Mycorrhizal specificity differences in epiphytic habitat: three epiphytic orchids harbor distinct ecological and physiological specificity

Orchidaceae has diversified in tree canopies and accounts for 68% of vascular epiphytes. Differences in mycorrhizal communities among epiphytic orchids can reduce species competition for mycorrhizal fungi and contribute to niche partitioning, which may be a crucial driver of the unusual species diversification among orchids. Mycorrhizal specificity-the range of fungi allowing mycorrhizal partnerships-was evaluated by assessment of mycorrhizal communities in the field (ecological specificity) and symbiotic cultures in the laboratory (physiological specificity) for three epiphytic orchids inhabiting Japan. Mycorrhizal communities were assessed with co-existing individuals growing within 10 cm of each other, revealing that ecological specificity varied widely among the three species, ranging from dominance by a single Ceratobasidiaceae fungus to diverse mycobionts across the Ceratobasidiaceae and Tulasnellaceae. In vitro seed germination tests revealed clear differences in physiological specificity among the three orchids, and that the primary mycorrhizal partners contributed to seed germination. In vitro compatibility ranges of three orchids strongly reflect the mycorrhizal community composition of wild populations. This suggests that differences in in situ mycorrhizal communities are not strongly driven by environmental factors, but are primarily due to physiological differences among orchid species. This study shows that the symbiotic strategy among the epiphytic orchid species varies from specialized to generalized association, which may contribute to biotic niche partitioning.

Dominant Dendrobium officinale mycorrhizal partners vary among habitats and strongly induce seed germination in vitro

Dendrobium officinale (Orchidaceae) is an endangered epiphytic orchid that has been well studied as a medicinal plant. Although previous studies have shown that various fungal isolates promote D. officinale seed germination and seedling development in vitro, mycorrhizal associations among its wild populations remain poorly understood. In this study, we identified mycorrhizal fungi associated with D. officinale (36 individuals from six sites) using Sanger sequencing and compared fungal communities among sites and habitats (lithophytic vs. epiphytic individuals). Among the obtained sequences, 76 belonged to orchid mycorrhizal fungi (OMF), among which Tulasnellaceae accounted for 45.8% and Serendipitaceae for 28.1%. The Serendipitaceae operational taxonomic unit (OTU) SE1 was the most dominant partner, accounting for 27.1% of all detected fungal sequences, followed by a Tulasnellaceae OTU, TU27, which accounted for 15.6%. The relative frequencies of Serendipitaceae and Tulasnellaceae differed greatly between lithophytic and epiphytic individuals. Serendipitaceae accounted for 47.3% of the OMF sequences among lithophytes, and Tulasnellaceae for 95.2% among epiphytes. Mycorrhizal community composition also varied among sites. We further conducted in vitro symbiotic culture from seeds with six fungal isolates. Two Serendipitaceae and two Tulasnellaceae isolates, including SE1 and TU27, significantly promoted seed germination and seedling development. These results indicate that D. officinale is mainly associated with Tulasnellaceae and Serendipitaceae as its main fungal partners, which strongly induced seed germination and seedling development in vitro, suggesting their association with D. officinale through its life cycle.

Strong primer bias for Tulasnellaceae fungi in metabarcoding: Specific primers improve the characterization of the mycorrhizal communities of epiphytic orchids

Primer bias toward Tulasnellaceae fungi during PCR is a known issue with metabarcoding analyses for the assessment of orchid mycorrhizal communities. However, this bias had not been evaluated for the fungal communities of epiphytic orchids, which account for 69% of all orchid species diversity. We compared the mycorrhizal communities detected using two primer pairs, a fungal universal primer pair (ITS86F/ITS4) and Tulasnella-specific primer pair (5.8STulngs/ITS4-Tul2), using a mock community of fungal isolates from epiphytic orchids and also environmental samples, including orchid roots and a tree bark tip from the host tree of an epiphytic orchid collected. The detected mycorrhizal communities differed widely depending on the primer pairs used. The fungal universal primer pair successfully identified Ceratobasidiaceae and Serendipitaceae fungi but did not reflect Tulasnellaceae diversity. Tulasnellaceae fungi were mainly detected using the Tulasnella-specific primer pair. These tendencies were observed in both the mock community and environmental samples. These results strongly suggest that the use of a Tulasnella-specific primer in combination with a fungal universal primer is essential for assessing the mycorrhizal communities of orchids through metabarcoding analysis, especially in epiphytic orchids. Our study contributes to further understanding of the diversity of mycorrhizal fungi in orchids.

