Studies On The Effects Of EPSPS Trangenic Herbicide-resistant Soybeans On Soil Nitrogen-fixing Bacteria

Since the first variety of genetically modified soybean cultivated commercially by Monsanto in the United States, genetically modified soybeans has made remarkable achievements in the commercialization and large-scale cultivation and brought huge economic benefits for farmers around the world. However, the ecological risks of genetically modified soybeans have caused widespread concern, especially impact on soil microbial communities. As a part of the most viable one in soil, soil microbes are the most abundant part in the terrestrial ecosystem and are the sum of all fungi and bacteria in the soil. Soil microbes play important roles in degradation of soil organic matter, nutrient mineralization and fixation, the regulation of plant pathology and soil structure improvement. Especially nitrogen-fixing bacteria group not only have a positive effect on nitrogen fixation, but also has a huge impact on the nitrogen cycle. Extensive planting of genetically modified soybeans may interact with soil microbes, and have an impact on soil microbial communities and their physiology and biochemistry. Herbicide-resistant transgenic soybeans have been the focus of study as the first commercialized modified soybean with widest range of cultivation.In this study, we chose a genetically modified herbicide-resistant soybean RNZ10-3-57and its receptor Mengdou12(M12) as well as a regional test variety of Inner Mongolia, Heihe43(H43) to analyze the effect of genetically modified soybeans on communities and functional diversity of soil nitrogen-fixing bacteria group. We also analyzed the impact of transgenic soybean on nitrogen-fixing bacteria using field surveys and plate culture, and analyzed the physicochemical properties and activity of soil microorganisms by some conventional methods and at last used real-time PCR and nifH-DGGE to analyze the changes of abundance of nitrogen-fixing bacteria group and the impact on the genetic diversity of bacterial communities.The results showed that parts of physical and chemical properties of transgenic variety RNZ10-3-57(including water content and pH), had no significant change compared with M12and H43. After planting genetically modified soybeans, there is no significant change with alkaline phosphatase, urease and FDA hydrolase activity and acid phosphatase activity was significantly higher than H43but had no significant differences with M12. It showed that acid phosphatase activity may be related to the genotype of soybeans.Results of investigation of nodulation effect of nitrogen-fixing bacteria group and culture and counting of rhizosphere soil rhizobia showed that nodules of RNZ10-3-57were larger and distributed with clumps, while nodules of M12and H43were smaller and discrete distributed but with a larger number. Besides, planting soybeans would affect the number of rhizobium in soil. The number of soil rhizobia in the rhizosphere in three periods presented a first-increase-then-decrease trend and reached the maximum number in flowering period. In addition, compared with three varieties of soybeans, we found there were significant differences in seedling and flowering periods while there was no significant difference in maturity period. It suggested that planting transgenic soybean would affect physiological and biochemical activities of soil rhizobia.Results of Real-time PCR showed that relative gene abundance of nifH in RNZ10-3-57was significantly higher than M12and H43. Because gene abundance determined the overall amount of the soil nitrogen-fixing bacteria, it showed that planting transgenic soybean could affect the overall amount of the soil nitrogen-fixing bacteria, leading to an increase of amount of nitrogen-fixing bacteria.Results of nifH-DGGE showed that there were some differences but not significantly in the number and location of nifH strips for transgenic soybean with their corresponding receipt M12. Appearance of characteristic microbes, such as Rhodopseudomonas sp. showed that planting transgenic soybean might cause changes of genetic diversity of soil nitrogen-fixing bacteria to a certain extent.All results suggested that planting transgenic soybean RNZ10-3-57would cause changes in the physiological and biochemical activities of soil nitrogen-fixing bacteria group, thereby affecting the nodulation effect of the nitrogen-fixing bacteria group, and also cause changes in soil community diversity of nitrogen-fixing bacteria group.