The rare Australian epiphytic orchid Sarcochilus weinthalii associates with a single species of Ceratobasidium

Efficiency of mycorrhizal fungi for seed germination and protocorms development of commercial Cattleya species (Orchidaceae)

Orchidaceae is one of the largest plant families and stands out for its wide variety of flowers with ornamental and environmental importance. Cattleya is one of the main commercial genera, presenting a great diversity of species and hybrids that attract the attention of collectors, orchid enthusiasts, and consumers. In their natural environment, orchids associate with mycorrhizal fungi, which are responsible for providing carbon and other nutrients during seed germination. This study investigated the potential of mycorrhizal fungi isolated from the genus Cattleya for in vitro symbiotic germination of seeds from three contrasting Cattleya species, comparing them with non-symbiotic germination in a commercially used culture medium for orchid propagation. The isolated fungi were molecularly identified through phylogenetic analyses of DNA sequences using the ITS (Internal Transcribed Spacer) region. Three isolates obtained were identified as Tulasnella amonilioides, and through microscopic evaluations, the formation of monilioid cells was observed, a morphological characteristic previously unknown for this species. The T. amonilioides isolates were efficient in promoting seed germination of Cattleya bicolor, Cattleya walkeriana and Cattleya jongheana and accelerated the germination process when compared with the non-symbiotic commercial medium, showing to be promised for commercial seed production of these orchids species.

Continental-scale distribution and diversity of Ceratobasidium orchid mycorrhizal fungi in Australia

BACKGROUND AND AIMS

Mycorrhizal fungi are a critical component of the ecological niche of most plants and can potentially constrain their geographical range. Unlike other types of mycorrhizal fungi, the distributions of orchid mycorrhizal fungi (OMF) at large spatial scales are not well understood. Here, we investigate the distribution and diversity of Ceratobasidium OMF in orchids and soils across the Australian continent.

METHODS

We sampled 217 Ceratobasidium isolates from 111 orchid species across southern Australia and combined these with 311 Ceratobasidium sequences from GenBank. To estimate the taxonomic diversity of Ceratobasidium associating with orchids, phylogenetic analysis of the ITS sequence locus was undertaken. Sequence data from the continent-wide Australian Microbiome Initiative were used to determine the geographical range of operational taxonomic units (OTUs) detected in orchids, with the distribution and climatic correlates of the two most frequently detected OTUs modelled using MaxEnt.

KEY RESULTS

We identified 23 Ceratobasidium OTUs associating with Australian orchids, primarily from the orchid genera Pterostylis, Prasophyllum, Rhizanthella and Sarcochilus. OTUs isolated from orchids were closely related to, but distinct from, known pathogenic fungi. Data from soils and orchids revealed that ten of these OTUs occur on both east and west sides of the continent, while 13 OTUs were recorded at three locations or fewer. MaxEnt models suggested that the distributions of two widespread OTUs are correlated with temperature and soil moisture of the wettest quarter and far exceeded the distributions of their host orchid species.

CONCLUSIONS

Ceratobasidium OMF with cross-continental distributions are common in Australian soils and frequently have geographical ranges that exceed that of their host orchid species, suggesting these fungi are not limiting the distributions of their host orchids at large spatial scales. Most OTUs were distributed within southern Australia, although several OTUs had distributions extending into central and northern parts of the continent, illustrating their tolerance of an extraordinarily wide range of environmental conditions.

