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Untangling factors that drive community composition of root associated fungal endophytes of Neotropical epiphytic orchids
Abstract and Figures
In orchids, most of the root-associated fungal endophytes remain undescribed as well as the drivers that affect their interactions with the plants. We characterized root-associated fungal endophytes of co-existing orchids across sites in two areas of montane rainforest in the southern Ecuadorian Andes. We amplified the nrDNA ITS2 region of 130 orchid individuals with Illumina MiSeq technology and tested whether changes in the structure of fungal communities are associated with hosts' phylogeny or the sites where the orchids grow. We identified 3492 OTUs corresponding to the Ascomycota, Basidiomycota, Chytridiomycota, Glomeromycota and Zygomycota phyla. Fungal communities associated with orchids at the lower geographic areas (between 2050 and 2800 m a.s.l.) showed that host evolution and sites are drivers that could shape distinct fungal communities, while at the highest geographic areas (between 3000 and 3500 m a.s.l.), no distinct fungal communities were found neither between co-existing orchid species nor between sites. These results suggested that among orchid species, abiotic and biotic factors do not influence the composition of fungal communities in the same way.
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... Finally, our results match the pattern of detecting a few highly abundant fungal taxa accompanied by a large number of OTUs with low abundances or representing rare taxa in orchid-fungal studies (Jacquemyn et al. 2011Okayama et al. 2012;Martos et al. 2012;Sakamoto et al. 2015;Waud et al. 2016;Cevallos et al. 2017Cevallos et al. , 2018aCevallos et al. , 2018bXing et al. 2017;Qin et al. 2019). Highly-abundant mycorrhizal fungi are expected to play a major part in nutrient and water acquisition and increase a plant's adaptability and resilience (Guimarães et al. 2011;Cevallos et al. 2017), while the role of sequence-scarce and rare OTUs is less clear. ...
... In particular, OTU23 was highly abundant in both species but mostly absent from E. odontochilum and O. klotzschianum (Supplementary Table 2). Although there is evidence that supports orchid niche partitioning through specific mycorrhizal associations (Jacquemyn et al. 2012Pellegrino et al. 2014;Cevallos et al. 2017), a few studies carried out in tropical epiphytic orchids have also encountered important OMF overlaps among coexisting orchid species (Cevallos et al. 2018b;Herrera et al. , 2019. For instance, sympatric individuals of Cyrtochilum pardinum and Epidendrum marsupiale associated with a similar OMF community in at least four different sites in the Andes forests (Cevallos et al. 2018b), which was later supported by Herrera et al. (2019). ...
... Although there is evidence that supports orchid niche partitioning through specific mycorrhizal associations (Jacquemyn et al. 2012Pellegrino et al. 2014;Cevallos et al. 2017), a few studies carried out in tropical epiphytic orchids have also encountered important OMF overlaps among coexisting orchid species (Cevallos et al. 2018b;Herrera et al. , 2019. For instance, sympatric individuals of Cyrtochilum pardinum and Epidendrum marsupiale associated with a similar OMF community in at least four different sites in the Andes forests (Cevallos et al. 2018b), which was later supported by Herrera et al. (2019). Tentatively, co-existing species are not capable of thriving if they use similar resources (Tilman 1982) unless characteristics of the microsite allow for habitat heterogeneity and the segregation of the niche; for example, separation by abiotic conditions of the canopy (Rasmussen and Rasmussen 2018). ...
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.
... Although the diversity and community assembly of root-associated endophytic fungi from epiphytic orchids are poorly understood, several studies have reported that members of Ascomycota and Basidiomycota are the dominant groups (e.g., [15,16]). Therefore, it is necessary to identify and classify the fungal diversity associated with epiphytic orchids [17] as a baseline for future investigations into the potential roles these fungi could be playing in the plant's lifecycle [8]. ...
