(Beyond Pesticides, April 25, 2012) Researchers at Portland State University have found that the cultivation of corn genetically engineered (GE) to express the insecticidal soil bacterium Bacillus thuringiensis (Bt) has negative impacts on beneficial soil life. The research team, led by PhD student Tanya Cheeke, was interested in determining whether the cultivation of Bt corn has a negative effect on arbuscular mycorrhizal fungal colonization of Bt corn or of crops subsequently planted in the same soil. Their findings, published in the April 2012 issue of the American Journal of Botany, show a decreased presence of the beneficial fungi in the roots of Bt corn when compared to non-Bt corn.
Bt corn is genetically engineered to express insecticidal toxins derived from Bt in an effort to protect it against common agricultural pests such as the corn root worm and European corn borer. Recent findings have shown, however, that insects are growing increasingly resistant to the toxin, due in part to a breakdown in resistance management implementation. Additionally, researchers in Europe recently found evidence that Bt is toxic to human cells in large doses.
Arbuscular mycorrhizal fungi (AMF) are ubiquitous microscopic soil fungi that form symbiotic relationships with the roots of most plants. Plants supply the fungi with carbon, and the fungi increase the host plant’s ability to uptake nutrients and water from the surrounding soil.
“Because these fungi rely on a plant host for nutrition and reproduction, they may be sensitive to genetic changes within a plant, such as insect-resistant Bt corn,” stated Ms. Cheeke. By experimentally planting seeds from several different lines of both Bt corn and non-Bt corn, and using local agricultural soil containing native mycorrhizal fungi, the authors were able to simulate what might happen naturally in an agricultural system.
“What makes our study unique is that we evaluated AMF colonization in 14 different lines of Bt and non-Bt corn under consistent experimental conditions in a greenhouse using locally collected agricultural field soil as the AMF inoculum,” said Ms. Cheeke. “The use of whole soil in this study allowed each Bt and non-Bt corn line to interact with a community of soil organisms, making this study more ecologically relevant than other greenhouse studies that use a single species of AMF,” she adds.
The authors of the study found that colonization of plant roots by symbiotic soil fungi was lower in the genetically modified Bt corn than in the non-modified control lines. However, there was no difference in root biomass or shoot biomass between the two types of corn at the time of harvest. Ms. Cheeke and co-authors also determined that the Bt-protein itself is not directly toxic to the fungi since AMF colonization of vegetable soybeans did not differ for those grown in soil previously containing Bt vs. non-Bt corn.
Together these findings contribute to the growing body of knowledge examining the unanticipated effects of Bt crop cultivation on non-target soil organisms. Examining non-target effects of genetically engineered crops on symbiotic soil organisms becomes even more important as acreage devoted to the cultivation of Bt crops continues to increase globally.
“In 2011, 88% of the corn cultivated in the United States was genetically modified to express insect resistance, herbicide tolerance, or some combination of stacked traits,” Ms. Cheeke commented. “Globally, genetically modified corn is cultivated in at least 16 different countries.”
Ms. Cheeke notes that the next step is to understand the ecological significance of this study. “In greenhouse studies Bt corn had lower levels AMF colonization, so now it is important to see if this pattern is also observed under field conditions.” She plans to use field experiments to test if planting a Bt crop for multiple years has an effect on the abundance or diversity of AMF in the soil ecosystem.
For more information on GE crops and their effects on human health and the environment, visit Beyond Pesticidesâ€™ genetic engineering page.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.