Effects of plant stress on mycorrhizal colonization of transgenic Bt corn

The relationship between genetically modified plants and the rhizosphere microbial communities is an important element of biotech risk assessment. General measures of microbial community structure and function, though important, frequently produce inconclusive results because of the myriad of possible microbial responses to any given perturbation or change. We have focused instead, on the interactions of arbuscular mycorrhizal fungi (AMF) with plant roots because these interactions are well-defined relative to plant health and they are amenable to relevant extrapolations to different ecosystems and agricultural practices. Results from our first experiment indicate that mycorrhizal colonization of Bt corn plant roots under conditions of plant stress is three times higher than non-Bt corn plants (Bt plants 9% colonized vs non-Bt plants 2.8% colonized, P = 0.0225 ), yet plant growth is not correspondingly stimulated (P > 0.10). This response may represent a potential unique advantage for Bt corn where it might be used under poor growing conditions (e.g., low nutrients, drought conditions, etc.) but it may also represent a breakdown in the AMF-plant root symbiosis of Bt corn that is exacerbated by certain plant stress conditions. Our next experiments focus on whether the differences in AMF colonization of Bt versus non-Bt parental corn plants are related to intrinsic properties of the plant or to diffusible components from the plant roots that affect microbial community composition and function. This study is one of the first to investigate the interactions between mycorrhizal fungi and transgenic Bt corn plants under different plant stress conditions.