... A great diversity of mycorrhizal and endophytic fungi was detected in association with the roots of M. acuminata. Despite the few samples, 843 OTUs were recovered, with a threshold of 96% or 97% sequence similarity, which is the commonly used threshold for the clustering of OTUs of fungi [15,27]. Using next-generation sequencing was advantageous for the amplification of fungi associated with the root tissue because far more sequences per sample were obtained compared to traditional methods (e.g., Sanger sequencing) [17,36], leading to a more accurate characterization of fungal communities [46]. ...
... Most of the detected endophytic fungi in this study (~50% of the fungi) belonged to the phylum Ascomycota, mainly in the class Dothideomycetes. These results are congruent with previous studies [15,50], likely due to the advantages of next-generation sequencing technology. In contrast, previous studies using Sanger sequencing reported Sordariomycetes as the most abundant class of endophytic fungi associated with other epiphytic and terrestrial orchid species from tropical and temperate areas (e.g., [8,51]). ...
In natural environments, it has been shown that orchids interact with multiple microorganisms including various species of fungi that colonize their tissues. The diversity of these fungi associated with orchid roots is still being described along with the ecological role they play when interacting with the orchids. In this study, we evaluated the richness and diversity of the endophytic and mycorrhizal fungi associated with the roots of Maxillaria acuminata, a common epiphytic orchid species from a tropical montane forest in southern Ecuador. We characterized the fungal communities by sequencing the ITS2 region of the nrDNA with Illumina MiSeq technology. In total, 843 fungal OTUs were uncovered using a 97% sequence similarity. The highest percentage of OTUs belonged to the Agaricomycetes class, Basidiomycota. Interestingly, the most frequent trophic guild from the analyzed OTUs was assigned as saprophytic. Also, some groups of orchid mycorrhizal-forming fungi were detected, including members within the families Ceratobasidicaceae, Serendipitaceae, Tulasnellaceae, and in the order Atractiellales. We discuss the potential influence of this diverse group of root-associated endophytic fungi on the development and survival of M. acuminata in the tropical forests of southern Ecuador.
... In particular, OTU23 was highly abundant in both species but mostly absent from E. odontochilum and O. klotzschianum (Supplemental Table 2). Although there is a large body of work that supports orchid niche partitioning trough speci c mycorrhizal associations, a few studies carried out in tropical epiphytic orchids have also encountered important OMF overlaps among coexisting orchid species (Cevallos et al. 2018b;Herrera et al. 2018Herrera et al. , 2019. For instance, sympatric individuals of Cyrtochilum pardinum and Epidendrum marsupiale associated with a similar OMF community in at least four different sites in the Andes forests (Cevallos et al. 2018b), which was later supported by Herrera et al. (2019). ...
... Although there is a large body of work that supports orchid niche partitioning trough speci c mycorrhizal associations, a few studies carried out in tropical epiphytic orchids have also encountered important OMF overlaps among coexisting orchid species (Cevallos et al. 2018b;Herrera et al. 2018Herrera et al. , 2019. For instance, sympatric individuals of Cyrtochilum pardinum and Epidendrum marsupiale associated with a similar OMF community in at least four different sites in the Andes forests (Cevallos et al. 2018b), which was later supported by Herrera et al. (2019). Tentatively, co-existing species are not capable of thriving if they use similar resources (Tilman, 1982) unless characteristics of the microsite allow for habitat heterogeneity and the segregation of the niche; for example, separation by abiotic conditions of the canopy (Rasmussen and Rasmussen 2018). ...
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. 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 structure 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 coexisting orchid species display distinct (OMF) communities while keeping a base of shared fungi. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time, a strategy that may serve as a complementary mechanism to fulfill the nutritional needs associated with reproduction. This study provides evidence for niche partitioning in coexisting tropical epiphytic orchids through the utilization of specific OMF, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.
