Soil persistence and biodiversity of ericoid mycorrhizal fungi in the absence of the host plant in a Mediterranean ecosystem

The occurrence of suitable mycorrhizal inocula may be an important factor affecting the dynamics of plant communities. We investigated the persistence and diversity of ericoid mycorrhizal fungi in the soil of a mature Quercus ilex forest where ericaceous hosts were absent. Erica arborea was used as a bait plant and results were compared to soil samples from experimental plots where cuttings had allowed reappearance of this ericaceous species. Fungal endophytes were isolated and tested in mycorrhiza resynthesis trials. Sterile mycorrhizal endophytes were assigned to morphotypes whose consistency was confirmed by ITS-RFLP. The ITS region of a representative of each morphotype was sequenced. BLAST searches and Neighbour-Joining analysis indicated taxonomic affinities with different classes within Ascomycota. Our results indicate that ericoid mycorrhizal fungi persist and maintain mycorrhizal ability in habitats lacking the ericaceous host. Their persistence could favour the establishment of E. arborea seedlings in pure Q. ilex forests after disturbance phenomena.     

Sebacinales are common mycorrhizal associates of Ericaceae

Previous reports of sequences of Sebacinales (basal Hymenomycetes) from ericoid mycorrhizas raised the question as to whether Sebacinales are common mycorrhizal associates of Ericaceae, which are usually considered to associate with ascomycetes. Here, we sampled 239 mycorrhizas from 36 ericoid mycorrhizal species across the world (Vaccinioideae and Ericoideae) and 361 mycorrhizas from four species of basal Ericaceae lineages (Arbutoideae and Monotropoideae) that do not form ericoid mycorrhizas, but ectendomycorrhizas. Sebacinales were detected using sebacinoid-specific primers for nuclear 28S ribosomal DNA, and some samples were investigated by transmission electron microscopy (TEM). Diverging Sebacinales sequences were recovered from 76 ericoid mycorrhizas, all belonging to Sebacinales clade B. Indeed, some intracellular hyphal coils had ultrastructural TEM features expected for Sebacinales, and occurred in living cells. Sebacinales belonging to clade A were found on 13 investigated roots of the basal Ericaceae, and TEM revealed typical ectendomycorrhizal structures. Basal Ericaceae lineages thus form ectendomycorrhizas with clade A Sebacinales, a clade that also harbours ectomycorrhizal fungi. This further supports the proposition that Ericaceae ectendomycorrhizas involve ectomycorrhizal fungal taxa. When ericoid mycorrhizas evolved secondarily in Ericaceae, a shift of mycobionts occurred to ascomycetes and clade B Sebacinales, hitherto not described as ericoid mycorrhizal fungi.     

Facilitated establishment of Quercus ilex in shrub-dominated communities within a Mediterranean ecosystem: do mycorrhizal partners matter?

Positive plant–plant interaction is a widespread phenomenon, especially in harsh environments, which can contribute to secondary successions. Here, we investigated whether Arbutus unedo positively influences Quercus ilex establishment in shrub communities by abiotic and/or biotic interactions in a Mediterranean forest ecosystem, where we previously showed that A. unedo and Q. ilex share numerous species of mycorrhizal fungi. In a first field experiment, patterns of Q. ilex survivorship were documented. During the summer following germination, a majority of seedlings survived in A. unedo chaparral (AU), whereas most of them died in previous succession stages dominated by Erica arborea (EA). These results showed that survival of the Q. ilex seedling is succession stage dependent, probably due to the differential effects of the summer drought. In a second experiment, Q. ilex seedlings were used as bait plants to investigate the mycorrhizal inoculum in EA and AU. Morphotyping and molecular typing revealed 2.5 times higher colonization in AU than in EA, with more diverse fungi. Our results demonstrate that A. unedo facilitates mycorrhization of Q. ilex by hosting compatible ectomycorrhizal symbionts and positively influences seedling survival by buffering abiotic conditions. A comprehensive understanding of facilitation should thus include investigations of the soil biological patterns.     

