Protocorm-Supporting Fungi Are Retained in Roots of Mature Tipularia discolor Orchids as Mycorrhizal Fungal Diversity Increases

Co-occurring orchid species associated with different low-abundance mycorrhizal fungi from the soil in a high-diversity conservation area in Denmark

Plant-fungal interactions are ubiquitous across ecosystems and contribute significantly to plant ecology and evolution. All orchids form obligate symbiotic relationships with specific fungi for germination and early growth, and the distribution of terrestrial orchid species has been linked to occurrence and abundance of specific orchid mycorrhizal fungi (OMF) in the soil. The availability of OMF can therefore be a habitat requirement that is relevant to consider when establishing management and conservation strategies for threatened orchid species, but knowledge on the spatial distribution of OMF in soil is limited. We here studied the mycorrhizal associations of three terrestrial orchid species (Anacamptis pyramidalis, Orchis purpurea and Platanthera chlorantha) found in a local orchid diversity hotspot in eastern Denmark, and investigated the abundance of the identified mycorrhizal fungi in the surrounding soil. We applied ITS metabarcoding to samples of orchid roots, rhizosphere soil and bulk soil collected at three localities, supplemented with standard barcoding of root samples with OMF specific primers, and detected 22 Operational Taxonomic Units (OTUs) putatively identified as OMF. The three orchid species displayed different patterns of OMF associations, supporting the theory that association with specific fungi constitutes part of an orchid's ecological niche allowing co-occurrence of many species in orchid-rich habitats. The identified mycorrhizal partners in the basidiomycete families Tulasnellaceae and Ceratobasidiaceae (Cantharallales) were detected in low abundance in rhizosphere soil, and appeared almost absent from bulk soil at the localities. This finding highlights our limited knowledge of the ecology and trophic mode of OMF outside orchid tissues, as well as challenges in the detection of specific OMF with standard methods. Potential implications for management and conservation strategies are discussed.

Habitat-related plastome evolution in the mycoheterotrophic Neottia listeroides complex (Orchidaceae, Neottieae)

BACKGROUND

Mycoheterotrophs, acquiring organic carbon and other nutrients from mycorrhizal fungi, have evolved repeatedly with substantial plastid genome (plastome) variations. To date, the fine-scale evolution of mycoheterotrophic plastomes at the intraspecific level is not well-characterized. A few studies have revealed unexpected plastome divergence among species complex members, possibly driven by various biotic/abiotic factors. To illustrate evolutionary mechanisms underlying such divergence, we analyzed plastome features and molecular evolution of 15 plastomes of Neottia listeroides complex from different forest habitats.

RESULTS

These 15 samples of Neottia listeroides complex split into three clades according to their habitats approximately 6 million years ago: Pine Clade, including ten samples from pine-broadleaf mixed forests, Fir Clade, including four samples from alpine fir forests and Fir-willow Clade with one sample. Compared with those of Pine Clade members, plastomes of Fir Clade members show smaller size and higher substitution rates. Plastome size, substitution rates, loss and retention of plastid-encoded genes are clade-specific. We propose to recognized six species in N. listeroides complex and slightly modify the path of plastome degradation.

CONCLUSIONS

Our results provide insight into the evolutionary dynamics and discrepancy of closely related mycoheterotrophic orchid lineages at a high phylogenetic resolution.

Co-occurring epiphytic orchids have specialized mycorrhizal fungal niches that are also linked to ontogeny

Mycorrhizal symbiosis has been related to the coexistence and community assembly of coexisting orchids in few studies despite their obligate dependence on mycorrhizal partners to establish and survive. In hyper-diverse environments like tropical rain forests, coexistence of epiphytic orchids may be facilitated through mycorrhizal fungal specialization (i.e., sets of unique and dominant mycorrhizal fungi associated with a particular host species). However, information on the role of orchid mycorrhizal fungi (OMF) in niche differentiation and coexistence of epiphytic orchids is still scarce. In this study, we sought to identify the variation in fungal preferences of four co-occurring epiphytic orchids in a tropical rainforest in Costa Rica by addressing the identity and composition of their endophytic fungal and OMF communities across species and life stages. We show that the endophytic fungal communities are formed mainly of previously recognized OMF taxa, and that the four coexisting orchid species have both a set of shared mycorrhizal fungi and a group of fungi unique to an orchid species. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time. This study provides evidence for the utilization of specific OMF that may be involved in niche segregation, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.

