A Preliminary Study On The Mechanism And Application Of Bacillus Megaterium A14 Strain To Block Cadmium Uptake In Peanut

With the accelerated industrialization,the accumulation of heavy metal pollution in agricultural land,especially cadmium(Cd)elements in soil,has caused widespread concern.As a major oil crop in China,peanut is also an important raw material for food production,and its yield and quality cannot be ignored.However,peanut has a large root density and root zone area,when exposed to Cd-contaminated soil,it is easy to accumulate Cd in the roots.At the same time,as a legume,it may accumulate more Cd at the nodulation or pod maturity stage.Cd accumulates in peanuts,which not only reduces the quality of peanuts,but also persecutes human health through diet.Microbial-mediated in-situ immobilization remediation aiming at imposing multiple barriers to heavy metals transfer from soil,rhizosphere,as well as internal transport in plant and other segments.It is more suitable for the remediation and safe use of moderately and lightly polluted farmland,and shows the characteristics of environmental friendliness,low cost and easy operation.Heavy metal passivating plant growth promoting bacteria can limit the migration of toxic metal elements in the soil,reduce the uptake and accumulation of heavy metals by crops,and promote crop growth,but the effects and mechanisms of functional bacteria in blocking Cd uptake in legume crops,especially peanut,are relatively weakly explored.The purpose of this paper is to apply Bacillus megaterium A14 to an outdoor pot experiment to observe the effect of A14 in blocking Cd uptake in peanut,and to clarify the mechanism of its action.Firstly,we isolated and screened a Cd-tolerant strain of bacteria A14 from acidic laterite with a high background value of Cd,and it performed the function of Cd fixation.The strain showed a Cd adsorption capacity of 54.25%in Cd-containing liquid medium at 5 mg L^-1,with the lowest degree of growth inhibition under Cd stress compared to other isolates.And it has higher ability to produce extracellular polysaccharides and glutathione under Cd stress.FTIR analysis showed the presence of functional groups such as-COOH,-OH and-NH₂on the surface of A14extracellular polysaccharide for Cd adsorption,thus giving the polysaccharide high Cd adsorption performance.In addition,we found that A14 has the growth-promoting properties of producing IAA(20.95 mg L^-1),siderophore,phosphate solubilization,and urease activity.16S r DNA sequence analysis showed that strain A14 belongs to Bacillus megaterium.Further outdoor pot experiment to explore the effect of strain A14 inoculation on alleviating soil Cd stress and reducing Cd accumulation in peanut.The results showed that in the high Cd pot treatment,inoculation with A14 reduced the proportion of soil exchangeable Cd from 42%to 24%,and Cd content in grains was reduced by 58.69%,while significantly increased peanut biomass.After verifying the effect of A14inoculation on the remediation of high Cd contamination of soil,A14 was continued to be added to the red soil potting treatment in the form of a bacterial agent.Compared with CK treatment,the yield of peanut pods with A14 inoculant increased by 186.08%and the Cd content in grains decreased by 84.01%.This indicates that strain A14 has a practical role in managing Cd contamination in peanuts at different Cd levels and the treatment effect is better than commom physico-chemical soil amendments.To further explore the potential mechanism of Cd fixation and growth promotion function after A14 inoculation,we selected the rhizosphere soil under the red soil pot experiment for 16S r RNA Illumina sequencing and soil physicochemical analysis,and also focused on the accumulation of antioxidant enzymes and the relative expression levels of Cd transport genes in peanut.It was found that inoculated with A14improved the physicochemical properties of rhizosphere soil including p H and soil enzyme activity,and enriched the bacterium groups including Proteobacteria and Firmicutes,which play the function of Cd-contaminated soil remediation.Moreover,a special interface of bacterial biofilm was formed on the root surface of peanut,which acted as a microbial barrier.In addition,antioxidant enzyme activities(SOD,POD,CAT)in peanut plants showed a systematic elevation,and the relative expression levels of Cd transporter genes were significantly up/down-regulated.In summary,this study verified the effectiveness of Bacillus megaterium A14 in treating Cd-contaminated soil and alleviating Cd stress in peanut through the process from laboratory to actual agricultural production,and explained the mechanism of A14 reducing the bioavailability of Cd from the perspectives of soil microbiology and plant physiology.The research results not only enrich the principle of combined phyto-microbial remediation,but also provide an effective technical way to prevent soil Cd pollution,reduce the risk of crop pollution and ensure safe production of peanuts.