Enigmatic Sebacinales

A historical retrospect and a taxonomic update will deal with Sebacina s.l. and s.str., Craterocolla, Efibulobasidium, Serendipita, Tremellodendron, Tremelloscypha, Tremellostereum, and Piriformospora, the Sebacinaceae, and the Sebacinales. Phylogenetic hypotheses for the order and subordinal taxa are discussed, including environmental sequence taxa. The cryptic biodiversity in Sebacinales is extensive but mostly unresolved with respect to the species involved. Trophic stages are manifold in Sebacinales but restricted to plant dependencies. Most of the species grow endophytically or form various mycorrhizae, but Craterocolla and Efibulobasidium species appear to be saprobic. The sebacinalean mycorrhizal diversity is unparalleled: ectomycorrhizae, ericoid and orchid mycorrhizae are frequent, both in autotrophs and heterotrophs, as well as mycothalli with Jungermanniales. Mycorrhizal community structures are difficult to evaluate in Sebacinales because of the high percentage of environmental sequence taxa lacking further characteristics. Nutritional requirements and exchanges have been studied extensively in Piriformospora indica, suggesting future possibilities for agricultural applications. The genomes of this species and of Sebacina vermifera have been sequenced recently, thus opening new fields in studying and understanding functional and evolutionary aspects.     

Communities and populations of sebacinoid basidiomycetes associated with the achlorophyllous orchid Neottia nidus-avis (L.) L.C.M. Rich. and neighbouring tree ectomycorrhizae

Several achlorophyllous orchids associate with ectomycorrhizal hymenomycetes deriving carbon from surrounding trees for the plant. However, this has not been shown for achlorophyllous orchids associating with species of Rhizoctonia, a complex of basal lineages of hymenomycetes that are the most common orchid partners. We analysed Neottia nidus-avis, an achlorophyllous orchid symbiotic with a Rhizoctonia, to identify its symbionts by internal transcribed spacer (ITS) sequencing. Analysis of 61 root systems from 23 French populations showed that N. nidus-avis associates highly specifically with a group of species of Sebacinaceae. Their diversity emphasizes the need for further investigations in the Sebacinaceae systematics. Sebacinoid ITS sequences were often identical within orchid populations and a trend to regional variation in symbionts was observed. Using ITS and intergenic spacer (IGS) polymorphism, we showed that each root system harboured a single species, but that several genets colonized it. However, no polymorphism of these markers was found among portions of each root: this is consistent with the putative mode of entry of the fungus, i.e. from the rhizome into roots but not repeatedly from the soil. In addition, ectomycorrhizae were always found within the N. nidus-avis root systems: 120 of the 144 ectomycorrhizae typed by ITS sequencing were colonized by a sebacinoid fungus identical in ITS sequence to the respective orchid symbiont (even for the IGS polymorphism in some cases). Because sebacinoids were demonstrated recently to be ectomycorrhizal, the orchid is likely to derive its resources from surrounding trees, a mycorrhizal cheating strategy similar to other myco-heterotrophic plants studied to date.     

Parallel evolutionary paths to mycoheterotrophy in understorey Ericaceae and Orchidaceae: ecological evidence for mixotrophy in Pyroleae

Several forest understorey achlorophyllous plants, termed mycoheterotrophs (MHs), obtain C from their mycorrhizal fungi. The latter in turn form ectomycorrhizas with trees, the ultimate C source of the entire system. A similar nutritional strategy occurs in some green forest orchids, phylogenetically close to MH species, that gain their C via a combination of MH and photosynthesis (mixotrophy). In orchid evolution, mixotrophy evolved in shaded habitats and preceded MH nutrition. By generalizing and applying this to Ericaceae, we hypothesized that green forest species phylogenetically close to MHs are mixotrophic. Using stable C isotope analysis with fungi, autotrophic, mixotrophic and MH plants as comparisons, we found the first quantitative evidence for substantial fungi-mediated mixotrophy in the Pyroleae, common ericaceous shrubs from boreal forests close to the MH Monotropoideae. Orthilia secunda, Pyrola chlorantha, Pyrola rotundifolia and Chimaphila umbellata acquired between 10.3 and 67.5% of their C from fungi. High N and ¹⁵N contents also suggest that Pyroleae nutrition partly rely on fungi. Examination of root fungal internal transcribed spacer sequences at one site revealed that 39 species of mostly endophytic or ectomycorrhizal fungi, including abundant Tricholoma spp., were associated with O. secunda, P. chlorantha and C. umbellata. These fungi, particularly ectomycorrhizal associates, could thus link mixotrophic Pyroleae spp. to surrounding trees, allowing the C flows deduced from isotopic evidence. These data suggest that we need to reconsider ecological roles of understorey plants, which could influence the dynamics and composition of forest communities.     

Saprotrophic fungal mycorrhizal symbionts in achlorophyllous orchids: Finding treasures among the ‘molecular scraps’?