Micronutrient transport in mycorrhizal symbiosis; zinc steals the show

Mycorrhizas are mutually beneficial associations between soil-borne fungi and plant roots. Mycorrhizal fungi provide their host plant with essential nutrients in exchange for sugars and/or lipids. Traditionally, transport and translocation of macronutrients, including nitrogen and phosphorus, throughout the fungal mycelium and towards the host plant are well studied. However, the regulation of nutrient exchange and their contribution in the morphogenesis and development of mycorrhizas remains unclear. In this Opinion, we argue that adding micronutrients in the current models of symbiotic transport is essential to fully understand the establishment, maintenance, and functioning of mycorrhizal associations. Homeostatic mechanisms at the cellular level and the first transport proteins involved have been recently documented for zinc (Zn) in arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal fungi. Mycorrhizal plants benefit from an improved Zn status in control conditions and are better protected when environmental Zn availability fluctuates. These recent progresses are paving the way for a better understanding of micronutrient allocation in mycorrhizas. Revising our vision on the role of micronutrients, particularly of Zn, in these interactions will allow a better use of mycorrhizal fungi in sustainable agriculture and forestry, and will increase management practices in waste land, as well as in agricultural and natural ecosystems.     

Ericoid mycorrhizal fungi: some new perspectives on old acquaintances

Many ericaceous species colonize as pioneer plants substrates ranging from arid sandy soils to moist mor humus, in association with their mycorrhizal fungi. Thanks to the symbiosis with ericoid mycorrhizal fungi, ericaceous plants are also able to grow in highly polluted environments, where metal ions can reach toxic levels in the soil substrate. For a long time this mycorrhizal type has been regarded as an example of a highly specific interaction between plants and fungi. More recent studies have been challenging this view because some ericoid mycorrhizal endophytes seem also able to colonise plants from very distant taxa. A molecular approach has allowed the investigation of genetic diversity and molecular ecology of ericoid mycorrhizal fungi, and has revealed that ericaceous plants can be very promiscuous, with multiple occupancy of their thin roots. The molecular analysis of sterile morphotypes involved in this symbiosis has also led to deeper understanding of the species diversity of ericoid fungi. Genetic polymorphism of ericoid fungi is wider than previously thought, and often increased by the presence of Group I introns in the nuclear small subunit rDNA.     

Mycorrhizas on nursery and field seedlings of Quercus garryana

Oak woodland regeneration and restoration requires that seedlings develop mycorrhizas, yet the need for this mutualistic association is often overlooked. In this study, we asked whether Quercus garryana seedlings in nursery beds acquire mycorrhizas without artificial inoculation or access to a mycorrhizal network of other ectomycorrhizal hosts. We also assessed the relationship between mycorrhizal infection and seedling growth in a nursery. Further, we compared the mycorrhizal assemblage of oak nursery seedlings to that of conifer seedlings in the nursery and to that of oak seedlings in nearby oak woodlands. Seedlings were excavated and the roots washed and examined microscopically. Mycorrhizas were identified by DNA sequences of the internal transcribed spacer region and by morphotype. On oak nursery seedlings, predominant mycorrhizas were species of Laccaria and Tuber with single occurrences of Entoloma and Peziza. In adjacent beds, seedlings of Pseudotsuga menziesii were mycorrhizal with Hysterangium and a different species of Laccaria; seedlings of Pinus monticola were mycorrhizal with Geneabea, Tarzetta, and Thelephora. Height of Q. garryana seedlings correlated with root biomass and mycorrhizal abundance. Total mycorrhizal abundance and abundance of Laccaria mycorrhizas significantly predicted seedling height in the nursery. Native oak seedlings from nearby Q. garryana woodlands were mycorrhizal with 13 fungal symbionts, none of which occurred on the nursery seedlings. These results demonstrate the value of mycorrhizas to the growth of oak seedlings. Although seedlings in nursery beds developed mycorrhizas without intentional inoculation, their mycorrhizas differed from and were less species rich than those on native seedlings.     

