| The combined effects of roots and microorganisms can change the rhizosphere environment, and then affect the bioavailability of heavy metal in soil. Reasonable regulation of roots and microbial community structure in the rhizosphere can improve the efficiency of ecological restoration. Rhizosphere is the most active region of the interaction between roots and microorganisms and the most sensitive region to heavy metal. Therefore, exploring the microbial ecology of soil around roots under heavy metal stress will contribute to help clarify the mechanisms between microorganisms and plants. Elsholtzia splendents was used in this study, polluted and unpolluted soils were collected from the heavy-metal-contaminated region near a copper smelter in Fuyang City of Zhejiang province. Soil samples around different rhizosphere zones of E. splendens were collected for the analysis of the speciation of heavy metal, dissolved organic carbon (DOC), bacterial community structure and the specific gene expression. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), molecular cloning and real time-polymerase chain reaction (RT-PCR) were used to explore the effects of heavy metal pollution on microbial ecology in rhizosphere. The main conclusions were as follows:(1) In polluted soil, the content of total Cu in the rhizosphere was significantly lower than that in the control soil. The speciation of Cu in different zones was various. Mature zone had the highest content of exchangeable Cu in our study, which was 4.24 mg/kg. The percentage of exchangeable, bound to carbonates and bound to Fe-Mn oxides to total Cu in the rhizosphere was higher than that in the control soil, and the zone from the interface of root and stem 1 cm had the highest percentage, which was up to 81.7%. But the percentage in control soil was just about 50%. So the zone from the interface of root and stem lcm had the strongest potential bioavailability of heavy metal.(2) Plant growth increased the content of DOC in rhizosphere. The zone from the interface of root and stem 1cm had the highest content of DOC in polluted soil, which was up to 471.0 mg/kg. However, meristematic zone was the highest zone in unpolluted soil, and the content of DOC decreased gradually from meristematic zone to the zone on the interface of root and stem 1cm. The difference in DOC content would affect microbial community structure of the rhizosphere.(3) From meristematic zone to the zone from the interface of root and stem 1cm, the Shannon diversity index showed a downward trend both in polluted and unpolluted soil, which indicated that bacterial community diversity declined. Changes in bacterial community diversity were relevant to the content of DOC. Principal component analysis (PCA) results showed that bacterial community structure in rhizosphere was significantly different from control soil. What'more, the zone from the interface of root and stem1cm was also separated from other zones.(4) Clone and sequence of the specific bands on the DGGE gel showed that proteobacteria and actinobacteria were the important bacterial groups. Compared to polluted soil, the bacterial species in unpolluted soil were more dispersed. Some species disappeared under heavy metal stress, such as Deinococcus-Thermus and Gpl of Acidobacteria; at the same time, Firmicutes and some other bacterial species were found in rhizosphere of E. splendens.(5) We analyzed the expression of encoded protein flagellin (fliC) and chemotaxis gene (cheA) in rhizosphere of E. splendens using RT-PCR. The difference in copy number of fliC between different rhizosphere zones was a way in which bacterial responded to external environment. The meristematic zone had the largest copy number of cheA both in polluted and unpolluted soil. The copy number of cheA was relevant to the content of DOC that was an important chemoattractant for bacterial movement. |
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