Plant protection from soil pathogens by arbuscular mycorrhizal fungal communities

Background/Question/Methods  Arbuscular mycorrhizal fungi (AMF) colonize the roots of many terrestrial plants and have been hypothesized to reduce susceptibility of host plants challenged by root pathogens.  Such effects may be the result of improved nutrition or altered physiology, and broadly influenced by root architecture.  These functional differences, however, have largely been demonstrated using isolates of individual AMF species. AM fungal species typically exist as communities and provide complementary or synergistic services to their host plant.  Functional variation in pathogen resistance therefore needs to be examined using the whole AMF community in order to better understand its ecological significance; few studies have done so.

In this study, we ask whether a) naturally occurring AMF communities differ functionally in protecting plant species with differing root architecture, and b) whether AMF communities are ecologically well matched to their local soil pathogen. We inoculated seedlings of Andropogon gerardii (big bluestem, fibrous roots) and Silphium integrifolium (compass plant, simple roots) with compositionally similar AMF communities from two sites (Morton, Dixon Prairies), and challenged seedlings with the root pathogen, Cylindrocarpon, isolated from Dixon Prairie soil.

Results/Conclusions A comparison between host species showed that Andropogon was more susceptible to Cylindrocarpon infection than Silphium.  Both sources of AMF significantly reduced the frequency of Cylindrocarpon infection (as hyphae), and we observed a positive correlation between AMF and pathogen hyphal abundance.  Our results also showed that interactions between host plant and AMF community were important determinants of pathogen abundance, plant growth, and plant nutrient status.  For fine-rooted Andropogon, the local AMF community (Dixon) resulted in a greater reduction in root pathogen abundance, and plants with greater root mass and P content than with the non-local AMF community (Morton).  In Silphium, however, both Dixon and Morton AMF communities effectively reduced root pathogen colonization, and significantly enhanced plant P content.  Functional differences between AMF communities for pathogen resistance should therefore be viewed in the context of their host plant species. That is, the application of non-local AMF communities might alter the relative resistance of co-existing plant species to pathogens that, in turn, may translate into changes in interspecific competitive ability