锦绣杜鹃菌根真菌rDNA ITS序列分析及接种效应研究

利用rDNA ITS序列对锦绣杜鹃菌根真菌的16个菌株进行了分类分析。根据菌株ITS序列全长计算各菌株间序列相似度和遗传距离,并与GenBank中最相似菌株序列构建系统发育树。结果表明：16个菌株在系统树上聚为3个大分支。其中7个菌株在支持率为100%的基础上与树粉孢属真菌Oidiodendron sp.聚为一类;2个菌株与未鉴定的杜鹃花科植物根系真菌unidentified root associated fungi聚为一类,支持率为100%;其他7个菌株在98%的支持率上与几种未命名的欧石楠类菌根真菌     

Molecular diversity of fungi from ericoid mycorrhizal roots

In order to investigate the diversity of fungal endophytes in ericoid mycorrhizal roots, about 150 mycelia were isolated from surface-sterilized roots of 10 plants of Calluna vulgaris. Each mycelium was reinoculated to C. vulgaris seedlings under axenic conditions, and the phenotype of the plant-fungus association assessed by light and electron microscopy. Many isolates that were able in vitro to produce typical ericoid mycorrhizae did not form reproductive structures under our culture conditions, whereas others could be identified as belonging to the species Oidiodendron maius. Morphological and molecular analysis of the fungal isolates showed that the root system of a single plant of C. vulgaris is a complex mosaic of several populations of mycorrhizal and non mycorrhizal fungi. PCR-RFLP techniques, used to investigate the mycorrhizal endophytes, revealed up to four groups of fungi with different PCR-RFLP patterns of the ITS ribosomal region from a single plant. Some of the mycorrhizal fungi sharing the same PCR-RFLP pattern showed high degree of genetic polymorphism when analysed with the more sensitive RAPD technique; this technique may prove a useful tool to trace the spread of individual mycorrhizal mycelia, as it has allowed us to identify isolates with identical RAPD fingerprints on different plants.     

Endo- and ectomycorrhizas in Quercus agrifolia Nee. (Fagaceae): patterns of root colonization and effects on seedling growth

We documented the patterns of root occupancy by Glomalean and ectomycorrhizal (EM) fungi in Quercus agrifolia, and host plant responses to inoculation with each mycorrhizal type alone or in combination. Glomalean hyphae, coils and vesicles, and EM root tips were recorded. Colonization patterns conformed to a succession from Glomalean and EM fungi in 1-year-old seedlings to predominantly EM in saplings (>11 years old); both mycorrhizal types were rarely detected within the same root segment. Inoculation of Q. agrifolia seedlings with EM or Glomalean fungi (AM) alone or in combination (EM+AM) altered the cost:benefit relationship of mycorrhizas to the host plant. Seedling survival, plant biomass, foliar nitrogen (N), and phosphorus (P) status were greatest in EM- or AM-only inoculated seedlings. Seedlings inoculated with both mycorrhizal types (AM+EM) exhibited the lowest survival rates, biomass, foliar N, and P levels. Roots of these plants were highly colonized by both EM (38% root length colonized) and Glomalean fungi (34%). Because these levels of colonization were similar to those detected in 1-year-old field seedlings, the presence of both mycorrhizal types may be a carbon cost and, in turn, less beneficial to oaks during establishment in the field. However, the shift to EM colonization in older plants suggests that mycorrhizal effects may become positive with time.     