... Two representatives of the genus Xylaria associated with D. andina roots were isolated (Table 1). Xylaria species have been commonly reported as endophytes from numerous orchid species from Costa Rica (Richardson and Currah 1995), Lepanthes species in Puerto Rico (Bayman et al. 1997), several species associated with volcanic areas of central Mexico (Beltrán-Nambo et al. 2018), several species of orchids from the Ecuadorian Andes (Cevallos et al. 2018), Vanilla species in Colombia (Gamboa-Gaitán 2014) and in medicinal Dendrobium species in China (Chen et al. 2011;Wang et al. 2017). Further, a study in Colombia found that Xylaria strains promoted vegetative growth of Vanilla planifolia Andrews seedlings, suggesting that they may be useful in the development of bioinoculants (Ordóñez et al., 2012). ...
... These findings are most evident when it comes to contrasting niches such as dry areas vs. humid areas (Jacquemyn et al. 2016) or different vegetation formations ). However, a study conducted in southern Ecuador (Cevallos et al. 2018), also found a differentiation in the community composition of root associated fungal endophytes from different epiphytic orchids. The authors suggested that despite being the same type of ecosystem (evergreen upper montane forests), each site had particular temperature and rain conditions. ...
The orchid–fungus relationship has been studied since the discovery that the minute seeds of orchids depend on fungi to support the germination process. With the aim of describing the biodiversity of cultivable endophytic and mycorrhizal fungi from the orchid Dichaea andina, we isolated pure fungal cultures from its roots and identified them by sequencing the internal transcribed spacer. We recorded 22 fungal operational taxonomic units belonging to eight orders of Ascomycota: Eurotiales, Hypocreales, Xylariales, Helotiales, Boliniales, Chaetothyriales, Chaetosphaeriales and Pleosporales. The only Basidiomycota isolated belonged to the genus Ceratobasidium from the order Cantharellales, whose members are known as orchid mycorrhizal fungi. At the genus level, we identified 16 genera, the most common of which were Byssochlamys, Camarops, Trichoderma, Cladophialophora, Fusarium and Xylaria; some of them had been reported previously as orchid endophytes. The relevance of endophytic fungi to their hosts is still unclear, but this widely distributed interaction deserves further investigation.
... The root is composed of a cortex covered by the velamen which serves to actively absorb water mixed with residues of organic matter. It has been shown that the velamen secretes metabolites that facilitate the recruitment of microorganisms tolerant to these conditions [32,39], which would obviously involve recruitment from the bark substrate to which the roots are attached. The thick, flat, elliptical leaves, covered with a layer of cuticular wax and stomata, receive a greater amount of light and eventually fall off to be recycled into the canopy or ground soil. ...
Orchids coexist with a diversity of endophytic fungi within their roots and other parts of the plant. These are presumed to contribute to nutrition, and may protect the plants against pathogens and herbivores; however, some may be latent pathogens and/or bring no benefit to the plant. Guarianthe skinneri is an epiphytic Central American threatened orchid used as an ornamental plant and in the rituals and celebrations of many communities. However, in the Soconusco region (Chiapas, Mexico), the pseudobulbs of mature plants are affected by the Lasiodiplodia theobromae fungus, causing the disease “black blotch”. We evaluated and compared the diversity of the endophytic fungal community within the leaves, pseudobulbs and roots of mature plants in two conditions, asymptomatic and symptomatic. Thirty samples from each condition and tissue were amplified with ITS and sequenced by Illumina MiSeq. Sequences were obtained and analyzed to determine taxonomic assignment and functionality with FUNGuild, obtaining 1857 amplicon sequence variants (ASVs). Alpha diversity was similar between plant conditions. In symptomatic plants, significant differences were found between the three types of tissue. According to the FUNGuild functionality analysis, 368 ASVs were determined to be endophytic fungi. The tissues of G. skinneri plants are reservoirs of fungal endophytes that should be considered for further exploration for research and conservation purposes.
