Germination of the fully myco-heterotrophic orchid Cyrtosia septentrionalis is characterized by low fungal specificity and does not require direct seed-mycobiont contact

We examined the level of specificity exhibited by seeds of Cyrtosia septentrionalis for germination-inducing fungi. Three wood-decomposing Armillaria species (A. gallica, A. mellea subsp. nipponica and A. tabescens) known to colonize adult plants and an unidentified species of Polyporales isolated from naturally growing protocorms were tested. Xylobolus annosus, a free-living decomposer of Russulales, was included as a control. Seed germination occurred in a sawdust-based medium in sealed and unsealed containers in the presence of all fungi, indicating low mycorrhizal specificity in germination. Moreover, germination occurred even in modified containers in which the fungus was physically isolated from the orchid seeds, indicating that direct seed-fungus contact is not required. Higher germination percentages were observed in sealed containers in which a modified atmosphere, consisting of a lower O2 and a higher CO2 concentration, had been established in the air above the inoculated medium as a result of the saprophytic activity of the fungus. In nature, atmospheric conditions more effective for seed germination might be established by the action of mycobionts in decomposing wood. Seeds germinated in the presence of the unknown Polyporales species underwent further growth and development, as compared with the other fungi tested, when covered with soil.

► Mycobionts induce germination of the myco-heterotrophic Cyrtosia septentrionalis.
► The orchid plant shows less specificity in both mycobionts and germination.
► The mycobiont isolated from growing protocorms was an unknown Polyporales fungus.
►  Triggers for germination are a modified atmosphere established by mycobionts.
► When the germinated seeds are covered with soil, they undergo further growth.