The relationship between genetically modified plants and arbuscular mycorrhizal fungi (AMF) is an important element of soil ecology research. Currently, 80% of all maize grown in the United States is genetically modified to express herbicide resistance, insecticidal traits derived from the soil bacterium Bacillus thuringiensis (Bt), or a combination of stacked traits. Transgenic crop cultivation continues to increase worldwide yet the effects of Bt crops on arbuscular mycorrhizal fungi have not been resolved. AMF are obligate plant symbionts that provide nutrients to plants in exchange for photosynthate. Mycorrhizal fungi contribute to soil structure and nutrient cycling and benefit plants by increasing nutrient uptake, providing protection against root pathogens, and improving drought tolerance. While AMF are sensitive to a variety of factors in agricultural ecosystems including tilling, fertilizer, and soil compaction, it is still not clear how AMF are affected by genetically modified crop plants such as Bt maize. In this experiment we evaluated the AMF colonization ability of nine different Bt maize isolines differing in number and type of engineered traits. Bt and non-Bt parental isolines were grown for 60 days under nutrient limited conditions using 50% local agricultural soil in the potting mix as the mycorrhizal inoculum.
Results from this experiment revealed that all nine isolines of Bt maize had reduced AMF colonization in their root systems. Five of the nine Bt isolines had significant reductions in total AMF colonization (all P < 0.04) and three of these had significant reductions in arbuscule formation (all P < 0.04). Reduced arbuscule formation was most prevalent in Bt isolines expressing more than one type of engineered trait (i.e. protection against the European corn borer, corn root worm, and herbicide tolerance). Because AMF are obligate symbionts they may be more sensitive to a change in physiology of a host plant than other soil organisms. The long-term implications of Bt crop cultivation have yet to be established, however, reductions in AMF could have significant effects on the soil ecosystem. A general reduction in AMF colonization of Bt plant roots may lead to reduced density of AMF propagules in the soil, thus impacting soil structure and function over time. As new genetically modified crops are developed, the Bt model system will be an important tool for resolving the nature of important mechanisms related to AMF-plant symbioses and evaluating the effects of Bt crop cultivation on symbiotic organisms in the soil.