Isotopic evidence of arbuscular mycorrhizal cheating in a grassland gentian species

Ubiquitous arbuscular mycorrhizal fungi in the roots of herbaceous understory plants with hyphal degeneration in Colchicaceae and Gentianaceae

Due to the loss of photosynthetic ability during evolution, some plant species rely on mycorrhizal fungi for their carbon source, and this nutritional strategy is known as mycoheterotrophy. Mycoheterotrophic plants forming Paris-type arbuscular mycorrhizas (AM) exhibit two distinctive mycorrhizal features: degeneration of fungal materials and specialization towards particular fungal lineages. To explore the possibility that some understory AM plants show partial mycoheterotrophy, i.e., both photosynthetic and mycoheterotrophic nutritional strategies, we investigated 13 green herbaceous plant species collected from five Japanese temperate forests. Following microscopic observation, degenerated hyphal coils were observed in four species: two Colchicaceae species, Disporum sessile and Disporum smilacinum, and two Gentianaceae species, Gentiana scabra and Swertia japonica. Through amplicon sequencing, however, we found that all examined plant species exhibited no specificity toward AM fungi. Several AM fungi were consistently found across most sites and all plant species studied. Because previous studies reported the detection of these AM fungi from various tree species in Japanese temperate forests, our findings suggest the presence of ubiquitous AM fungi in forest ecosystems. If the understory plants showing fungal degeneration exhibit partial mycoheterotrophy, they may obtain carbon compounds indirectly from a wide range of surrounding plants utilizing such ubiquitous AM fungi.

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.

Impact of mycoheterotrophy on the growth of Gentiana zollingeri (Gentianaceae), as suggested by size variation, morphology, and 13C abundance of flowering shoots

Gentiana zollingeri is an annual photosynthetic plant that employs a mycoheterotrophic growth strategy during its underground seedling stage (initial mycoheterotrophy). Notably, the morphological characteristics of its flowering shoots, such as shoot size, leaf size, and leaf color, are highly variable, and it was hypothesized that these variations may be linked to nutritional mode. The morphological characteristics of G. zollingeri individuals were thus investigated alongside environmental factors, 13C abundance, and diversity of colonizing arbuscular mycorrhizal (AM) fungi. The majority of G. zollingeri flowering individuals were found to exhibit a high affinity for the specific AM fungi that exclusively colonize roots of the mycoheterotrophic seedlings, while other phylogenetically diverse AM fungi could also be detected. The leaves to shoot dry weight ratio (leaf ratio) was negatively correlated with the canopy openness in the habitat, suggesting that leaf development is impeded in sunny conditions. Furthermore, the shoot weight of G. zollingeri was positively correlated with leaf 13C abundance. Given that 13C enrichment can provide indirect evidence of mycoheterotrophy in AM plants, the results suggest that the utilization of carbon obtained through mycoheterotrophy, at least during the underground seedling stage, is crucial for G. zollingeri.

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.

Evolutionary histories and mycorrhizal associations of mycoheterotrophic plants dependent on saprotrophic fungi

Mycoheterotrophic plants (MHPs) are leafless, achlorophyllous, and completely dependent on mycorrhizal fungi for their carbon supply. Mycorrhizal symbiosis is a mutualistic association with fungi that is undertaken by the majority of land plants, but mycoheterotrophy represents a breakdown of this mutualism in that plants parasitize fungi. Most MHPs are associated with fungi that are mycorrhizal with autotrophic plants, such as arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. Although these MHPs gain carbon via the common mycorrhizal network that links the surrounding autotrophic plants, some mycoheterotrophic lineages are associated with saprotrophic (SAP) fungi, which are free-living and decompose leaf litter and wood materials. Such MHPs are dependent on the forest carbon cycle, which involves the decomposition of wood debris and leaf litter, and have a unique biology and evolutionary history. MHPs associated with SAP fungi (SAP-MHPs) have to date been found only in the Orchidaceae and likely evolved independently at least nine times within that family. Phylogenetically divergent SAP Basidiomycota, mostly Agaricales but also Hymenochaetales, Polyporales, and others, are involved in mycoheterotrophy. The fungal specificity of SAP-MHPs varies from a highly specific association with a single fungal species to a broad range of interactions with multiple fungal orders. Establishment of symbiotic culture systems is indispensable for understanding the mechanisms underlying plant-fungus interactions and the conservation of MHPs. Symbiotic culture systems have been established for many SAP-MHP species as a pure culture of free-living SAP fungi is easier than that of biotrophic AM or ECM fungi. Culturable SAP-MHPs are useful research materials and will contribute to the advancement of plant science.

Stable C and N isotope natural abundances of intraradical hyphae of arbuscular mycorrhizal fungi

Data for stable C and N isotope natural abundances of arbuscular mycorrhizal (AM) fungi are currently sparse, as fungal material is difficult to access for analysis. So far, isotope analyses have been limited to lipid compounds associated with fungal membranes or storage structures (biomarkers), fungal spores and soil hyphae. However, it remains unclear whether any of these components are an ideal substitute for intraradical AM hyphae as the functional nutrient trading organ. Thus, we isolated intraradical hyphae of the AM fungus Rhizophagus irregularis from roots of the grass Festuca ovina and the legume Medicago sativa via an enzymatic and a mechanical approach. In addition, extraradical hyphae were isolated from a sand-soil mix associated with each plant. All three approaches revealed comparable isotope signatures of R. irregularis hyphae. The hyphae were 13C- and 15N-enriched relative to leaves and roots irrespective of the plant partner, while they were enriched only in 15N compared with soil. The 13C enrichment of AM hyphae implies a plant carbohydrate source, whereby the enrichment was likely reduced by an additional plant lipid source. The 15N enrichment indicates the potential of AM fungi to gain nitrogen from an organic source. Our isotope signatures of the investigated AM fungus support recent findings for mycoheterotrophic plants which are suggested to mirror the associated AM fungi isotope composition. Stable isotope natural abundances of intraradical AM hyphae as the functional trading organ for bi-directional carbon-for-mineral nutrient exchanges complement data on spores and membrane biomarkers.

The effects of gibberellin on the expression of symbiosis-related genes in Paris-type arbuscular mycorrhizal symbiosis in Eustoma grandiflorum

Arbuscular mycorrhiza (AM) is a symbiotic interaction in terrestrial plants that is colonized by fungi in the Glomeromycotina. The morphological types of AM, including the Arum-type and Paris-type, are distinct, depending on the host plant species. A part of the regulatory pathways in Arum-type AM symbiosis has been revealed because most model plants form the Arum-type AM with a model AM fungus, Rhizophagus irregularis. Moreover, gibberellin (GA) is known to severely inhibit AM fungal colonization in Arum-type AM symbiosis. Recently, we showed that exogenous GA treatment significantly promoted AM fungal colonization in Paris-type AM symbiosis in Eustoma grandiflorum. In this study, we focused on the transcriptional changes in AM symbiosis-related genes in GA-treated E. grandiflorum. The expression levels of all examined E. grandiflorum genes were maintained or increased by GA treatment compared with those of the control treatment. Our new results suggest that signaling pathway(s) required for establishing AM symbiosis in E. grandiflorum may be distinct from the well-characterized pathway for that in model plants.