An increasing number of crops commercially grown today are genetically modified (GM) to resist insect pests and/or tolerate herbicides. Although Bt corn is one of the most commonly grown GM crops in the United States, little is known about its effects on the long-term health of soils. Although there are many benefits to using biotechnology in agriculture, such as potentially reducing insecticide use, there may be unintended side effects as well—does GM corn impact non-target soil organisms, such as arbuscular mycorrhizal fungi, or affect plants subsequently grown in the same field?
Bt corn is genetically engineered to express insecticidal toxins derived from a soil bacterium, Bacillus thuringiensis, to protect it against common agricultural pests such as the corn root worm and European corn borer. Tanya Cheeke and her colleagues (at Portland State University, Oregon) were 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. They published their findings in a recent issue of the American Journal of Botany (http://www.amjbot.org/content/99/4/700.full).
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 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 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.
Interestingly, the authors found that colonization of plant roots by symbiotic soil fungi was lower in the genetically modified Bt corn than in the control lines. However, there was no difference in root biomass or shoot biomass between the two types of corn at the time of harvest.
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," Cheeke commented. "Globally, genetically modified corn is cultivated in at least 16 different countries."
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.
Tanya E. Cheeke, Todd N. Rosenstiel, and Mitchell B. Cruzan. 2012. Evidence of reduced arbuscular mycorrhizal fungal colonization in multiple lines of Bt maize. American Journal of Botany 99(4): 700-707. DOI: 10.3732/ajb.1100529
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