... The ITS2 amplicons were prepared for Illumina sequencing using a multiplex strategy in two rounds PCR(Cevallos et al. 2018). The rst round of PCR was performed using forward (fITS7 -GTGARTCATCGAATCTTTG) and reverse (ITS4 -TCCTCCGCTTATTGATATGC) primers containing the proper overhang sequence for Nextera XT DNA Library Preparation Kit. ...
Cattleya milleri is a microendemic orchid of iron-rich rupestrian grasslands in the Brazilian savanna hotspot. It is under critical threat due to illegal collections and habitat destruction. As endophytic and mycorrhizal fungi have potential application during C. milleri propagation and conservation, we investigated its root fungal community. C. milleri roots were sampled in five natural sites and at a greenhouse. Fungal root endophytes were isolated for cultural characterization and molecular ITS (Internal Transcribed Spacer) identification. Total DNA was extracted from root endorhiza and rhizosphere to ITS amplification and sequencing. Sixteen fungal isolates were clustered in 6 Operational Taxonomic Units (OTUs), while endorhizal and rhizospheric sequences were clustered in 327 OTUs. Endorhiza richness (OTUs number) ranged from 25 to 89 OTUs, and rhizosphere presented 56 OTUs. Cluster analysis showed high divergence between natural and greenhouse environments, but a small distance among natural samples. Four phyla, 48 orders, 81 families, and 94 genera were annotated. The putative role of 134 OTUs was annotated, and 24 genera were endophytes, 2 mycorrhizas, 33 pathotrophs, 40 saprotrophs and 17 symbiotrophs. Three orders containing endophytes (Capnodiales, Hypocreales, and Pleosporales) and one containing mycorrhizae (Sebacinales) occurred in all sites. The mycorrhizae Tulasnella occurred in all natural samples. The presence of only two mycorrhizal taxa suggested the mycorrhizae may limit C. milleri distribution. However, many fungi can be recruited from the environment as non-mycorrhizal endophytes. Considering their abundance and role in orchid development, Pleosporales, Tulasnella , and Sebacinalles may be considered for C. milleri propagation and conservation.
... As reported in parasitic plants, the microbial transfer might also occur between the epiphytic plants and their host. Studies in epiphytic orchids reported that the root fungal composition of epiphytic orchids is dominated by Ascomycota and Basidiomycota (Cevallos et al. 2018;Maldonado et al. 2020;Cevallos et al. 2022), while the bacterial composition in the root is dominated by Cyanobacteria (Tsavkelova et al. 2001;Tsavkelova et al. 2003a;Tsavkelova et al. 2003b;Tsavkelova et al. 2022). However, whether the microbial community in the root of epiphytic orchids originated from the host plants is unknown since there is still no study on microbial correlation and interaction between epiphytic plants and their host currently Very few studies have investigated the microbial composition in vascular epiphytic and parasitic plants and their respective host. ...
Differences in bacterial composition between vascular epiphyte and parasitic plants living on the same host plants. Biodiversitas 23: 5798-5805. Epiphytic and parasitic plants can grow and complete their life cycle while attached to the host. Therefore, the interactions between these plants and their host provide profound evidence for co-evolution. During these symbiotic interactions, bacteria are actively exchanged between parasitic and epiphytic plants with their hosts. Since epiphytes and parasitic plants have different ways of life, they might assemble their bacterial community differently despite living in the same host. However, direct microbiome comparisons between epiphytic and parasitic plants colonizing the same host have never been evaluated. In this study, we examined the bacterial compositions of the epiphytic Hoya sp. and parasitic Dendrophthoe sp. that grow in two host species, frangipani (Plumeria sp.) and teak (Tectona grandis). The results revealed that bacterial compositions in the root of Hoya sp. are highly similar to the peripheral tissue of the host stem. In contrast, bacterial composition in the haustoria of Dendrophthoe sp. is quite distinct from the host. These results revealed that epiphytes and parasitic plants acquire and assemble their microbiome differently, despite living in the same host species. These differences might originate from different nutrient acquisition strategies between the two plants.