Mycoheterotrophic plants are achlorophyllous plants that obtain carbon from their mycorrhizal fungi. They are usually considered to associate with fungi that are (1) specific of each mycoheterotrophic species and (2) mycorrhizal on surrounding green plants, which are the ultimate carbon source of the entire system. Here we review recent works revealing that some mycoheterotrophic plants are not fungal-specific, and that some mycoheterotrophic orchids associate with saprophytic fungi. A re-examination of earlier data suggests that lower specificity may be less rare than supposed in mycoheterotrophic plants. Association between mycoheterotrophic orchids and saprophytic fungi arose several times in the evolution of the two partners. We speculate that this indirectly illustrates why transition from saprotrophy to mycorrhizal status is common in fungal evolution. Moreover, some unexpected fungi occasionally encountered in plant roots should not be discounted as ‘molecular scraps’, since these facultatively biotrophic encounters may evolve into mycorrhizal symbionts in some other plants.Key words: endophytic fungi, evolution of mycorrhizae, mycoheterophy, mycorrhizae, saprophytic fungi, specificityConsiderable advances were recently made in the ecology of achlorophyllous, heterotrophic plants that obtain carbon from their mycorrhizal fungi (Fig. 1). Most plants have contact with soil through mycorrhizal symbioses, in which roots associate with a suitable fungal partner. Fungi utilize soil mineral nutrients, and while sharing them with host plants, they generally receive carbon as a reward. In contrast, some achlorophyllous plants living in the shaded forest understorey have reversed the process. They receive carbon from their mycorrhizal fungi exclusively, hence the designation ‘mycoheterotrophic’ (MH) plants.¹ Mycoheterotrophy has appeared several times during the evolution of land plants, and more than 20 times among orchids that encompass half of all MH species.² In the last decade, the development of molecular tools has enabled researchers to identify many fungal symbionts, which are often uncultivable. The fungi occurring in the densely colonized roots of MH species often produce a stronger PCR signal than any fungal contaminant, making molecular tools very effective for this field of study.Open in a separate windowFigure 1Wullschlaegelia aphylla, a mycoheterotrophic orchid unspecifically associated with saprotrophic Mycena and Gymnopus species. (A) Whole plant at flowering time, with reduced, tuberoid root system at that period. (B) Section of mycorrhizal root showing intracellular hyphal pelotons at early stage (p), or late stage (undergoing lysis, lp); among orchids, the colonization of dead cortical cell (cc) is a unique feature to some saprotrophic fungi (picture by A. Faccio, University of Torino).     

Species diversity and drivers of spread of alien fungi (<Emphasis Type="Italic">sensu lato</Emphasis>) in Europe with a particular focus on France

A first comprehensive inventory of alien fungi and fungal-like organisms (in Stramenopila) recorded in France since 1800 was established, comprising 227 species, with 64.7% plant pathogens, 29.5% saprotrophic species, 3.5% ectomycorrhizal fungi, 1.3% animal parasites and 0.9% mycopathogenic fungi. Using this and a previously built European dataset, correlates of invasion success in fungi (sensu lato) were investigated, especially for pathogenic species occurring in wild environments (mostly forest tree pathogens). Several common features were demonstrated at the two spatial scales. Some taxonomic/phylogenetic orders were shown to be over-represented in alien fungi and Stramenopila pseudo-fungi, e.g. Peronosporales and to have faster spread, e.g. Erysiphales. Residence time and economic variables, especially imports, were important explaining variables of the levels of invasion. The influence of climatic factors was also suggested.     

Fungal associates of <Emphasis Type="Italic">Pyrola rotundifolia</Emphasis>, a mixotrophic Ericaceae,from two Estonian boreal forests

Pyrola rotundifolia (Ericaceae, Pyroleae tribe) is an understorey subshrub that was recently demonstrated to receive considerable amount of carbon from its fungal mycorrhizal associates. So far, little is known of the identity of these fungi and the mycorrhizal anatomy in the Pyroleae. Using 140 mycorrhizal root fragments collected from two Estonian boreal forests already studied in the context of mixotrophic Ericaceae in sequence analysis of the ribosomal DNA internal transcribed spacer region, we recovered 71 sequences that corresponded to 45 putative species in 19 fungal genera. The identified fungi were mainly ectomycorrhizal basidiomycetes, including Tomentella, Cortinarius, Russula, Hebeloma, as well as some ectomycorrhizal and/or endophytic ascomycetes. The P. rotundifolia fungal communities of the two forests did not differ significantly in terms of species richness, diversity and nutritional mode. The relatively high diversity retrieved suggests that P. rotundifolia does not have a strict preference for any fungal taxa. Anatomical analyses showed typical arbutoid mycorrhizae, with variable mantle structures, uniseriate Hartig nets and intracellular hyphal coils in the large epidermal cells. Whenever compared, fungal ultrastructure was congruent with the molecular identification. Similarly to other mixotrophic and autotrophic pyroloids in the same forests, P. rotundifolia shares its mycorrhizal fungal associates with surrounding trees that are likely a carbon source for pyroloids.