Tritrophic interactions with Silphium integrifolium at the axis : elucidating the role of arbuscular mycorrhizal fungal species on plant phenotype

Abstract: A fundamental understanding of the intricate interactions between crops and their environment may help guide the development of ecologically intensified agriculture. Arbuscular mycorrhizal fungi (AMF) are root symbionts that can increase a plant’s access to soil resources and have been found to reprogram plant defensive mechanisms, potentially influencing the performance of plant antagonists. The effect of mycorrhizal association on plant phenotype may thereby depend on fungal genotype, as AMF strains can differ in functional traits. In the presented experiment the impact of native prairie AMF inocula on growth and volatile organic compound (VOC) production of two silflower (Silphium integrifolium) accessions was assessed. Plants were grown in the greenhouse with a single species AMF inoculum, a mixture of all strains, or mock-inoculated. To assess if any of the volatile production was mediated by different AMF inocula, plants were stressed with the chewing herbivore Epicauta funebris or mechanically wounded prior to VOC measurements. The presence but not the type of native mycorrhizal inoculum was found to be crucial for plant growth as it resulted in a nearly fivefold increase in aboveground biomass compared to mock-inoculated plants. The two silflower accessions also showed marked growth differences in the presence of AMF, indicating that the mycorrhizal responsiveness (MR) of silflower is dependent on plant genotype. Fungal treatment did not lead to distinct VOC profiles, yet the measured VOCs differed among fungal strains and non-mycorrhizal silflower specimen, the latter being found to be more prolific in VOC production per unit biomass than plants grown with some of the AMF strains. Without a direct measure of herbivore performance or attraction of its natural enemies, however, little can be inferred about the actual impact of the mycorrhizal phenotype on crop-antagonist interactions. While further research is required for a better understanding of the role of mycorrhizae in applied settings and if selection for this trait is desirable, these findings suggest that native AMF are an important component of silflower ecology.