Fungal diversity driven by bark features affects phorophyte preference in epiphytic orchids from southern China

Epiphytic orchids exhibit varying degrees of phorophyte tree specificity. We performed a pilot study to investigate why epiphytic orchids prefer or avoid certain trees. We selected two orchid species, Panisea uniflora and Bulbophyllum odoratissimum co-occurring in a forest habitat in southern China, where they showed a specific association with Quercus yiwuensis and Pistacia weinmannifolia trees, respectively. We analysed a number of environmental factors potentially influencing the relationship between orchids and trees. Difference in bark features, such as water holding capacity and pH were recorded between Q. yiwuensis and P. weinmannifolia, which could influence both orchid seed germination and fungal diversity on the two phorophytes. Morphological and molecular culture-based methods, combined with metabarcoding analyses, were used to assess fungal communities associated with studied orchids and trees. A total of 162 fungal species in 74 genera were isolated from bark samples. Only two genera, Acremonium and Verticillium, were shared by the two phorophyte species. Metabarcoding analysis confirmed the presence of significantly different fungal communities on the investigated tree and orchid species, with considerable similarity between each orchid species and its host tree, suggesting that the orchid-host tree association is influenced by the fungal communities of the host tree bark.

Progress and Prospects of Mycorrhizal Fungal Diversity in Orchids

Orchids form mycorrhizal symbioses with fungi in natural habitats that affect their seed germination, protocorm growth, and adult nutrition. An increasing number of studies indicates how orchids gain mineral nutrients and sometime even organic compounds from interactions with orchid mycorrhizal fungi (OMF). Thus, OMF exhibit a high diversity and play a key role in the life cycle of orchids. In recent years, the high-throughput molecular identification of fungi has broadly extended our understanding of OMF diversity, revealing it to be a dynamic outcome co-regulated by environmental filtering, dispersal restrictions, spatiotemporal scales, biogeographic history, as well as the distribution, selection, and phylogenetic spectrum width of host orchids. Most of the results show congruent emerging patterns. Although it is still difficult to extend them to all orchid species or geographical areas, to a certain extent they follow the "everything is everywhere, but the environment selects" rule. This review provides an extensive understanding of the diversity and ecological dynamics of orchid-fungal association. Moreover, it promotes the conservation of resources and the regeneration of rare or endangered orchids. We provide a comprehensive overview, systematically describing six fields of research on orchid-fungal diversity: the research methods of orchid-fungal interactions, the primer selection in high-throughput sequencing, the fungal diversity and specificity in orchids, the difference and adaptability of OMF in different habitats, the comparison of OMF in orchid roots and soil, and the spatiotemporal variation patterns of OMF. Further, we highlight certain shortcomings of current research methodologies and propose perspectives for future studies. This review emphasizes the need for more information on the four main ecological processes: dispersal, selection, ecological drift, and diversification, as well as their interactions, in the study of orchid-fungal interactions and OMF community structure.

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.

Reproductive and dispersal strategies shape the diversity of mycobiont-photobiont association in Cladonia lichens

Ecological preferences, partner compatibility, or partner availability are known to be important factors shaping obligate and intimate lichen symbioses. We considered a complex of Cladonia species, traditionally differentiated by the extent of sexual reproduction and the type of vegetative propagules, to assess if the reproductive and dispersal strategies affect mycobiont-photobiont association patterns. In total 85 lichen thalli from 72 European localities were studied, two genetic markers for both Cladonia mycobionts and Asterochloris photobionts were analyzed. Variance partitioning analysis by multiple regression on distance matrices was performed to describe and partition variance in photobiont genetic diversity. Asexually reproducing Cladonia in our study were found to be strongly specific to their photobionts, associating with only two closely related Asterochloris species. In contrast, sexually reproducing lichens associated with seven unrelated Asterochloris lineages, thus being photobiont generalists. The reproductive mode had the largest explanatory power, explaining 44% of the total photobiont variability. Reproductive and dispersal strategies are the key factors shaping photobiont diversity in this group of Cladonia lichens. A strict photobiont specialisation observed in two studied species may steer both evolutionary flexibility and responses to ecological changes of these organisms, and considerably limit their distribution ranges.

