Can orchid mycorrhizal fungi be persistently harbored by the plant host?

Abstract

The environmental distribution of non-obligate orchid mycorrhizal (OM) symbionts belonging to the ‘rhizoctonia’ complex remains elusive. Some of these fungi, indeed, are undetectable in soil outside the host rhizosphere. A manipulation experiment was performed to assess the importance of neighbouring non-orchid plants and soil as possible reservoirs of OM fungi for Spiranthes spiralis , a widespread photosynthetic European terrestrial orchid species. Fungi of S. spiralis roots were identified by DNA metabarcoding before and 4 months after the removal of the surrounding vegetation and soil. Although such a treatment significantly affected fungal colonization of newly-formed orchid roots, most OM fungi were consistently associated with the host roots. Frequency patterns in differently aged roots suggest that these fungi colonize new orchid roots from either older roots or other parts of the same plant, which may thus represent an environmental source for the subsequent establishment of the OM symbiosis. • We addressed the importance of vegetation and soil as reservoirs of orchid mycobionts. • Removal of other plants and soil impacted Spiranthes spiralis root fungal community. • Most orchid mycorrhizal fungi were consistently associated with the host roots. • Some fungi may colonize mature orchid roots from other parts of the same plant. • Our findings have implications for the interpretation of orchid distribution.