Isolation and identification of beneficial orchid mycorrhizal fungi in Paphiopedilum barbigerum (Orchidaceae)

Seed germination and seedling development in nearly all orchid species rely on a symbiotic relationship with mycorrhizal fungi; however, this is not the case with all mycorrhizal fungi. This study aims to provide an understanding about the important role of mycorrhiza in seed germination and growth of Paphiopedilum barbigerum. Therefore, we isolated and identified endophytic fungi from the roots of wild P. barbigerum. The beneficial mycorrhizal fungi Epulorhiza sp. FQXY019 and Tulasnella calospora FQXY017 were screened by seed symbiotic germination tests and found to promote seed germination. However, only the seeds inoculated with FQXY019 progressed from the seed germination to rooting stage. This shows that mycorrhizal fungi and P. barbigerum have a specific relation at different growth phases. In addition, we selected FQXY019 and inoculated it into MS medium, B5 medium, OMA medium, and PDA medium. The results showed that FQXY019 co-cultured on PDA significantly promoted the increase in seedling fresh weight, leaf length, and root length (p < .01). Furthermore, it significantly promoted the root number and leaf number of seedlings compared with those co-cultured on MS, B5, and OMA media and control (p < .05). Thus, this study demonstrated the promoting effect of Epulorhiza sp. FQXY019 on seed germination and seedling development, making it an alternative method for the artificial propagation of P. barbigerum.

The challenges of growing orchids from seeds for conservation: An assessment of asymbiotic techniques

Lewis Knudson first successfully germinated orchid seeds asymbiotically on artificial medium in 1922. While many orchid species have since been grown asymbiotically, the tremendous variation in how species respond to artificial medium and growth conditions ex situ has also become apparent in the past century. In this study, we reviewed published journal articles on asymbiotic orchid seed germination to provide a summary of techniques used and to evaluate if these differ between terrestrial and epiphytic species, to identify areas where additional research is needed, and to evaluate whether asymbiotic germination could be used more often in ex situ conservation. We found articles reporting successful asymbiotic germination of 270 species and 20 cultivars across Orchidaceae. Researchers often used different techniques with epiphytic versus terrestrial species, but species-specific responses to growth media and conditions were common, indicating that individualized protocols will be necessary for most species. The widespread success in generating seedlings on artificial media suggests that asymbiotic techniques should be another tool for the conservation of rare orchid species. Further advances are needed in understanding how to introduce mycorrhizae to axenically grown orchids and to maximize the viability of seedlings reintroduced into natural habitats to fully utilize these methods for conservation.

How Does Deforestation Affect the Growth of Cypripedium (Orchidaceae) Species? A Simulation Experiment in Northeast China

Due to wild habitat destruction, Cypripedium is among the most endangered groups in China. Determining how Cypripedium respond to environmental changes is curial to their conservation. However, less is known about the effect of deforestation on the growth of Cypripedium. In this study, we selected four Cypripedium species in Northeast China, and conducted conservation-based transplantation simulating deforestation to explore the impact of increased light intensity on the growth of Cypripedium. After three years, the maximum net photosynthetic rate was decreased by 15.9%, 11.5%, 13.6% and 5.3% for C. calceolus L., C. guttatum Sw., C. macranthos Sw. and C.×ventricosum Sw., respectively, resulting in poor viability, manifesting as shorter and thinner shoots, and smaller leaves. Unexpectedly, no significant traits shifts were found in the roots across four species, which may be related to the long root lifespan and conservation. Our research confirmed that increased light intensity caused by deforestation would lead to an increase in respirate cost and a decrease in photosynthate accumulation, and consequently the recession of plant growth. Except for habitat loss, individual plant reduction caused by deforestation could be responsible for the population decline of Cypripedium.

Mycorrhizal Fungal Partners Remain Constant during a Root Lifecycle of Pleione bulbocodioides (Orchidaceae)

Mycorrhizal mutualisms are vital for orchids through germination to adulthood. Fungal species diversity and community composition vary across seasons and plant development stages and affect plant survival, adaptation, and community maintenance. Knowledge of the temporal turnover of mycorrhizal fungi (OMF) remains poorly understood in the eco-physiologically diverse orchids (especially in epiphytic orchids), although it is important to understand the function and adaptation of mycorrhizae. Some species of Pleione are epiphytic plants with annual roots and may recruit different fungal partners during their root lifecycle. Based on continuous samplings of Pleione bulbocodioides during a whole root lifecycle, we characterized the fungal temporal dynamics using Illumina sequencing of the ITS2 region. Our data showed that the plants of P. bulbocodioides were quickly colonized by OMF at root emergence and had a constant OMF composition throughout one root lifecycle, although the OMF richness declined with root aging after a peak occurrence during root elongation. In contrast, the richness of root-inhabiting fungal endophytes kept increasing with root aging and more drastic turnovers were found in their species compositions. Our findings of OMF temporal turnover contribute to further understanding of mycorrhizal associations and adaptation of Orchidaceae and will benefit orchid resource conservation and utilization.