Isotopic and molecular data support mixotrophy in Ophioglossum at the sporophytic stage

Mycoheterotrophy in the wood-wide web

The prevalence and potential functions of common mycorrhizal networks, or the 'wood-wide web', resulting from the simultaneous interaction of mycorrhizal fungi and roots of different neighbouring plants have been increasingly capturing the interest of science and society, sometimes leading to hyperbole and misinterpretation. Several recent reviews conclude that popular claims regarding the widespread nature of these networks in forests and their role in the transfer of resources and information between plants lack evidence. Here we argue that mycoheterotrophic plants associated with ectomycorrhizal or arbuscular mycorrhizal fungi require resource transfer through common mycorrhizal networks and thus are natural evidence for the occurrence and function of these networks, offering a largely overlooked window into this methodologically challenging underground phenomenon. The wide evolutionary and geographic distribution of mycoheterotrophs and their interactions with a broad phylogenetic range of mycorrhizal fungi indicate that common mycorrhizal networks are prevalent, particularly in forests, and result in net carbon transfer among diverse plants through shared mycorrhizal fungi. On the basis of the available scientific evidence, we propose a continuum of carbon transfer options within common mycorrhizal networks, and we discuss how knowledge on the biology of mycoheterotrophic plants can be instrumental for the study of mycorrhizal-mediated transfers between plants.

Prevalent arbuscular mycorrhizae in roots and highly variable mycobiome in leaves of epiphytic subtropical fern Ophioderma pendulum

PREMISE

Endophytic and mycorrhizal fungi are crucial in facilitating plant nutrition acquisition and stress tolerance. In epiphytic habitats, plants face nutrition and water stress, but their roots are mostly nonmycorrhizal and especially lacking in arbuscular mycorrhizal associations. Ophioderma pendulum is an epiphytic fern with a partially mycoheterotrophic lifestyle, likely heavily reliant on symbiotic fungi. To characterize fungal associations in the sporophyte of O. pendulum, we focused on leaves and roots of O. pendulum, seeking to reveal the fungal communities in these organs.

METHODS

Roots and leaves from O. pendulum in a subtropical forest were examined microscopically to observe the morphology of fungal structures and determine the percentage of various fungal structures in host tissues. Fungal composition was profiled using metabarcoding techniques that targeted ITS2 of the nuclear ribosomal DNA.

RESULTS

Roots were consistently colonized by arbuscular mycorrhizal fungi (Glomeromycota), especially Acaulospora. Unlike previous findings on epiphytic ferns, dark septate endophytes were rare in O. pendulum roots. Leaves were predominantly colonized by Ascomycota fungi, specifically the classes Dothideomycetes (46.88%), Eurotiomycetes (11.51%), Sordariomycetes (6.23%), and Leotiomycetes (6.14%). Across sampling sites, fungal community compositions were similar in the roots but differed significantly in the leaves.

CONCLUSIONS

Ophioderma pendulum maintains stable, single-taxon-dominant communities in the roots, primarily featuring arbuscular mycorrhizal fungi, whereas the leaves may harbor opportunistic fungal colonizers. Our study underlines the significance of mycorrhizal fungi in the adaptation of epiphytic ferns.

Organellar phylogenomics of Ophioglossaceae fern genera

Previous phylogenies showed conflicting relationships among the subfamilies and genera within the fern family Ophioglossaceae. However, their classification remains unsettled where contrasting classifications recognize four to 15 genera. Since these treatments are mostly based on phylogenetic evidence using limited, plastid-only loci, a phylogenomic understanding is actually necessary to provide conclusive insight into the systematics of the genera. In this study, we have therefore compiled datasets with the broadest sampling of Ophioglossaceae genera to date, including all fifteen currently recognized genera, especially for the first time the South African endemic genus Rhizoglossum. Notably, our comprehensive phylogenomic matrix is based on both plastome and mitogenome genes. Inferred from the coding sequences of 83 plastid and 37 mitochondrial genes, a strongly supported topology for these subfamilies is presented, and is established by analyses using different partitioning approaches and substitution models. At the generic level, most relationships are well resolved except for few within the subfamily Ophioglossoideae. With this new phylogenomic scheme, key morphological and genomic changes were further identified along this backbone. In addition, we confirmed numerous horizontally transferred (HGT) genes in the genera Botrypus, Helminthostachys, Mankyua, Sahashia, and Sceptridium. These HGT genes are most likely located in mitogenomes and are predominately donated from angiosperm Santalales or non-Ophioglossaceae ferns. By our in-depth searches of the organellar genomes, we also provided phylogenetic overviews for the plastid and mitochondrial MORFFO genes found in these Ophioglossaceae ferns.

