Co-Inoculation with a Dark Septate Endophyte Alters Arbuscular Mycorrhizal Colonization of Two Widespread Plants of the Sagebrush Steppe

In sagebrush steppes of Western North America, dark septate endophytic fungi (DSEs) are typical components of the soil microbiota. To study the response of native plants to these fungi, a DSE was first isolated from the roots of the dominant shrub Artemisia tridentata. The isolate was a non-described species within the Darksidea or a closely related sister group. In vitro and pot experiments were then conducted to determine the effect of the isolate on root tissue integrity, colonization by the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, and plant biomass. Two plant species, A. tridentata, and the native grass Poa secunda, were each exposed to one of four treatments: no inoculation, inoculation with DSE, inoculation with R. irregularis, and inoculation with both fungi. Microscopic observations revealed that the DSE hyphae grew along the root surface and penetrated epidermal and cortical cells without apparent damage to the roots. The DSE isolate did not affect total AMF colonization of A. tridentata; however, it increased AMF colonization of P. secunda from 19 to 43% (p = 0.004). Also, in both species, the presence of the DSE more than double the frequency of AMF intraradical storage structures, which consisted of vesicles plus intraradical spores. Alone or co-inoculated with AMF, the DSE isolate did not affect plant biomass. This result indicates that under the greenhouse growth conditions used, well-watered plants exposed to light intensities below those typical of field situations, the symbiosis was commensalistic.