A Symbiosis Between a Dark Septate Fungus, an Arbuscular Mycorrhiza, and Two Plants Native to the Sagebrush Steppe

Abstract

Plant roots form symbioses with various fungi, including arbuscular mycorrhizae (AMFs) and dark septate endophytes (DSEs). The symbiosis between plants and AMFs has been extensively studied and is generally considered to be mutualistic. Much less is known about the symbiosis between plants and DSE. In sagebrush habitats, DSEs are common, but their effects on the vegetation are unclear. As a first step to study these effects, I isolated and cultured a DSE from the roots of the shrub Artemisia tridentata. Based on partial sequences of five genes and phylogenetic analyses, the isolated fungus was a non-described species within the Darksidea or a closely related sister group. Subsequently, I performed experiments in vitro and in potted plants to determine the effect of the isolated DSE on root tissue integrity, colonization by the AMF Rhizophagus irregularis, and plant biomass. These experiments were conducted in two plant species, A. tridentata and the native grass Poa secunda. Plants were exposed to one of four treatments: no inoculation (-AMF-DSE), inoculation with the DSE isolate (-AMF+DSE), inoculation with R. irregularis (+AMF-DSE), and inoculation with both fungi (+AMF+DSE). Microscopic observations revealed that the DSE hyphae grew along the root surface and penetrated epidermal and cortical cells without damage to them. In A. tridentata, the hyphae also reached the stele. For both species, total DSE colonization in the –AMF+DSE treatment was similar to that in the +AMF+DSE treatment, indicating the presence of AMF did not alter DSE colonization. Inoculation with DSE did not affect total AMF colonization of A. tridentata; however, it increased total colonization of P. secunda from 16.9 (+5.6%) in the +AMF-DSE treatment to 42.6 (+2.9%) in the +AMF+DSE treatment. Also, in both species, the presence of the DSE more than doubled the frequency of AMF intraradical storage structures, which consisted of vesicles plus intraradical spores. These results suggest that via increases in AMF colonization, DSE could lead to a beneficial effect on the host plants. However, neither on its own nor through co-inoculation with AMF, did the DSE isolate affect plant biomass. Thus, under the two conditions tested, the symbiosis was commensalistic. Further work is needed to evaluate the symbiosis in settings that better mimic the natural environment.