University of Arkansas, Fayetteville Division of Agriculture


Currently, the majority of soybean, corn, and cotton crops grown in the U.S. is RoundupReady® (RR) varieties. RR crops are resistant to the active ingredient, glyphosate [N-phosphonomethylglycine], in the herbicide Roundup®. RR crops have been genetically modified by the addition of an enzyme found in Agrobacterium sp. strain CP4 EPSPS that produces an essential protein, involved with aromatic amino-acid production, that is resistant to glyphosate. Glyphosate translocates via phloem from plant leaf tissues to other areas including the root system, and is thus able to affect the rhizosphere microbial community, including mycorrhizae, which are not resistant to glyphosate. A greenhouse experiment was conducted to determine response of mycorrhizal infection and plant nutrients to glyphosate and phosphorus (P) applications to RR soybean, corn, and cotton. Crops were untreated, or treated with glyphosate in low-P soil or in Pfertilized soil, and grown for 6 weeks, after which roots and shoots were harvested and analyzed for mycorrhizal infection and P concentrations. Plant roots were cleared and stained with Trypan Blue dye and analyzed with a dissecting microscope for mycorrhizae on a percent-root basis. Phosphorus had significant positive effects on plant shoot P concentrations for all crops. Mycorrhizal infection rates showed a negative effect in soybean with reduced infection in the P treatment. Glyphosate for all crops and all treatments showed no effect on mycorrhizal infections or plant shoot-P concentrations. Therefore, our results indicate that glyphosate generally may be disregarded in terms of potential detrimental effects on mycorrhizal-plant interactions or plant-P uptake by soybean, corn, and cotton crops in low-P soil.