Effects of nutrient stress on the colonization of mycorrhizal fungi in transgenic Bt corn

Background/Question/Methods

Transgenic Bt crops are genetically engineered to express an insecticidal toxin derived from the spore-forming soil bacterium Bacillus thuringiensis (Bt) to protect plants from particular agricultural pests. We used Bt corn as a model system to evaluate the effects of transgenic crops on beneficial organisms in the rhizosphere. Arbuscular mycorrhizal fungi (AMF) are beneficial soil organisms that are dependent on a plant host for nutrition and reproduction. This mutualistic relationship is especially important to the host plants when grown in low nutrient or drought conditions. Because of their close association with plants, changes in the molecular structure of the plant genome may have more significant impacts on mycorrhizal fungi than on other soil organisms that do not rely on a specific plant host. To determine whether observed reductions of AMF in transgenic Bt corn are consistent across different soil and environmental conditions we compared the level of AMF colonization in Bt and non-Bt corn in three different nutrient treatments (No, Low, and High) and with three different inoculation levels (0, 40, or 80 spores) of Glomus mosseae. Nutrient treatments were applied weekly, and after 60 days, plants were harvested and AMF structures (hyphae, arbuscules, and vesicles) in the root system were stained and quantified. Chlorophylls a and b were extracted to determine the extent of nutrient stress in plants. 
Results/Conclusions

Results from this experiment revealed that Bt plants had reduced AMF colonization across all nutrient treatments, but this effect was most significant under severe nutrient stress. In the ‘High’ nutrient treatment very little AMF colonization was observed in either plant type, in ‘Low’ nutrient conditions Bt plants were 3.2% colonized while non-Bt plants were 21.4% colonized, and in the ‘No’ nutrient treatment Bt plants were 13.4% colonized while non-Bt plants were 35% colonized by AMF (p= 0.0058). Percent colonization was positively correlated with root biomass (p = 0.0118) and spore levels (0.0180), and negatively correlated with increasing nutrient levels. The reduction in AMF colonization may represent a potential disadvantage for Bt crops under poor growing conditions, but it may also represent a breakdown in the AMF-plant root symbiosis of Bt plants that is exacerbated by certain plant stress conditions. The long-term implications of Bt crop production on the soil ecosystem have yet to be established, however reductions in AMF could have significant ecological implications in the future as more Bt crops are developed and the potential for outcrossing increases.