An Evaluation of the Nontarget Effects of Transgenic Bacillus thuringiensis Maize on Arbuscular Mycorrhizal Fungi in the Soil Ecosystem

摘要

My dissertation research examined the effect of the cultivation of insect-resistant Bacillus thuringiensis (Bt) maize on the soil environment with a goal of understanding how to obtain a balance between technological advancement and maintenance of a healthy soil ecosystem. Although Bt plants may help to reduce pesticide use, conferring benefits to farm workers and the environment, there are still unresolved questions about how the cultivation of Bt plants affects soil organisms. For this dissertation project, I used 14 different genotypes of Bt maize and non-Bt maize (Zea mays) to investigate the effects of transgenic Bt plants on the colonization ability, abundance, and diversity of symbiotic arbuscular mycorrhizal fungi (AMF) in the soil ecosystem over time. My greenhouse studies demonstrated that Bt maize plants exhibited reduced AMF colonization across multiple Bt genotypes and that effects were most pronounced when fertilizer levels were limited and spore density was high. In addition, I found that although differences in AMF colonization between Bt and non-Bt maize were difficult to detect in the field, spore density was reduced in Bt field plots after just one growing season. When I tested the effect of plot history on AMF and plant growth, I found that Bt and non-Bt maize plants had higher leaf chlorophyll content when grown in plots previously cultivated with the same maize line as the previous year, indicative of a positive feedback effect. I also examined potential mechanisms contributing to the reduced AMF colonization observed in Bt maize in greenhouse studies and determined that follow-up experiments should continue to investigate differences in root apoplastic invertase activity and root permeability in Bt and non-Bt maize. Future investigations would also benefit from examining potential differences in root exudate profiles and volatile organic compounds between Bt and non-Bt cultivars. Taken together, my dissertation results suggest that, while difficult to detect in the field, reductions in AMF colonization in Bt maize roots may be ecologically significant as they could lead to a decrease in the abundance of AMF propagules in the soil over time, potentially impacting soil structure and function in areas where Bt crop cultivation is high.