Influence of host tree species on isolation and communities of mycorrhizal and endophytic fungi from roots of a tropical epiphytic orchid, Dendrobium sinense (Orchidaceae)

Most studies on the host preference of orchids have focused on the association between orchids and host characteristics, but little is known about the differences of mycorrhizal and endophytic fungal communities in epiphytic orchids growing on different host tree species. We selected Dendrobium sinense, a tropical epiphytic orchid, to determine if fungal endophytes from the roots of D. sinense were preferentially correlated with host tree species. Fifty-six fungal operational taxonomic units (OTUs) from 36 host trees were identified. The results indicated that the species richness and diversity of mycorrhizal and endophytic fungal communities isolated from D. sinense roots were strongly influenced by host tree species. Both species richness and diversity indices showed that D. sinense roots on Syzygium buxifolium harbored the most diverse and abundant endophytic fungi. Species of Tulasnellaceae were dominant on S. buxifolium and Rhododendron moulmainense but infrequent on Cyclobalanopsis disciformis and Podocarpus neriifolius. Our results provide evidence for distinct mycorrhizal and endophytic fungal communities on different host tree species. Further research focusing on fungi-orchid-host preference could be conducted to increase our understanding for the in situ conservation of epiphytic orchids.

Mycorrhizal preferences and fine spatial structure of the epiphytic orchid Epidendrum rhopalostele

PREMISE OF THE STUDY

The presence of compatible fungi is necessary for epiphytic orchid recruitment. Thus, identifying associated mycorrhizal fungi at the population level is essential for orchid conservation. Recruitment patterns may also be conditioned by factors such as seed dispersal range and specific environmental characteristics.

METHODS

In a forest plot, all trees with a diameter at breast height >1 cm and all individuals of the epiphytic orchid Epidendrum rhopalostele were identified and mapped. Additionally, one flowering individual of E. rhopalostele per each host tree was randomly selected for root sampling and DNA extraction.

KEY RESULTS

A total of 239 E. rhopalostele individuals were located in 25 of the 714 potential host trees. Light microscopy of sampled roots showed mycorrhizal fungi in 22 of the 25 sampled orchids. Phylogenetic analysis of ITS1-5.8S-ITS2 sequences yielded two Tulasnella clades. In four cases, plants were found to be associated with both clades. The difference between univariate and bivariate K functions was consistent with the random labeling null model at all spatial scales, indicating that trees hosting clades A and B of Tulasnella are not spatially segregated. The analysis of the inhomogenous K function showed that host trees are not clustered, suggesting no limitations to population-scale dispersal. χ(2) analysis of contingency tables showed that E. rhopalostele is more frequent on dead trees than expected.

CONCLUSIONS

EPIDENDRUM RHOPALOSTELE establishes mycorrhizal associations with at least two different Tulasnella species. The analysis of the distribution patterns of this orchid suggests a microsite preference for dead trees and no seed dispersal limitation.

The host bias of three epiphytic Aeridinae orchid species is reflected, but not explained, by mycorrhizal fungal associations

PREMISE OF THE STUDY

The three co-occurring epiphytic orchid species, Sarcochilus hillii, Plectorrhiza tridentata, and Sarcochilus parviflorus vary in host specificity; all are found predominantly on the tree Backhousia myrtifolia but some also associate with a broad range of species. Despite this specialization, no fitness advantage has been detected for adult orchid plants growing on the preferred host. Therefore, we predicted that the host specialization of these orchid species is a consequence of a bias toward particular orchid mycorrhizal fungi, which are in turn biased toward particular woody plant species.

METHODS

To test this hypothesis, we sampled representatives of each orchid species on B. myrtifolia and other host species across sites. Rhizoctonia-like fungi were isolated from orchid roots and identified using molecular markers.

KEY RESULTS

Three groups of fungi were identified, and the orchid species varied in their specificity for these. All fungal groups were found on the host B. myrtifolia; yet at all sites, only one orchid species, S. hillii, associated with all three groups.

CONCLUSIONS

Our results demonstrate that these orchid species did vary in their mycorrhizal specificity; however, the distribution of their mycorrhizal associates did not directly explain their host associations. Rather, we propose that the mycorrhizal relationship of these orchid species is complex and have suggested future avenues of research.