Breaking boundaries: fungi in the “rhizoctonia” species complex exhibit systemic colonization in three terrestrial orchid species

Most green orchids associate with orchid mycorrhizal (OrM) fungi belonging to the ‘rhizoctonia’ complex, a polyphyletic group of Tulasnellaceae, Ceratobasidiaceae and Serendipitaceae (Agaricomycotina), which are generally assumed to live as saprotrophs in soil. However, OrM rhizoctonias 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 niche 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 therefore investigated by metabarcoding. To test the possibility of a vertical transmission of OrM fungi, reproductive structures (capsules, as well as seeds in S. spiralis ) were also analyzed in a subset of plants.

In all orchid species, a broad majority of OrM fungi found in roots was also detected in either stems or leaves of the same plant. OrM fungi were also detected in capsules/seeds.

Systemic colonization of orchid tissues by OrM symbionts is a novel finding that raises important questions on the plant-fungus relationship in the aerial organs and opens intriguing perspectives on the potential modes of fungal transmission to the orchid progeny.