Mycoheterotrophy

Merckx and Gomes introduce mycoheterotrophy, the ability of a plant to take up its carbon from a close fungal partner.

Mycobiome detection from a single subterranean gametophyte using metabarcoding techniques

PREMISE

Several ferns and lycophytes produce subterranean gametophytes, including the Ophioglossaceae, Psilotaceae, and some members of the Schizaeaceae, Gleicheniaceae, and Lycopodiaceae. Despite the surge in plant-microbiome research, which has been particularly boosted by high-throughput sequencing techniques, the microbiomes of these inconspicuous fern gametophytes have rarely been examined. The subterranean gametophytes are peculiar due to their achlorophyllous nature, which makes them rely on fungi to obtain nutrients. Furthermore, the factors that shape the fungal communities (mycobiomes) of fern gametophytes have not been examined in depth.

METHODS AND RESULTS

We present a workflow to study the mycobiome of the achlorophyllous gametophytes of Ophioderma pendulum using a high-throughput metabarcoding approach. Simultaneously, each gametophyte was investigated microscopically to detect fungal structures. Two primer sets of the nuclear ITS sequence targeting general fungi were applied, in addition to a primer set that specifically targets the nuclear small subunit ribosomal rDNA region of arbuscular mycorrhizal fungi. Both the microscopic and metabarcoding approaches revealed many diverse fungi inhabiting a single gametophyte of O. pendulum.

DISCUSSION

This study provides methodological details and discusses precautions for the mycobiome investigation of achlorophyllous gametophytes. This research is also the first to uncover the mycobiome assembly of an achlorophyllous gametophyte of an epiphytic fern.

Mycobiont diversity and first evidence of mixotrophy associated with Psathyrellaceae fungi in the chlorophyllous orchid Cremastra variabilis

Mixotrophy (MX, also called partial mycoheterotrophy) in plants is characterized by isotopic abundances that differ from those of autotrophs. Previous studies have evaluated mycoheterotrophy in MX plants associated with fungi of similar ecological characteristics, but little is known about the differences in the relative abundances of ¹³C and ¹⁵N in an orchid species that associates with several different mycobionts species. Since the chlorophyllous orchid Cremastra variabilis Nakai associates with various fungi with different ecologies, we hypothesized that it may change its relative abundances of ¹³C and ¹⁵N depending on the associated mycobionts. We investigated mycobiont diversity in the chlorophyllous orchid C. variabilis together with the relative abundance of ¹³C and ¹⁵N and morphological underground differentiation (presence or absence of a mycorhizome with fungal colonization). Rhizoctonias (Tulasnellaceae, Ceratobasidiaceae, Sebacinales) were detected as the main mycobionts. High differences in δ¹³C values (- 34.7  to - 27.4 ‰) among individuals were found, in which the individuals associated with specific Psathyrellaceae showed significantly high relative abundance of ¹³C. In addition, Psathyrellaceae fungi were always detected on individuals with mycorhizomes. In the present study, MX orchid association with non-rhizoctonia saprobic fungi was confirmed, and the influence of mycobionts on morphological development and on relative abundance of ¹³C and ¹⁵N was discovered. Cremastra variabilis may increase opportunities to gain nutrients from diverse partners, in a bet-hedging plasticity that allows colonization of various environmental conditions.

Mycoheterotrophic seedling growth of Gentiana zollingeri, a photosynthetic Gentianaceae plant species, in symbioses with arbuscular mycorrhizal fungi

We found mycoheterotrophic seedling growth (initial mycoheterotrophy) of Gentiana zollingeri, a spring-flowering photosynthetic species of Gentianaceae family. Small seeds (about 300 µm in length) were buried in a habitat by using seed packets, and development of the subterranean seedlings to form shoots, more than 3 cm in length, was observed in symbiosis with arbuscular mycorrhizal (AM) fungi in the dark (i.e., underground of a field). Hyphal coils and their degenerations were observed in the root cortical cells of the subterranean seedlings as well as those of adult plants. Among the mycobionts identified on the basis of partial small subunit rDNA sequences, it was found that AM fungi of a lineage in Glomeraceae dominantly colonized, and the AM fungi were also dominant in adult individuals of G. zollingeri in three habitats separated one another by 17.2, 34.7, and 49.6 km. Though initial mycoheterotrophy in symbioses with AM fungi has been observed in some pteridophytes, this is the first study to demonstrate this type of symbiosis in a photosynthetic seed plant. The mycoheterotrophy means that an energy distribution occurs through the hyphal bridges of AM fungi among different photosynthetic seed plants, which may be important in constructing plant species diversity in some ecosystems.

