Fungal association and root morphology shift stepwise during ontogenesis of orchidCremastra appendiculatatowards autotrophic nutrition

Abstract

Background and Aims The chlorophyllous, terrestrial orchid Cremastra appendiculata from East Asia is unique concerning its fungal mycorrhiza partners. The initially mycoheterotrophic protocorms exploit rather specialised 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. Methods We compared different development stages of C. appendiculata to surrounding autotrophic reference plants based on multi-element natural abundance stable isotope analyses (δ 13C, δ 15N, δ 2H, δ 18O) 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. Key Results 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 colonised 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 15N, 13C and 2H compared to reference plants. Seedlings showed an intermediate isotopic signature, underpinning the differences in the fungal community depending on their subterranean morphology. Conclusions 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 role of mycorrhizal fungi in driving ecotype formation in mycoheterotrophic plants

“…3d), a fully mycoheterotrophic species, and its photosynthetic sister species, Cremastra appendiculata (Fig. 3c), within the C. appendiculata species complex (Yagame et al 2021, Zahn et al 2022. It has been observed that C. aphylla, which inhabits dark…”

“…3d), a fully mycoheterotrophic species, and its photosynthetic sister species, Cremastra appendiculata (Fig. 3c), within the C. appendiculata species complex (Yagame et al 2021, Suetsugu et al 2022, Zahn et al 2022). It has been observed that C. aphylla , which inhabits dark shaded understory, is consistently associated with wood‐decaying Psathyrellaceae, whereas C. appendiculata , which inhabits somewhat more open habitats, is mainly associated with rhizoctonias.…”