Submitted to: European Journal of Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2009
Publication Date: 9/21/2009
Citation: Kremer, R.J., Means, N.E. 2009. Glyphosate and Glyphosate-Resistant Crop Interactions with Rhizosphere Microorganisms. European Journal of Agronomy. 31(3):153-161. Interpretive Summary: The widespread use of glyphosate and glyphosate-resistant crops represents a major advancement in effective weed management in conventional agricultural systems. “Roundup Ready [RR] Production System” may alter microbial ecology and biology of the rhizosphere environment of the crop resulting in potential effects on plant nutrient availability, development of plant disease, and activities and composition of beneficial microorganisms in the soil. We report research results that illustrate approaches for understanding impacts of genetically-modified (GM) crops (including glyphosate-resistant varieties) on soil biological interactions; defining mechanisms responsible for the observed effects; and determining strategies to overcome potential production-limiting factors associated with GM crop-soil biological interactions. We conducted studies during 1997-2007 showing Fusarium root colonization was consistently higher on RR soybean treated with glyphosate than on RR soybean without glyphosate and on conventional soybean. Colonization of roots by Fusarium spp., a group of fungi with potential to incite various root and stem diseases, was generally two to five times higher with glyphosate compared to no herbicide or with a conventional (non-glyphosate) herbicide. Thus, Fusarium was a good indicator of potential impacts of RR soybean on the rhizosphere ecosystem by responding to alterations (introduction of glyphosate and altered root exudation) in the rhizosphere. Additional examinations of the structure and functions of other microbial groups were conducted for a more complete assessment of RR crops to better understand mechanisms contributing to the impacts of GM plants on the soil environment and on crop productivity. These analyses included manganese (Mn)-transforming bacteria, nitrogen-fixing bacteria, and pseudomonad bacterial communities, which are interactive and could explain linkages of shifts in microbial communities with functional alterations observed in the cropping system. A key indicator of potential impacts of RR soybean was the appearance of root nodules (tumor-like growths containing beneficial nitrogen-fixing bacteria). Nodulation was always lower on RR soybean than on conventional varieties (i.e., Williams 82), suggesting an altered nodulation response in the GM soybean. A negative relationship between population size of fluorescent pseudomonads (bacteria that can counter effects of fungal disease agents) and Fusarium root colonization further demonstrated that RR soybean and/or glyphosate may be involved in shifting the rhizosphere microbial balance. The research strategy designed in this study for examining key indicators in the rhizosphere and linking to microbial structure is an effective aid for sorting out possible impacts of RR soybean. The results are useful to other researchers, extension personnel, and farmers by providing an understanding of complex factors in the rhizosphere interacting with root-associated microorganisms, which is essential for developing approaches to minimize or eliminate potential adverse effects of GM crops in a crop management system.
Technical Abstract: Current crop production relies heavily on use of transgenic, glyphosate-resistant cultivars (GR). The impacts of widespread cultivation of genetically-modified (GM) crops and the use of one herbicide class on agroecosystems, especially considering potential effects on biological processes, have received considerable attention. This paper reviews impacts of glyphosate on rhizosphere microorganisms and activities and summarizes both previously published and new data from a long-term field project that documents effects of glyphosate applied to GR soybean and maize cultivars on rhizosphere microorganisms including Fusarium spp., pseudomonads, Mn-transforming bacteria, and agrobacteria and soybean root nodulation. We conducted field studies in Missouri, U.S.A. during 1997-2007 to determine effects of glyphosate applied to GR soybean and maize cultivars on root colonization and soil populations of Fusarium spp. and selected rhizosphere bacteria. Fusarium is typically prevalent in the soybean rhizosphere but can become dominant and pathogenic on susceptible plants, often in response to root exudation. We found the frequency of root-colonizing Fusarium increased significantly within one week after glyphosate application throughout the growing season in each year at all sites. Roots of GR soybean and maize cultivars treated with glyphosate were colonized at a considerably higher density by Fusarium spp. than were GR or non-GR cultivars not treated with glyphosate. Several additional microbial groups and functions were affected by glyphosate and GR crops including Mn transformation and plant availability; potential phytopathogen and antagonistic bacterial interactions; and reduction in nodulation. Glyphosate in root exudates may not only serve as a nutrient source for fungi but may also stimulate propagule germination and enhance early growth. The specific microbial indicator groups and processes used in our study demonstrated sensitivity to impacts of GR crops and are considered part of an evolving framework for developing a polyphasic microbial analysis to provide a complete assessment of GR technology that is more reliable than single techniques or general microbial assays.