The community composition and function of arbuscular mycorrhizal (AM) fungi can vary in response to many different ecological factors including host plant identity. Different plant functional groups or distantly related plant species are often reported to host distinct AM fungal communities, but little is known about how AM fungi may respond to subtle genetic differences in closely related plant species. In this study using a model Populus system in a common garden, we examined the species composition of AM fungi associated with narrowleaf cottonwood (Populus angustifolia James), Fremont cottonwood (P. fremontii S. Wats) and their natural F1 hybrids, and tested the functional differences among mycorrhizal communities on these genetically related crosstypes. To characterize AM species composition, we 1) morphologically and molecularly identified fungal spores extracted from greenhouse trap cultures, and 2) cloned and sequenced AM fungi that were present inside field cottonwood roots after PCR amplification with AM fungal specific primers. To investigate the functional variation in AM fungal communities, we inoculated narrowleaf cottonwood cuttings in the greenhouse with whole spore communities originating from all three cottonwood crosstypes separately. Plant growth responses were measured at harvest.
Five species of AM fungal spores were identified from trap cultures: Glomus intraradices, G. constrictum, G.etunicatum, G. claroideum, and an unknown Acaulospora species. All five species were observed on all crosstypes, but relative abundance varied among crosstypes for two AM fungal species. Three of the five AM fungal species observed as spores in trap cultures were also found using molecular assessment of cottonwood roots, including G. intraradices, G. etunicatum, G. claroideum. Glomus intraradices dominated the root samples and showed substantial intraspecific variation in DNA sequences. Cottonwood cuttings of the same genotype varied significantly in total biomass when inoculated with fungal spore communities isolated from narrowleaf, Fremont and F1 soils relative to sterile inoculated controls (P = 0.013). Plants inoculated with AM fungal spores from F1 trees grew significantly larger than control plants while plants receiving narrowleaf and Fremont spore inocula had similar biomass to control plants. Our results suggest that genetic differences among host plants can lead to functionally distinct AM fungal communities. The causes and consequences of such functional variation need further investigation.