The cultivable endophytic community of Norway spruce ectomycorrhizas from microhabitats lacking ericaceous hosts is dominated by ericoid mycorrhizal Meliniomyces variabilis

Most of the temperate conifers associate with ectomycorrhizal fungi, but their roots also harbour a wide range of endophytes. We focused on ascomycetes associating with basidiomycetous ectomycorrhizas of Norway spruce in a temperate montane forest in central Europe and found that the majority of the co-associated fungi belonged to the Rhizoscyphus ericae aggregate (REA), being dominated by Meliniomyces variabilis. We further tested the ability of representative isolates to colonize spruce root tips and European blueberry (Vaccinium myrtillus) hair roots in an agar system as well as their effect on blueberry growth in a peat-agar system. M. variabilis intracellularly colonized spruce (Picea abies) root tip cortex, formed ericoid mycorrhizas in blueberry and enhanced blueberry shoot and root growth in comparison with non-inoculated plants. Our findings suggest that spruce ectomycorrhizas may represent selective niches for ericoid mycorrhizal fungi in habitats lacking suitable ericaceous hosts.     

Waiting for fungi: the ectomycorrhizal invasion of lowland heathlands

1.  In England, the loss of lowland heathland, a habitat of global conservation importance, is primarily due to the invasion of birch and pine. This encroachment has been researched in depth from a plant perspective but little is known about the role of mycorrhizal fungi. In lowland heathlands the resident dwarf shrubs form ericoid mycorrhizas whereas invading trees form ectomycorrhizas. Therefore, tree encroachment into heathlands can be regarded as the replacement of a resident mycorrhizal community by an invading one.
2 . This study examined how fungi form mycorrhizas with Betula and Pinus in lowland heathlands. We addressed the question of whether there are mycorrhizal fungi that mediate invasion using a molecular ecology approach to compare the mycorrhizal inoculum potential of soil at three levels of invasion (uninvaded heathland, invaded heathland and woodland) and the fungi forming mycorrhizas on tree seedlings and trees across diverse sites.
3.  We show that in lowland heathlands: (i) seedlings have severely limited access to ectomycorrhizal fungi relative to woodlands, (ii) there are few keystone spore-dispersed ectomycorrhizal fungi that can mediate tree invasion, (iii) tree seedlings can remain non-mycorrhizal for at least one year when no inoculum is present, even near saplings, and (iv) mycorrhizal seedlings achieve greater biomass than non-mycorrhizal seedlings. Within uninvaded heathland we detected only Rhizopogon luteolus , Suillus variegatus , S. bovinus ( Pinus symbionts) and Laccaria proxima (primarily a Betula symbiont).
4. Synthesis . Overall, ectomycorrhizal inoculum in lowland heathlands is rare; most tree seedlings growing in heathland soil are not mycorrhizal due to limited spore dispersal, poorly developed spore banks and weak common mycorrhizal networks. These seedlings can persist awaiting mycorrhization to boost their growth.     

Root soluble carbohydrates of Afzelia africana Sm. seedlings and modifications of mycorrhiza establishment in response to the excision of cotyledons

Stem length, number of secondary lateral roots, shoot dry weight and reducing sugar concentrations of root were significantly reduced when translocation of reserves from cotyledons to the roots of Afzelia africana seedlings was interrupted by complete or partial cotyledon excision. The sucrose but not the glucose concentration of lateral roots also decreased significantly after complete cotyledon excision. Hartig net development rather than fungal sheath formation was affected after inoculation with the early fungal isolate E1 and by both late-stage fungal isolates L1 and L2 after partial or complete cotyledon excision. However, mycorrhizal colonization by the early fungal isolate E2 was not affected by cotyledonary reserves, suggesting that this fungal isolate has a lower carbohydrate requirement than fungal isolates E1, L1 and L2. The late-stage fungal isolates L1 and L2 induced a hypersensitivity reaction by epidermal cell walls of the host plant after complete cotyledon excision, suggesting they are more dependent than the early fungal isolate E1 on available root carbohydrate substrates for ectomycorrhizal colonization. These results are discussed in the light of the hypothesis that early and late-stage fungi were different carbohydrate requirements, and that the time sequence of colonization was related to the root carbohydrate status, which increased with time.     

