| Cadmium(Cd)is a typical heavy-metal,which can be enriched in soil and crops and pose a serious threat to human health.Therefore,controlling the accumulation of Cd in soil and crops is very important to human health and environmental safety.Traditional technologies for Cd contamination control include precipitation,ion-exchange,membrane separation,adsorption and so on.However,these traditional methods may encounter some problems in economic cost,efficiency and environmental friendliness.Agroecosystems are home to a large number of microbial communities,especially root microbes,which play an important role in reducing soil and crop Cd toxicity.It can not only reduce the bioavailability of Cd,reduce the transport of Cd from soil to plants,alleviate the oxidative stress response and systemic damage caused by Cd,but also secrete growth-promoting substances,directly promote crop growth.Therefore,it shows important potential in reducing cd toxicity in soils and crops.Based on the positive influence of root microorganisms on Cd remediation,some microbes with Cd-resistance have been isolated from nature.In this study,bacterial strains with certain tolerance to cadmium were isolated and screened from cadmium polluted rice root samples,and the strains were identified.The adsorption effect and tolerance mechanism of cadmium resistant strains were determined.In order to evaluate the potential of the selected strains to remediate soil cadmium pollution,the effects of the selected strains on cadmium uptake by rice were studied.The main results are as follows:(1)After screening and purification,five Cd resistant strains were obtained from rice roots.Among them,strain YDR is an endophytic strain in roots,and has the best tolerance to Cd.Its minimum inhibitory concentration(MIC)on Cd is 80 mg/L.The colonies of strain YDR on solid medium are opaque,round,neat edges,smooth surface,moist,sticky and easy to provoke.After cultured in liquid medium for 24 hours,the liquid medium became turbid,and the bacteria were evenly distributed in the medium without obvious precipitation.At the same time,strain YDR can grow normally in the range of p H value of 3-10 and culture time of 12-24 h.The physiological characteristics of strain YDR can make it well adapt to the environment polluted by Cd.(2)By determining the 16 S r RNA gene sequence and comparing the measured sequence with the known nucleic acid sequence in NCBI database,the evolutionary relationship between the screened strain and the known bacteria was determined.The results showed that the strain with the closest homology between the 16 S r RNA sequence of strain YDR and the16 S r RNA sequence of bacteria in NCBI database was bacterial strain,and the homology reached 99.58%,It was preliminarily identified as a bacterial strain.(3)In the treatment of different Cd concentrations,the inoculation of strain YDR will change the p H value of Cd containing bacterial suspension.With the extension of culture time,the p H value of the control group did not change significantly,but after inoculation with strain YDR,the p H value of Cd containing bacterial suspension continued to increase,and the variation range was between 5.00-10.00.The increase of p H value may increase the adsorption of Cd on the cell surface or the accumulation of Cd in the cell,resulting in the reduction of Cd concentration and toxicity in the solution,which is also consistent with the research results of the adsorption and accumulation ability of strain YDR to Cd in the experiment.That is,the longer the culture time,the higher the extracellular and intracellular total Cd content of strain YDR and the lower the Cd concentration in the solution.The content of intracellular Cd was higher than that of extracellular Cd under all treatments.In order to understand the effect of Cd on the morphology of strain YDR cells and further determine the adsorption mechanism of strain YDR on Cd,scanning electron microscopy was used to observe.The observation results showed that the strain YDR cells without Cd treatment showed a dense accumulation structure,the cell surface was smooth,the cell body was full,the cell morphology was thin strip,while the strain YDR cells treated with Cd adhered together,the cells showed irregular depression,and the cytoplasm gathered in the middle.Some cells have vacuoles of different sizes,and the surface of the cell is not smooth.These results also show that a certain concentration of Cd will damage the cell.Therefore,we speculate that strain YDR is more inclined to accumulate in the cell about the mechanism of intracellular accumulation and extracellular adsorption of heavy metals.(4)Through the inoculation experiment of strain YDR and rice seedlings,the potential of strain YDR in reducing Cd toxicity of crops was studied.Compared with the control without bacteria,the inoculation of strain YDR could significantly promote the stem length,root length,stem fresh weight and root fresh weight of rice.In addition,as a member of the antioxidant system,the study found that compared with the control,the activities of SOD,pod and cat of rice seedlings inoculated with strain YDR increased,and the content of MDA,the final product of lipid peroxidation,decreased,indicating that the inoculation of strain YDR improved the antioxidant system,reduced cell membrane damage and maintained cell integrity.At the same time,strain YDR enhances the absorption capacity of Cd in the roots of rice seedlings,changes the distribution of Cd in rice seedlings,inhibits the transport of Cd from roots to leaves,and reduces the content of Cd in rice plants.The above research shows that strain YDR can promote the growth of rice seedlings under Cd stress,and can be used for bioremediation of Cd contaminated farmland to improve crop growth and yield. |
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