Abstract A key feature of Orchidaceae is the production of dust‐like seeds that depend on fungal carbon during early development. Although protocorms and mature green orchids typically associate with rhizoctonia fungi, many non‐photosynthetic orchids and some photosynthetic green orchids maintain symbioses with saprotrophic (SAP), non‐rhizoctonia fungi throughout their lives. We examined the mycorrhizal communities of four green Calypsoinae orchids ( Calypso bulbosa , Cremastra variabilis , Oreorchis patens and Tipularia japonica ) at germination. Using seed baiting combined with molecular barcoding, we tested for potential associations with SAP, non‐rhizoctonia fungi at the protocorm stage, given that conspecific adults with coralloid rhizomes, structures morphologically resembling protocorms, often associate with these fungi. We also measured natural abundances of 13 C and 15 N in T. japonica protocorms, coralloid rhizome‐bearing adults, individuals lacking coralloid rhizomes and autotrophic reference plants to quantify the contribution of fungal carbon to plant nutrition. Germination was observed exclusively near adult plants with coralloid rhizomes and decomposing wood, whereas seeds placed near adults lacking coralloid rhizomes failed to germinate. Molecular barcoding revealed that protocorms of all four species were predominantly associated with wood‐decaying fungi. Their mycorrhizal communities closely matched those of conspecific adults with coralloid rhizomes, and the dominant operational taxonomic unit (OTU) was even shared across protocorms and coralloid rhizomes of C. variabilis and O. patens . Stable isotope analyses further indicated that T. japonica protocorms derived all their carbon from wood‐decaying fungi, while coralloid rhizome‐bearing adults obtained more than half of their carbon from these fungi. These results underscore the role of SAP, non‐rhizoctonia fungi in germination and suggest that wood‐derived carbon facilitates seedling establishment in shaded forests. This broadens our understanding of plant adaptation to low‐light environments and deepens our knowledge of the functional diversity of mycorrhizal interactions. Moreover, because many fully mycoheterotrophic Calypsoinae species possess coralloid rhizomes exclusively colonized by SAP non‐rhizoctonia fungi, such germination‐stage associations in green Calypsoinae may have promoted the repeated evolution of full mycoheterotrophy in this group. Read the free Plain Language Summary for this article on the Journal blog.
Suetsugu et al. (2025) studied this question.