Breaking boundaries: fungi in the “rhizoctonia” complex occur systemically in three European orchid species

Mycorrhizal fungi are an important component of the root microbiota in most land plants as they play fundamental roles in plant nutrition and health. Orchids, one of the largest plant families, are particularly dependent on mycorrhizal fungi for seed germination and plant development. Orchid mycorrhizal (OrM) fungi identified in the roots of most green orchids belong to the ‘rhizoctonia’ complex, a polyphyletic group of Tulasnellaceae, Ceratobasidiaceae, Serendipitaceae and Sebacinaceae (Agaricomycotina), and are generally assumed to colonize their host from soil, where they live as saprotrophs. However, some rhizoctonia OrM fungi were rarely detected by metabarcoding in soil around orchid roots, and we have tested the hypothesis that these fungi may use adult orchid plants as a habitat by colonizing not only their roots, but also other organs. The occurrence of OrM rhizoctonias inside roots, stems and leaves of three terrestrial orchid species ( Spiranthes spiralis , Serapias vomeracea and Neottia ovata ) was investigated by metabarcoding. In all orchid species, a broad majority of putative OrM fungi found in roots was also detected in either stems or leaves of the same plant. Putative OrM fungi were also detected in the capsules of all three species and in seeds of S. spiralis . Systemic occurrence of mycorrhizal fungi in plants is a novel finding. In particular, identification of OrM symbionts in all orchid tissues raises important questions on the plant-fungus relationship in the aerial organs.