Plant-rhizobia mutualism influences aphid abundance on soybean

The mutualism between legumes and nitrogen-fixing bacteria (rhizobia) is a key component of many ecological and agricultural systems, particularly because of the resulting enrichment of soil nitrogen. While many aspects of this interaction have been well characterized, little is known about the effects of the legume-rhizobia mutualism on other organisms, particularly herbivores. Changes that occur in the host plant upon inoculation, such as altered ratios of different nitrogen forms and changes in leaf chemistry, may affect plant quality from the perspective of herbivores. Such changes are influenced by the genotype of the symbiotic bacteria. We investigated the effects of different rhizobial sources on the population levels of a specialized legume herbivore feeding on soybean. Plants associating with indigenous rhizobial strains supported reduced aphid populations compared to plants inoculated with a commercial rhizobial preparation. Plant size, total leaf nitrogen content, and the general intensity of nodulation were similar in both inoculated treatments, suggesting that these plant attributes cannot explain the observed differences in aphid abundance. Genetic analyses confirmed that the commercial rhizobial strains were distinct from indigenous strains. These results suggest that the plant-rhizobial interaction can result in improved herbivore resistance and that particular rhizobial strains confer greater resistance to their mutualist partners than others.