Submitted to: International Plant Nutrition Institute (IPNI) Agronomy Information Bulletin
Publication Type: Popular Publication
Publication Acceptance Date: April 15, 2008
Publication Date: April 30, 2008
Citation: Kremer, R.J. 2008. Glyphosate and glyphosate-resistant crop interactions with rhizosphere microorganisms. 2008. In: T. Tamada and S. Stipp e Abdalla (eds.) Proceedings of the Symposium on Problems in Plant Nutrition and Diseases in Modern Agriculture. International Plant Nutrition Institute (IPNI) Agronomy Information Bulletin No. 119. International Plant Nutrition Institute, Piracicaba, Brazil (In Portuguese) p. 15-16. Technical Abstract: Glyphosate and glyphosate-resistant crops represent a major advancement in effective weed management that is now widely used in many crop production systems. Studies conducted during 1997-2007 showed that Fusarium root colonization was consistently higher on Roundup Ready (RR) soybean treated with glyphosate, compared with RR soybean without glyphosate or in conventional soybean weed management. Colonization of roots by Fusarium spp. was generally two- to five-times higher with glyphosate, compared to no-herbicide or with a conventional herbicide. These findings indicate that use of glyphosate with RR soybeans may alter microbial ecology and biology of the rhizosphere environment of the crop, and that important soil functions including nutrient cycling and nutrient availability, potential phytopathogen and antagonist interactions, and the activities and composition of beneficial microorganisms could also be affected. As a result, an approach is needed for understanding the impacts of genetically-modified (GM) crop production on soil biological interactions that defines the mechanisms responsible for observed effects and to provide strategies to overcome potential production-limiting factors. Comprehensive studies of the structure and functions of the broader microbial community were conducted in 2006 and 2007 to provide a more complete assessment of RR crop production. Specific analyses include fungal root colonization, Mn-transforming bacteria, symbiotic nitrogen-fixing bacteria, and pseudomonad communities to begin explaining linkages of shifts in microbial communities with functional alterations. As a key indicator of potential impacts of RR soybean production, nodulation was always lower on RR soybean compared to conventional varieties (i.e., Williams 82), suggesting an altered nodulation response in the GM soybean. The ratio of Mn-reducing to Mn-oxidizing bacteria was consistently lower in RR soybean, especially with glyphosate, suggesting low Mn availability due to microbial transformation, glyphosate immobilization, or the genetics of the RR soybean. The relative proportion of fluorescent pseudomonads was always higher in conventional soybean rhizosphere, relative to the RR soybean treatments. This microbial imbalance may be an effect of glyphosate, the genetically-modified RR variety, or their combination. A negative relationship between population size of fluorescent pseudomonads and Fusarium root colonization further demonstrated that RR soybean or glyphosate was involved in shifting the rhizosphere microbial balance. Further verification of the interaction of the microbial groups was that most fluorescent pseudomonads were antagonistic toward Fusarium. Understanding complex factors in the rhizosphere that involve root exudation and glyphosate release that interact with root-associated microorganisms is essential for developing approaches to manipulate the rhizosphere environment to reduce or eliminate potential adverse effects of GM crops as part of a crop management system. The multifunctional framework approach used in this study based on examining key indicators in the rhizosphere and linking these to microbial structure was an effective strategy for sorting out possible impacts of RR technology within soybean production systems.