Structural characteristics of root–fungus associations in two mycoheterotrophic species, Allotropa virgata and Pleuricospora fimbriolata (Monotropoideae), from southwest Oregon, USA

All members of the Monotropoideae (Ericaceae), including the species, Allotropa virgata and Pleuricospora fimbriolata, are mycoheterotrophs dependent on associated symbiotic fungi and autotrophic plants for their carbon needs. Although the fungal symbionts have been identified for A. virgata and P. fimbriolata, structural details of the fungal–root interactions are lacking. The objective of this study was, therefore, to determine the structural features of these plant root–fungus associations. Root systems of these two species did not develop dense clusters of mycorrhizal roots typical of some monotropoid species, but rather, the underground system was composed of elongated rhizomes with first- and second-order mycorrhizal adventitious roots. Both species developed mantle features typical of monotropoid mycorrhizas, although for A. virgata, mantle development was intermittent along the length of each root. Hartig net hyphae were restricted to the host epidermal cell layer, and fungal pegs formed either along the tangential walls (P. fimbriolata) or radial walls (A. virgata) of epidermal cells. Plant-derived wall ingrowths were associated with each fungal peg, and these resembled transfer cells found in other systems. Although the diffuse nature of the roots of these two plants differs from some members in the Monotropoideae, the structural features place them along with other members of the Monotropoideae in the “monotropoid” category of mycorrhizas.