Inlfuence Of Bt Rice Plants On Nitrogen Fixation Bacteria And Horizontal Transfer Of The Gene For Nitrogenase Iron Protein

Rice is considered as one of the most important grain crops in theworld, more than the half people live on rice. However, with influencedby kinds of diseases, insects, bad weather and other adversities, Theproduction growth in rice has been strictly prohibited. With thedevelopment of transgenic technology，transgenic rice’ environmentsafety is One of the focuses from more and more researchers.Cry-proteins from Bacillus thuringiensis (Bt) are by far the most commoninsecticidal proteins that have excellent resistable significance for eightlepidopteran pest species in paddy field. Transgenic rice can not onlyimprove the plants production, but also produce uncertain impact toecological environment. In China, Bt rice is still not allowed forcommercial cultivation, although a small-scale pilot planting had beenundertaken in certain parts of China for more than10years. China isreluctant for an introduction of Bt rice for the commercial purposebecause of two reasons. Firstly, cultivating Bt rice may lead todevelopment of resistance of the target pests of Bt rice. Secondly, thereare the presumed potential ecological and environmental risks,including effects on non-target organisms and on soil ecosystems. Thereports about influence of transgenic rice on soil microorganism are few,although soil microorganism is of great signifance to ecologicalenvironment and agriculture, especially nitrogen fixing microorganismwhich can convert about200million tons of nitrogen to ammonia eachyear. Any changes, despite small, in the composition of the microbialcommunity should be considered as an early warning indicator for riskassessment.This study mainly investgated the microbial diversity in soils oftransgenic rice by using the traditional culturable method andnon-culturable molecular method based on the analysis of the16S rDNA gene and the nifH gene. The relative contents and main resultsare as following: through the plate count method of bacteria andnitrogen fixing bacteria among various soil samples collected from bothBt and non-Bt rice plots, we found that the planting of transgenic ricemay have inhibitory effect on diversity of microorganism fromrhizosphere soil, which permeated from shallowlayer soil to deep layer soil. However, such impact was oftentransient in duration, the impact significance might be reduced with anincrease in the years of planting. Similar results were obtained from theplanting of ordinary rice in the different transgenic and non-transgenicsoil types and the measuring of Shoot lengths after3-5days of seedsgermination. Phylogenetic tree of16S rDNAs shows a great geneticdiversity of our isolated nitrogen fixation bacteria strains, and found thatthe majority of clones were related to two groups: the classes ofActinobacteria. The other guoup is Proteobacteria including α-Proteobacteri, More than half of nitrogen fixation (92％) in therhizosphere belong to Actinobacteria. Eight of seventeennitrogen-fixing bacteria strains were chosen because of their distinctivephylogenic analysis and their phenotypic dissimilarity as judged bymorphological characteristics. Comparison of phylogenetic trees forthe16S rDNAs and nifH genes of the eight strains illustrated that thephylogenies of the nifH genes and16SrDNAs might not congruent, nifHgenes were seem to evolutionarily less diverged than the16S rDNAs,would constitute evidence for relatively recent in situ horizontal transferof nifH between bacteria indigenous in the study field site.This study simply showed the nitrogen fixing bacteria communitiesstructure from various rhizosphere soil of transgenic rice andnon-transgenic rice, in the absence of evidence that the transgenicrice had an effect on Environmental security.