... Sanger sequencing is the most widespread approach and is still used today for studies in which the strains are not required to be used in germination experiments or to be conserved in mycelium banks for later purposes. Although recent next-generation sequencing (NGS) techniques with Illumina technology offer more complex and robust information on fungi associated with orchids (Cevallos et al. 2016(Cevallos et al. , 2018, this approach is not the most used due to the costs involved, which are not necessarily within the common financial source availability of researchers from developing countries in the region. ...
South America is undoubtedly the cradle of orchid diversity. However, few aspects of the biology of this plant group have been explored in the region. Orchids establish an important relationship with fungi that supply them with nutrients in the early stages of development to stimulate the germination of their tiny seeds. These fungi are called orchid mycorrhizal fungi (OMF), and their interaction with orchids forms a particular group of mycorrhizae: the orchid mycorrhiza (OM). In this chapter, we present the advances of the research conducted in South America, which explores some aspects of this interesting interaction. We have noticed that most studies on OM are academic documents deposited in university library repositories or published in local scientific journals. About half of the studies have focused on determining the diversity of OMF associated with a few orchid species of interest. Studies of phylogeny, morphological, and symbiotic seed germination are other of the main topics addressed. Research on ultrastructure and community ecology is hardly representative, while evolutionary implications, mutualistic networks, and metabolic aspects are the least explored topics. We encourage collaborations with the international scientific community to continue investigating complex questions that allow us to understand the role of mycorrhizas in the evolutionary success of tropical orchids. Moreover, we believe that it is important to propose attractive research that generates interest not only in the academic community but also in ordinary people who have traditionally been related to orchids (e.g., growers) in order to develop the enormous potential of the region in this field.KeywordsMycorrhizal fungi Seed germination Orchidaceae Symbiosis Rhizoctonia-like fungi
... Considering the enormous number of orchid species, their symbionts remain poorly recognized. Noteworthy, most of the experiments on orchid endophytic fungi included only root tissue, [76,77] while in traditional medicine, stems, and leaves are organs usually used for therapies [43,48,49,[78][79][80][81]. The importance of recognition of orchid endophytic fungi for secondary metabolites synthesis and their potential application in medicine were summarized by Sarsaiya et al. [39] and Pant et al. [82]. ...
Species of orchids, which belong to the largest family of flowering plants, are commonly used in folk medicine for the treatment of infections and tumors. However, little is known about the actual chemical composition of these plants and their anticancer properties. In this paper, the most recent literature on orchid-derived bioactive substances with anticancer properties is reviewed. For the assessment, previous papers on the anticancer activity of Orchidaceae published since 2015 were considered. The papers were found by exploring electronic databases. According to the available data, many species of orchids contain potential antitumor chemicals. The bioactive substances in a relatively insignificant number of orchids are identified, and most studies are on Asian taxa. Broader research on American and African species and the correct identification of samples included in the experiments are essential for evaluating the usefulness of orchids as a plant family with vast anticancer potential.
... Methods for proposing functional roles based on fungal classification have been developed (http://www.stbates.org/guilds/app.php), and permit a partial understanding of the role of each fungus detected in a sample of an environment (Cevallos et al. 2018). ...
