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Early diverging taxa of Ascomycota and Basidiomycota share similarities in subcellular characters of the spindle pole body (SPB), nuclear division, and septal pore apparatus, but our understanding of character evolution is incomplete because of the limited number of structural studies within the earliest diverging subphyla of Dikarya, Taphrinomycotina and Pucciniomycotina. Two species of Helicogloea (Atractiellomycetes) were analyzed for these characters and provide data on SPB and nuclear division for an additional class of Pucciniomycotina. A detailed analysis of septal pore apparatus for the Helicogloea species permits comparisons with those of other Pucciniomycotina and Ascomycota. The endogenous origin of hyphal branches is shown to occur in a third class of Pucciniomycotina. The full set of characters supports a close relationship between Atractiellomycetes and Pucciniomycetes.

Among fungi isolated from healthy root mycobiomes of Populus, we discovered a new endorrhizal fungal species belonging to the rust lineage Pucciniomycotina, described here as Atractiella rhizophila. We characterized this species by transmission electron microscopy (TEM), phylogenetic analysis, and plant bioassay experiments. Phylogenetic sequence analysis of isolates and available environmental and reference sequences indicates that this new species, A. rhizophila, has a broad geographic and host range. Atractiella rhizophila appears to be present in North America, Australia, Asia, and Africa and is associated with trees, orchids, and other agriculturally important species, including soybean, corn, and rice. Despite the large geographic and host range of this species sampling, A. rhizophila appears to have exceptionally low sequence variation within nuclear rDNA markers examined. With inoculation studies, we demonstrate that A. rhizophila is nonpathogenic, asymptomatically colonizes plant roots, and appears to foster plant growth and elevated photosynthesis rates.