Investigating the effects of herbicide on interactions between beneficial soil bacteria, herbicide-resistant soybeans, and insect herbivores

Use of genetically modified (GM) plants in agriculture is increasingly widespread, and the impacts of genetic engineering on interactions between crop plants and other organisms warrant investigation. GM soybean plants (Glycine max) engineered for herbicide-resistance facilitate the use of herbicides like Roundup for weed control. However, herbicides can adversely impact the mutualistic nitrogen-fixing bacteria (rhizobia) that colonize soybean roots, with potentially harmful effects on plant performance. Soybeans commonly interact both with rhizobia from commercial inoculants applied by growers and with rhizobia strains occurring naturally in the soil. We recently found that rhizobia, in addition to providing nitrogen in a form usable by plants, can confer enhanced resistance against phloem-feeding herbivores and that the strength of resistance conferred varies among rhizobia strains. In the present study we are exploring variation among rhizobia strains in their resistance to Roundup and the implications of such variation for plant performance and plant-herbivore interactions. Under field conditions, GM soybeans were either treated with a commercial rhizobia inoculant at planting or allowed to associate with naturally occurring rhizobia strains. Roundup was applied to half of the plants in each rhizobia treatment and we assayed the effects on nodulation intensity, plant biomass, foliar nitrogen content, aphid population density, and crop yield. The results of this study will elucidate important interactions between plants, insects, and mutualistic rhizobia and provide information that may inform the development of rhizobia inoculants or the design of weed management strategies.