Orchids depend on mycorrhizal fungi for their nutrition, at least in the early stages of their growth and
development, and in many cases throughout the life. In spite of the increasing number of studies describing fungal
diversity in orchids, there is still more to be learnt about the identity of fungal partners and specificity in orchid
mycorrhizal associations. We investigated the fungal communities associated with the roots of Cephalanthera
damasonium (Mill.) Druce and C. longifolia (L.) Fritsch adult plants, using morphological methods and fungal internal transcribed spacer-DNA polymerase chain reaction amplification, cloning, and sequencing. A range of fungi belonging to Basidiomycota and Ascomycota was uncovered in the roots of the two investigated orchid species, showing a low degree of mycorrhizal specificity. At least 11 fungal taxa, including Cenococcum geophilum Fr., Ceratobasidium sp., Exophiala salmonis J. W. Carmich., Hymenogastraceae, and Sebacinaceae colonized
C. damasonium roots, while approximately 9 fungal types, such as Bjerkandera adusta (Willd.) P. Karst., Phlebia
acerina Peck, Sebacinaceae, Tetracladium sp., and Tomentella sp. associated with C. longifolia. Phylogenetic and
statistical analyses indicated significant differences in the fungal communities associated with the two studied
Cephalanthera species, as well as distinct mycorrhizal partners associated with each orchid plant. Our results
strongly suggest that both C. damasonium and C. longifolia are generalist in their mycorrhizal associations.
In epiphytic orchids, distinctive groups of fungi are involved in the symbiotic association. However, little is known about the factors that determine the mycorrhizal community structure. Here, we analyzed the orchid mycorrhizal fungi communities associated with three sympatric Cymbidieae epiphytic tropical orchids (Cyrtochilum flexuosum, Cyrtochilum myanthum, and Maxillaria calantha) at two sites located within the mountain rainforest of southern Ecuador. To characterize these communities at each orchid population, the ITS2 region was analyzed by Illumina MiSeq technology. Fifty-five mycorrhizal fungi operational taxonomic units (OTUs) putatively attributed to members of Serendipitaceae, Ceratobasidiaceae and Tulasnellaceae were identified. Significant differences in mycorrhizal communities were detected between the three sympatric orchid species as well as among sites/populations. Interestingly, some mycorrhizal OTUs overlapped among orchid populations. Our results suggested that populations of studied epiphytic orchids have site-adjusted mycorrhizal communities structured around keystone fungal species. Interaction with multiple mycorrhizal fungi could favor orchid site occurrence and co-existence among several orchid species.
Plants associate through their roots with fungal assemblages that impact their abundance and productivity. Non-mycorrhizal endophytes constitute an important component of such fungal diversity, but their implication in ecosystem processes is little known. Using a selection of 128 root-endophytic strains, we defined functional groups based on their traits and plant interactions with potential to predict community assembly and symbiotic association processes. In vitro tests of the strains’ interactions with Arabidopsis thaliana, Microthlaspi erraticum and Hordeum vulgare showed a net negative effect of fungal colonization on plant growth. The effects partly depended on the phylogenetic affiliation of strains, but also varied considerably depending on the plant-strain combination. The variation was partly explained by fungal traits shared by different lineages, like growth rates or melanization. The origin of strains also affected their symbioses, with endophytes isolated from Microthlaspi spp. populations being more detrimental to M. erraticum than strains from other sources. Our findings suggest that plant–endophyte associations are subject to local processes of selection, in which particular combinations of symbionts are favored across landscapes. We also show that different common endophytic taxa have differential sets of traits found to affect interactions, hinting to a functional complementarity that can explain their frequent co-existence in natural communities.
Biotic interactions play an important role in the assembly and stability of communities. All orchids depend on mycobionts for early establishment, but whether individual orchid species depend on a specific or broad spectrum of mycobionts is still a matter of debate. Tulasnellaceae (Basidiomycota) is the richest and most widespread mycobiont worldwide. We assessed Tulasnellaceae richness in epiphytic and terrestrial orchids in different habitats, and evaluated the degree of generalism in orchid-Tulasnellaceae interactions and the robustness of this mutualistic system to the extinction of mycobiont partners. We sampled 114 orchid individuals including all common and rare species in 56 plots of 1 m² in 3 habitats: pristine forest, regenerating forest and a landslide site in a tropical montane rainforest in Southern Ecuador. We found 52 orchid and 29 Tulasnellaceae species. The composition of Tulasnellaceae OTUs was moderately to highly similar across habitats and between orchid growth forms. A significantly nested network architecture indicated the existence of a core of generalist Tulasnellaceae OTUs interacting with both rare and common orchids. Terrestrial and epiphytic orchids showed significant differences in robustness to the extinction of their Tulasnellaceae mycobionts. Thus, generalist mycobionts may be relevant for the preservation of hyperdiverse orchid communities in the tropics.
