Does plant chemistry mediate eco-evolutionary feedbacks in the rhizosphere?

Lauren Carley and Anita Simha, University Program in Ecology, Duke University, Durham, NC, Lauren Carley and Anita Simha, Biology Department, Duke University, Durham, NC, Lauren Carley, Department of Plant and Microbial Biology, University of Minnesota, Twin Cities, St. Paul, MN, Amanda Carmichael and Maggie Wagner, Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, Amanda Carmichael, Atlanta Botanical Garden, Atlanta, GA, Maggie Wagner, Kansas Biological Survey, Lawrence, KS

Background/Question/Methods
Plant defensive chemicals can influence belowground and aboveground interactions jointly. Root exudates in particular have been repeatedly shown to impact the microbial communities that assemble in rhizosphere soil. Thus, it is likely that above- and belowground selective pressures both influence natural selection on plant chemistry. Despite this, few studies have studied above- and belowground effects of plant chemistry jointly. Furthermore, because plant chemistry may both alter biotic interactions in the rhizosphere and experience selection by microbial interactions, secondary metabolites are ideal focal traits for studying feedbacks between ecological and evolutionary processes.
In the ecological model system Boechera stricta (Brassicaceae), a wild relative of Arabidopsis, the aboveground effects of glucosinolates on herbivore defense and fitness are well described. In this study, we utilized two B. stricta genotypes producing contrasting glucosinolate profiles to determine whether and how secondary metabolite variation influences plant-microbe associations and their effects on fitness. Specifically, we performed three cycles of rhizosphere selection to ask whether defensive chemical profile alters microbial community assembly in the rhizosphere. Then, using genotype-specific rhizosphere communities as inocula for a reciprocal transplant experiment, we asked whether trait-mediated filtering of microbial communities feeds back to influence plant fitness. Together, these processes complete an eco-evo feedback.
Results/Conclusions
Preliminary evidence shows that soil microbial communities influence aboveground performance and fitness. First, soil microbial treatment in the reciprocal transplant influenced germination proportion. Second, plant size in early life was influenced by a genotype*treatment interaction; genotypes differing in glucosinolate profiles showed strongest differences in size when grown in soil inoculated with random microbial communities or mismatched microbial communities than when grown in matched microbial communities or in sterile soil. The effects of microbial community on size differences between genotypes dissipated over time. These results indicate that microbial community composition may influence fitness by affecting germination, and may alter selection by affecting the expression of genetic differences in aboveground phenotypes, although the magnitude of these effects may vary across the life cycle. Combined with past evidence that glucosinolate profile influences aboveground selection by herbivores, these findings suggest that pleiotropic ecological effects across tissue types (here, leaves and roots) may cause complex patterns of natural selection on plant secondary chemistry. Characterization of the effects of glucosinolate profile on rhizosphere communities is ongoing, and will contextualize these results in the framework of an eco-evolutionary feedback.