Partial mycoheterotrophy is common among chlorophyllous plants with Paris-type arbuscular mycorrhiza

BACKGROUND AND AIMS

An arbuscular mycorrhiza is a mutualistic symbiosis with plants as carbon providers for fungi. However, achlorophyllous arbuscular mycorrhizal species are known to obtain carbon from fungi, i.e. they are mycoheterotrophic. These species all have the Paris type of arbuscular mycorrhiza. Recently, two chlorophyllous Paris-type species proved to be partially mycoheterotrophic. In this study, we explore the frequency of this condition and its association with Paris-type arbuscular mycorrhiza.

METHODS

We searched for evidence of mycoheterotrophy in all currently published 13C, 2H and 15N stable isotope abundance patterns suited for calculations of enrichment factors, i.e. isotopic differences between neighbouring Paris- and Arum-type species. We found suitable data for 135 plant species classified into the two arbuscular mycorrhizal morphotypes.

KEY RESULTS

About half of the chlorophyllous Paris-type species tested were significantly enriched in 13C and often also enriched in 2H and 15N, compared with co-occurring Arum-type species. Based on a two-source linear mixing model, the carbon gain from the fungal source ranged between 7 and 93 % with ferns > horsetails > seed plants. The seed plants represented 13 families, many without a previous record of mycoheterotrophy. The 13C-enriched chlorophyllous Paris-type species were exclusively herbaceous perennials, with a majority of them thriving on shady forest ground.

CONCLUSIONS

Significant carbon acquisition from fungi appears quite common and widespread among Paris-type species, this arbuscular mycorrhizal morphotype probably being a pre-condition for developing varying degrees of mycoheterotrophy.

Mycorrhizal communities of two closely related species, Pyrola subaphylla and P. japonica, with contrasting degrees of mycoheterotrophy in a sympatric habitat

Mycoheterotrophic plants typically form associations with a narrow range of mycorrhizal fungi. Consequently, mycorrhizal specialization is often considered to be an important step in mycoheterotrophic evolution. However, it remains unclear whether such specialization is likely to occur in plants of the genus Pyrola, which are generally associated with fungi in multiple ectomycorrhizal families. Here, we investigated the mycorrhizal communities of a nearly fully mycoheterotrophic Pyrola species (Pyrola subaphylla), a closely related partially mycoheterotrophic Pyrola species (Pyrola japonica), and a co-occurring autotrophic ectomycorrhizal tree, Quercus crispula, which is their potential carbon source, in a cool-temperate Japanese forest. High-throughput DNA sequencing revealed that numerous common ectomycorrhizal OTUs interact with the two Pyrola species and Q. crispula, thereby providing an opportunity to exploit a certain amount of carbon from common mycorrhizal networks. In addition, not only P. japonica but also P. subaphylla exhibited exceptionally high alpha mycobiont diversity, with 52 ectomycorrhizal OTUs belonging to 12 families being identified as P. subaphylla mycobionts and 69 ectomycorrhizal OTUs in 18 families being detected as P. japonica mycobionts. Nonetheless, the beta mycobiont diversity of P. subaphylla and P. japonica individuals was significantly lower than that of Q. crispula. Moreover, the beta mycobiont diversity of P. subaphylla was found to be significantly lower than that of P. japonica. Therefore, despite their seemingly broad mycorrhizal interactions, the two Pyrola species (particularly P. subaphylla) showed consistent fungal associations, suggesting that mycorrhizal specialization may have developed during the course of mycoheterotrophic evolution within the genus Pyrola.

Evidence for mycorrhizal cheating in Apostasia nipponica, an early-diverging member of the Orchidaceae

Most land plants, from liverworts to angiosperms, form mutualistic mycorrhizal symbioses with fungal partners. However, several plants known as mycoheterotrophs exploit fungal partners by reversing the polarity of carbon movement, which usually moves from plant to fungus. We investigated the physiological ecology of a photosynthetic orchid, Apostasia nipponica, which belongs to the first branching group within the Orchidaceae, to improve our understanding of mycoheterotrophic evolution in orchids. The fungal symbionts and nutrition modes of A. nipponica were investigated using molecular barcoding and carbon-13 (¹³ C) and nitrogen-15 (¹⁵ N) measurements, respectively. Community profiling based on a metabarcoding technique revealed that A. nipponica associates with specific Ceratobasidium spp. within ectomycorrhizas-forming clades, whereas isotope analysis revealed that A. nipponica was similar to fully mycoheterotrophic orchids in its ¹³ C signature and was even more enriched in ¹⁵ N than most of the fully mycoheterotrophic orchids that exploit ectomycorrhizal fungi. Our molecular and mass-spectrometric approaches demonstrated, for the first time, that a member of the Apostasioideae, the earliest-diverging lineage of the Orchidaceae, gains carbon through both photosynthesis and fungal cheating (i.e. partial mycoheterotrophy) during the adult stage.