Effect And Mechanism Of Endophyte Inoculation On Improving Phytoremediation Of Cadmium Contaminated Soil By Phytolacca Acinosa

In China,the situation of soil heavy metal contamination is severe,resulting in the increasing food quality and safety problems.As an in-situ soil remediation technology,microbial assisted phytoremediation has great advantages in cost,environmental protection and applicability,and has attracted extensive attention.In this study,functional endophytes with potential to enhance phytoremediation were screened from Phytolacca acinosa.The effects of endophyte inoculation on phytoremediation efficiency and the related mechanisms were further studied.Firstly,51 endophytes were isolated from Phytolacca acinosa grown in the heavy metal contaminated soil.Five of them have great potential to enhance phytoremediation as they possess great cadmium resistance and plant growth promoting traits.Then the plant growth and cadmium accumulation promoting performance of the above five strains were studied by microfluidic and pot experiment,and the results showed that EL3 increased phytoremediation by 46.33%and 55.49%,respectively.There was a significant positive correlation between the phytoremediation parameters in the two systems,indicating the feasibility of microfluidic technology for studying the endophyte assisted phytoremediation.Besides,the effects of endophyte colonization on the soil aggregate properties were studied.The results showed that endophyte increased the contents of available nitrogen,phosphorus,potassium and organic matter,as well as the bioavailable cadmium in macro-aggregates and micro-aggregates(increased by 4.92-15.00%and 0-9.37%respectively).Both multiple linear regression and structural equation modeling analysis showed that the cadmium accumulation in P.acinosa was mainly depended on the cadmium bioavailability in macro-aggregates and microaggregates.Among the above five strains,the highest colonization rate in plants was observed in EL3,and the colonization rate can be affected by organic acids,amino acids and sugars.Glycine,which could increase the specific growth rate of EL3 from 0.59 to 0.66 and enhance the colonization of endophyte EL3 in the stem and leaf tissues of P.acinosa plant,had the strongest chemotactic effect on EL3 with the relative chemotaxis index of 2.43.In addition,it was found that the improvement of EL3 colonization rate could also promote phytoremediation efficiency and compared with the control group,the addition of glycine could increase the biomass of P.acinosa plant by 14.75%and the heavy metal accumulation by 16.86%.The study on the regulation mechanisms of endophyte EL3 colonization on phytoremediation showed that EL3 inoculation increased chlorophyll content and antioxidant performance of plants,and thus alleviate the oxidative damage of heavy metals in plants.Additionally,the enzyme activities of rhizosphere soil,the content of bioavailable Cd and available P at root-soil interface were also improved with endophyte inoculation.Moreover,EL3 inoculation improved the metabolic capacity of microorganisms and increased the relative abundance of ecologically beneficial microorganisms including Bacillus and Lysobacter.Therefore,the phytoremediation hotspot area at root-soil interface increased by 6.51-15.19%.Endophyte colonization improved phytoremediation efficiency by the regulation of rhizosphere/endophytic microecological properties.Plant growth and heavy metal accumulation parameters were significantly and positively correlated with soil nutrients,enzyme activities and the relative abundance of ecologically beneficial microbes.This study could provide theoretical direction for the development of microbial assisted phytoremediation and technical support for the practical application of cadmium contaminated soil bioremediation.