Effect Of Bt Insecticidal Gene On Rice Photosynthetic Carbon Sequestration And Microbial Diversity In Paddy Soil

Use of transgenic crops is increasing at a rapid rate in worldwide. Field andlaboratory studies of transgenic Bt crops have been carried out to assess their risks toenvironment. The insertion of exogenous gene (including the target gene, the markergene and promoter) disrupted the existing network of plants, which may alter thephysiological and biochemical pathway or the process of metabolism and may resultin potential risks to non-target organism or beneficial microorganisms, and direct orindirect effect on soil health and agricultural ecosystem. In this thesis, stable carbonisotope pulse labeling had been developed for the research on variation ofphotosynthates distribution between transgenic Bt rice and its parental variety. It iscleared that transgenic Bt rice has some temporary effect on photosynthatesdistribution in rice plant. Combined with the ¹³C-PLFA technology, the soil organiccarbon sequestration and microbial community structure dynamics in rhizosphere soilwere studied. Based on these findings, seasonal effects of transgenic rice and theinsecticide triazophos on soil enzyme activities and microbial communities werecompared under field conditions. These results provided strong evidence on riskassessment of transgenic Bt rice application and provided technologies for the samekind of studies.The greenhouse experiment was carried out to investigate the effects oftransgenic Bt gene on photosynthesis and enzyme activities of rice flag leaves. Theresult indicated that there were significant and temporary differences betweentransgenic Bt rice and non-transgenic parental rice on photosynthesis and chlorophyllcontent of flag leaves. There was no significant difference between transgenic Bt andnon-transgenic parental rice of the same index at maturing stage. It was suggested theincorporation of crylAb gene resulted in some changes in rice plant. More works needto be done under to assess the risk in field condition.Based on the study of physiological differences between transgenic Bt andnon-transgenic parental rice, the temporal effects of gene transformation on carbon partitioning in rice plant and rhizosphere microbial communities were investigatedunder greenhouse conditions using the ¹³C pulse-chase labeling method andphospholipid fatty acid (PLFA) analysis. It was found that the ¹³C contents in leavesof transgenic (Bt) and non-transgenic (Ck) rice were significantly different at theseedling, booting and heading stages. There were no detectable differences of ¹³Ccontent in rice leaves at the maturing stage of rice development. Although asignificantly lower amount of Gram-positive bacterial PLFAs and a higher amount ofGram-negative bacterial PLFAs were observed in transgenic Bt rice rhizosphere ascompared with non-transgenic parental rice at the seedling, booting and headingstages of rice development, there were no significant differences in the amount ofindividual ¹³C-PLFA between transgenic Bt and non-transgenic parental ricerhizosphere soil at any growing stage. There were no significant differences in theamount of fungal and actinomycete related actinomycete related PLFAs betweentransgenic Bt and non-transgenic parental rice rhizosphere soil. These findingsindicated that the insertion of crylAb gene into rice had temporary effect on thephotosynthate distribution in rice or the microbial community composition in itsrhizosphere, however, it did not last until the maturing stage of rice development.A flooded soil experiment was conducted in which ¹³C-enriched rice straw(transgenic and non-transgenic parental rice)was added to evaluate the effects oftransgenic Bt rice and straw application on the soil C pool and greenhouse gas (CH₄and CO₂) dynamics in the root zone (soil surrounding rice roots) and the non-rootzone (soil outside the root zone). The result showed that rice straw additionsignificantly increased the methane and carbon dioxide emissions in flooding soil.Meanwhile, the rhizosphere microenvironment stimulated straw decompositionsignificantly. The presence of root exudates increased the soil organic carbon (SOC),dissolve organic carbon (DOC), microbial biomass carbon (MBC) concentration andmethane, carbon dioxide emission rate significantly, relative to the soil with strawaddition in non-root zone. The incorporation of ¹³C-label into SOC, DOC, MBC, CH₄and CO₂ was not affected by the Bt gene insert not only in root zone but in non-root zone. The succession profiles of methanogenic archaeal communities associated withrice straw decomposition in root and non-root zone were studied by polymerase chainreaction - denaturing gradient gel electrophoresis (PCR-DGGE) analysis followed by16S rDNA sequencing. Principal components analysis based on DGGE patternsindicated that straw incorporation affected the methanogenic archaeal communitiesirrespective of their location on rice straw (root and non-root zone) compared with therice plant cultivation only. However, no significant differences between transgenic Btrice straw and non-transgenic parental rice straw were observed of the PCA analysison DGGE pattem.Effects of transgenic rice expressing the Cry1Ab insecticidal protein activeagainst lepidoperan pests and the insecticide triazophos [3-(o,o-diethyl)-1-phenylthiophosphoryl-1,2,4-triazol] on soil enzyme activities and bacterial communitieswere compared under field conditions. Results showed that seasonal variations in theselected enzyme activities of transgenic, non-transgenic and non-transgenic parentalrice with triazophos application were clearly detected. However, there were nostatistically significant differences (P>0.05) in phosphatase activity, dehydrogenaseactivity, respiration or methanogenesis in rhizosphere soil between transgenic,non-transgenic and non-transgenic Bt rice with triazophos application over the ricecropping cycle. Denaturing gradient gel electrophoresis (DGGE) analyses were usedto compare rhizosphere bacterial composition among transgenic Bt rice,non-transgenic parental rice-and non-transgenic parental rice with triazophosapplication. Some occasional and inconsistent effects of the application of triazophoson the bacterial composition in the rhizosphere soil of rice plant were found at thebooting and heading stages as compared with that of transgenic or non-transgenicparental rice. There was no significant difference of the bacterial composition in therhizosphere soil of transgenic and non-transgenic parental rice. These resultssuggested KMD1 (Bt) rice expressing the crylAb gene had no measurable adverseeffect on the key microbial processes or microbial community composition inrhizophere soil.