Localization of helotialean fungi on ectomycorrhizae of Castanopsis cuspidata visualized by in situ hybridization

Non-ectomycorrhizal fungi that associate with typical ectomycorrhizae often remain hidden, and their localization inside ectomycorrhizal (ECM) roots has remained uncharacterized. In this study, the fungal community associated with the ectomycorrhizae of Castanopsis cuspidata was investigated using a culture-dependent isolation technique. Additionally, the species composition and localization were determined using molecular techniques. The results of the isolation and identification of fungal species revealed the predominance of a few species belonging to the order Helotiales. Furthermore, the fungal community structures were significantly different depending on the taxa of the ectomycorrhiza-forming fungi. A taxon-specific probe was developed to analyze the localization of one dominant Hyaloscyphaceae (Helotiales) species in ECM tissues by in situ hybridization. Hybridization signals were detected on the surface of the fungal mantle and around the ECM fungal cells within the mantle. Hyphal penetration into ECM hyphal cells of fungal mantles was also observed. Signals were not detected in the Hartig net or plant tissues inside the mantle in healthy ectomycorrhizae. These findings suggest that the analyzed species interact not only with host plant as root endophyte but also directly with the ECM fungi.