Here we tested the hypothesis that species of the soil-inhabiting insect-pathogenic fungus Metarhizium are not randomly distributed in soils but show plant-rhizosphere-specific associations. We isolated Metarhizium from plant roots at two sites in Ontario, Canada, sequenced the 5′ EF-1α gene to discern Metarhizium species, and developed an RFLP test for rapid species identification. Results indicated a non-random association of three Metarhizium species (Metarhizium robertsii, Metarhizium brunneum and Metarhizium guizhouense) with the rhizosphere of certain types of plant species (identified to species and categorized as grasses, wildflowers, shrubs and trees). M. robertsii was the only species that was found associated with grass roots, suggesting a possible exclusion of M. brunneum and M. guizhouense. Supporting this, in vitro experiments showed that M. robertsii conidia germinated significantly better in Panicum virgatum (switchgrass) root exudate than did M. brunneum or M. guizhouense. M. guizhouense and M. brunneum only associated with wildflower rhizosphere when co-occurring with M. robertsii. With the exception of these co-occurrences, M. guizhouense was found to associate exclusively with the rhizosphere of tree species, predominantly Acer saccharum (sugar maple), while M. brunneum was found to associate exclusively with the rhizosphere of shrubs and trees. These associations demonstrate that different species of Metarhizium associate with specific plant types.
Evolutionary Divergences in Root Exudate Composition among Ecologically-Contrasting Helianthus Species
