Fungal association and root morphology shift stepwise during ontogenesis of orchid Cremastra appendiculata towards autotrophic nutrition

The chlorophyllous, terrestrial orchid Cremastra appendiculata from East Asia is unique concerning its fungal mycorrhiza partners. The initially mycoheterotrophic protocorms exploit rather specialized non-rhizoctonia saprotrophic Psathyrellaceae. Adult individuals of this orchid species are either linked to Psathyrellaceae being partially mycoheterotrophic or form mycorrhiza with fungi of the ubiquitous saprotrophic rhizoctonia group. This study provides new insights on nutrition mode, subterranean morphology and fungal partners across different life stages of C. appendiculata. We compared different development stages of C. appendiculata to surrounding autotrophic reference plants based on multi-element natural abundance stable isotope analyses (delta C-13, delta N-15, delta H-2, delta O-18) and total N concentrations. Site- and sampling-time-independent enrichment factors of stable isotopes were used to reveal trophic strategies. We determined mycorrhizal fungi of C. appendiculata protocorm, seedling and adult samples using high-throughput DNA sequencing. We identified saprotrophic non-rhizoctonia Psathyrellaceae as dominant mycorrhizal fungi in protocorm and seedling rhizomes. In contrast, the roots of seedlings and mature C. appendiculata were mainly colonized with fungi belonging to the polyphyletic assembly of rhizoctonia (Ceratobasidium, Thanatephorus and Serendipitaceae). Mature C. appendiculata did not differ in isotopic signature from autotrophic reference plants suggesting a fully autotrophic nutrition mode. Characteristic of orchid specimens entirely relying on fungal nutrition, C. appendiculata protocorms were enriched in N-15, C-13 and H-2 compared to reference plants. Seedlings showed an intermediate isotopic signature, underpinning the differences in the fungal community depending on their subterranean morphology. In contrast to the suggestion that C. appendiculata is a partially mycoheterotrophic orchid species, we provide novel evidence that mature C. appendiculata with rhizoctonia mycobionts can be entirely autotrophic. Besides an environmentally driven variability among populations, we suggest high within-individual flexibility in nutrition and mycobionts of C. appendiculata, which is subject to the ontogenetic development stage. The green orchid Cremastra appendiculatafrom East Asia is unique, because it changes fungal mycorrhiza partners during development. In the early seedling stage this orchid exploits wood- or litter-decomposing fungi, while adults either continue being linked to these wood- or litter-decomposers or switch towards ubiquitous saprotrophic fungi of the rhizoctonia group. This study provides new insights on changes in subterranean morphology and nutrition mode accompanying this fungal partner switch. It turned out that only rhizomes were colonized by the wood- or litter-decomposing fungi while roots of adults were colonized by rhizoctonia. While early seedlings fully relied on fungi as nutrient source, adults without rhizomes were fully autotrophic.