrogen and ammonium nitrogen in soil increased by 22.0%and 21.4%,and that of alkali-hydrolyzed nitrogen also slightly increased.Sequencing results of rhizosphere microbial community showed that strain Y4 treatment increased the relative abundance of Burkholderia from 9.6%to11.5%in wheat maturation stage and changed the community structure of indigenous microorganisms.Under strain Y4 application,the relative abundances of Gemmatimonadales,Pseudomonadales,Chitinophagales increased,while those of Acidobacteriota decreased.（3）The results showed that the root application of strain Pseudomonas sp.W112 significantly reduced the Cd content in wheat root and grain by 10.2%and 33.0%,respectively.Under strain W112 treatment,the transfer factor of Cd from basal node to internode decreased by 63.5%,and Cd content in wheat internode stem was also reduced by 29.5%.The study on Cd occurrence morphology showed that the treatment of strain W112 increased Cd content and the proportion of residual Cd in root and basal segment,while decreased those of inorganic Cd and water-soluble Cd in roots.The subcellular distribution of Cd was further detected,and the content of Cd in root cell wall and basal node cytosol increased by 19.6%and 61.8%,respectively,indicating that the treatment of strain W112 improved the fixation ability of Cd in root cell wall and basal node cytosol.At the same time,the p H value of rhizosphere soil increased by 3.1%after strain W112 application.The content of nitrate nitrogen increased by 30.8%,while that of ammonium nitrogen decreased by26.6%.The contents of exchangeable Cd,carbonate-bound Cd and Fe-Mn oxide bound Cd in soil decreased by 44.4%,21.7%and 15.9%,respectively,while the proportion of residual Cd increased by 65.2%.The ratio of nitrate nitrogen to ammonium nitrogen was increased.Further study of rhizosphere soil showed that the treatment of strain W112 increased the relative abundance of W112by 534%,and changed the community structure of indigenous microorganisms.Under strain W112treatment,the relative abundances of Gemmatimonadales,Burkholderiales and Bacteroidota increased,while those of Acidobacteria decreased.