The mycorrhizal community of the epiphytic orchid Thrixspermum japonicum is strongly biased toward a single Ceratobasidiaceae fungus, despite a wide range of fungal partners

PREMISE

Orchids depend primarily on mycorrhizal fungi to obtain nutrients throughout their life cycle. Epiphytic orchids account for 69% of orchid diversity. The unstable availability of water and nutrients in their arboreal habitats often results in severe water and nutrient stresses. Consequently, mycorrhizal associations may be important for the survival of epiphytic orchids, but our understanding thereof remains limited. Here, we investigated the mycorrhizal community in a single epiphytic orchid species, using more samples than in any previous study.

METHODS

We assessed the mycorrhizal communities of Thrixspermum japonicum, one of the most common epiphytic orchids in the temperate region of Japan. In total, 144 individuals were collected from 28 host tree species at 20 sites across 1300 km. The mycorrhizal fungi were identified based on nuclear ribosomal DNA internal transcribed spacer sequences and assigned operational taxonomic units (OTUs) based on 97% sequence similarity.

RESULTS

We obtained 24 OTUs; 9 belonged to the Ceratobasidiaceae and 15 to the Tulasnellaceae. These OTUs are widely distributed throughout the phylogenetic trees of the two fungal families. However, a single Ceratobasidiaceae OTU accounted for 49.7% of all fungal sequences and was predominant in samples from 15 host tree species and 12 sites.

CONCLUSIONS

Our results imply that despite having a broad range of mycorrhizal partners, T. japonicum was predominantly associated with a single fungal taxon at most of the sites among the host-tree species investigated. These findings contribute to elucidating mycorrhizal symbiosis in epiphytic habitats.

A leafless epiphytic orchid, Taeniophyllum glandulosum Blume (Orchidaceae), is specifically associated with the Ceratobasidiaceae family of basidiomycetous fungi

Leafless epiphytes in the Orchidaceae undergo a morphological metamorphosis in which the root has chloroplast-containing cortical cells and is the sole photosynthetic organ for carbon gain. All orchids are entirely dependent on mycorrhizal fungi for their carbon supply during seed germination, and this mycorrhizal association generally persists in adult plants. However, our knowledge of the mycorrhizal association of leafless epiphytic orchids remains limited, and the contribution of the mycorrhizal association to nutrient acquisition in these orchid species is largely unknown. In this study, the mycorrhizal fungi of a leafless epiphytic orchid, Taeniophyllum glandulosum, were identified molecularly using 68 mature plants and 17 seedlings. In total, 187 fungal internal transcribed spacer sequences were obtained, of which 99% were identified as Ceratobasidiaceae. These sequences were classified into five operational taxonomic units (OTUs) based on 97% sequence similarity. The most frequent sequence was OTU1, which accounted for 91% of all Ceratobasidiaceae sequences, although other phylogenetically distinct Ceratobasidiaceae fungi were detected. These results show that T. glandulosum is specifically associated with a particular group of Ceratobasidiaceae. All mycorrhizal fungi found in T. glandulosum seedlings belonged to OTU1, which was also found in adult plants on the same host tree. The mycorrhizal fungi from 13 host tree species were compared, and T. glandulosum was preferentially associated with OTU1 on 11 tree species. In conclusion, T. glandulosum is specifically associated with Ceratobasidiaceae fungi and this specific association remains throughout the orchid life cycle and is found on divergent host tree species.

A method for facilitating the seed germination of a mycoheterotrophic orchid, Gastrodia pubilabiata, using decomposed leaf litter harboring a basidiomycete fungus, Mycena sp

BACKGROUND

Mycoheterotrophic plants are one of the most difficult plant groups to conserve because they are entirely dependent on symbiotic fungi. Establishment of viable culture systems would greatly aid their conservation. We describe a simple culture system for the mycoheterotrophic orchid, Gastrodia pubilabiata, that does not require laboratory facilities. The orchid is symbiotic with leaf-litter-decomposing fungi.

RESULTS

Gastrodia pubilabiata seeds were incubated in plastic boxes or glass bottles filled with leaf litter collected from the natural habitat of the species. Seed germination was observed after 35 days and seedling development followed. Fungal isolates from seedlings were identified as Mycenaceae (Basidiomycota), a leaf-litter-decomposing mycorrhizal fungus of Gastrodia species.

CONCLUSION

Our method can be used to conserve endangered mycoheterotrophic plants associated with leaf litter-decomposing fungi efficiently, and can also serve as a model system for physiological and molecular studies of such plants.