Mutualisms in novel environments: ecological and evolutionary implications of herbicide on plant-rhizobia interactions

Suwa, T., J.T. Lennon, and J.A. Lau

Presented at the All Scientist Meeting (2011-04-15 to 2011-04-15 )

Exposure to novel environmental stressors, such as chemical pesticides, has been shown to reduce population growth rates of plants, animals, and microorganisms; however, these novel stressors could also alter the evolution of exposed populations if genotypes differ in their response to the stress. Moreover, these evolutionary changes could potentially feedback to influence ecological processes including competition, predation, and mutualisms. Here we conducted experiments to examine the ecological and evolutionary consequences of a novel environmental stress, herbicide application, on the soybean (Glycine max) and rhizobia (Bradyrhizobium japonicum) mutualism. Using eleven isogenic rhizobium strains, we quantified variation in tolerance to glyphosate, an active ingredient of the herbicide, Roundup ®. We then examined how glyphosate-induced changes in rhizobia influenced soybean fitness and how the effects of glyphosate varied across strains by inoculating replicated soybean seedlings in the greenhouse with each rhizobium strain and imposing glyphosate
(presence/absence) treatments. We found that rapid evolution of glyphosate tolerance is likely and that these evolutionary changes could have indirect negative effects on soybean fitness. We found that glyphosate reduced the maximum growth rate of many, but not all rhizobia strains. Surprisingly, plants inoculated with rhizobium populations that had a long history of glyphosate exposure produced more nodules in both the presence and absence of glyphosate. We conclude that glyphosate application could potentially influence both the ecology and evolution of field rhizobium populations. Such rapid evolutionary changes could influence soybean yields if glyphosate tolerance is costly in terms of reduced population growth rates and/or reduced nitrogen fixation.

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