Knowledge of fungal root-associates is essential for effective conservation of tropical epiphytic orchids. We investigated the diversity of root-associated fungi of Cyrtochilum myanthum, Scaphyglottis punctulata and Stelis superbiens from a tropical mountain rainforest in southern Ecuador, using a culture dependent approach. We identified 115 fungal isolates, corresponding to 49 fungal OTUs, based on sequences of the nrDNA ITS and partial 28S region. Members of Ascomycota were unambiguously dominant (37 OTUs), including Trichoderma sp. as the most frequent taxon. Members of Basidiomycota (Agaricales and Polyporales) and Mucoromycota (Umbelopsidales and Mortierellales) were also identified. Four potential mycorrhizal OTUs of Tulasnellaceae and Ceratobasidiaceae were isolated from C. myanthum and S. superbiens. Fungal community composition was examined using Sørensen and Jaccard indices of similarity. Alfa diversity was significantly different between C. myanthum and S. superbiens. No difference in beta diversity of the fungal communities between the 3 orchid species and the collecting sites was detected. The study revealed a high diversity of fungi associated with orchid roots. Our results contribute to a better understanding of specific relationships between epiphytic orchids and their root-associated fungi.
Healthy plant roots usually host heterogeneous communities of root endophytes that are abundant in all plants and ecosystems. Despite their abundance, current understanding of the distribution of these endophytes is superficial at the best. A comprehensive treatise of the distribution of variety of fungi described as root endophytes would most likely prove an exhausting exercise in futility, given the large gaps in current data. As a result, we broadly target the helotialean endophytes that are commonly observed in temperate and boreal, forested ecosystems and the pleosporalean endophytes commonly observed in grassland ecosystems. To explore the sparse data available for the distribution of these root-associated fungal endophytes, we combine references from current literature and a case study to evaluate whether or not the endophyte communities (1) shift over environmental gradients, (2) respond to some specific drivers, or (3) seem limited by dispersal by showing distance-decay responses. The data indicate that communities of endophytic fungi indeed change over large geographical scales. The available data suggest a Helotiales-dominated endophyte guild in forested ecosystems and a Pleosporales-dominated assemblage in grassland ecosystems. Further, large-scale studies and our preliminary analyses of communities across North American grassland ecosystems suggest some shifts corresponding to geographical/environmental gradients. However, specific drivers are difficult to distinguish because abiotic gradients are often correlated, resulting in complex model selection problems. Finally, we observed that our own and published data reach no agreement on distance decay, even though both agree that the distance decay may be of rather minor importance.
Orchids are highly dependent on their mycorrhizal fungal partners for nutrient supply, especially during early developmental stages. In addition to organic carbon, nitrogen (N) is probably a major nutrient transferred to the plant because orchid tissues are highly N‐enriched. We know almost nothing about the N form preferentially transferred to the plant or about the key molecular determinants required for N uptake and transfer.
We identified, in the genome of the orchid mycorrhizal fungus Tulasnella calospora , two functional ammonium transporters and several amino acid transporters but found no evidence of a nitrate assimilation system, in agreement with the N preference of the free‐living mycelium grown on different N sources.
Differential expression in symbiosis of a repertoire of fungal and plant genes involved in the transport and metabolism of N compounds suggested that organic N may be the main form transferred to the orchid host and that ammonium is taken up by the intracellular fungus from the apoplatic symbiotic interface.
This is the first study addressing the genetic determinants of N uptake and transport in orchid mycorrhizas, and provides a model for nutrient exchanges at the symbiotic interface, which may guide future experiments.