Patterns of plant naturalization show that facultative mycorrhizal plants are more likely to succeed outside their native Eurasian ranges

The naturalization of an introduced species is a key stage during the invasion process. Therefore, identifying the traits that favor the naturalization of non-native species can help understand why some species are more successful when introduced to new regions. The ability and the requirement of a plant species to form a mutualism with mycorrhizal fungi, together with the types of associations formed may play a central role in the naturalization success of different plant species. To test the relationship between plant naturalization success and their mycorrhizal associations we analysed a database composed of mycorrhizal status and type for 1981 species, covering 155 families and 822 genera of plants from Europe and Asia, and matched it with the most comprehensive database of naturalized alien species across the world (GloNAF). In mainland regions, we found that the number of naturalized regions was highest for facultative mycorrhizal, followed by obligate mycorrhizal and lowest for non-mycorrhizal plants, suggesting that the ability of forming mycorrhizas is an advantage for introduced plants. We considered the following mycorrhizal types: arbuscular, ectomycorrhizal, ericoid and orchid mycorrhizal plants. Further, dual mycorrhizal species were those that included observations of arbuscular mycorrhizas as well as observations of ectomycorrhizas. Naturalization success (based on the number of naturalized regions) was highest for arbuscular mycorrhizal and dual mycorrhizal plants, which may be related to the low host specificity of arbuscular mycorrhizal fungi and the consequent high availability of arbuscular mycorrhizal fungal partners. However, these patterns of naturalization success were erased in islands, suggesting that the ability to form mycorrhizas may not be an advantage for establishing self-sustaining populations in isolated regions. Taken together our results show that mycorrhizal status and type play a central role in the naturalization process of introduced plants in many regions, but that their effect is modulated by other factors.     

Afforestation of abandoned farmland with conifer seedlings inoculated with three ectomycorrhizal fungi—impact on plant performance and ectomycorrhizal community

The aim of a 3-year study was to investigate whether inoculation of Pinus sylvestris L. and Picea abies (L.) Karst. seedlings with mycorrhizas of Cenococcum geophilum Fr., Piceirhiza bicolorata, and Hebeloma crustuliniforme (Bull.) Quel. has any impact on: 1) survival and growth of outplanted seedlings on abandoned agricultural land, and 2) subsequent mycorrhizal community development. For inoculation, the root system of each plant was wrapped in a filter paper containing mycelium, overlaid with damp peat–sand mixture and wrapped in a paper towel. In total, 8,000 pine and 8,000 spruce seedlings were planted on 4-ha of poor sandy soil in randomized blocks. Already after the first year natural mycorrhizal infections prevailed in the inoculated root systems, and introduced mycorrhizas were seldom found. Yet, the seedlings that had been pre-inoculated with C. geophilum and the P. bicolorata during the whole 3-year period showed significantly higher survival and growth as compared to controls. Moreover, the independent colonization of roots by C. geophilum and the P. bicolorata from natural sources was also observed. A diverse mycorrhizal community was detected over two growing seasons in all treatments, showing low impact of inoculation on subsequent fungal community development. A total of 19 additional ectomycorrhizal morphotypes was observed, which clustered into two well-separated groups, according to host tree species (pine and spruce). In conclusion, the results showed limited ability to increase tree survival and growth, and to manipulate the mycorrhizal community even by extensive pre-inoculations, indicating that fungal community formation in root systems is governed mainly by environmental factors.     

Diversity and Spatial Structure of Belowground Plant–Fungal Symbiosis in a Mixed Subtropical Forest of Ectomycorrhizal and Arbuscular Mycorrhizal Plants

Plant–mycorrhizal fungal interactions are ubiquitous in forest ecosystems. While ectomycorrhizal plants and their fungi generally dominate temperate forests, arbuscular mycorrhizal symbiosis is common in the tropics. In subtropical regions, however, ectomycorrhizal and arbuscular mycorrhizal plants co-occur at comparable abundances in single forests, presumably generating complex community structures of root-associated fungi. To reveal root-associated fungal community structure in a mixed forest of ectomycorrhizal and arbuscular mycorrhizal plants, we conducted a massively-parallel pyrosequencing analysis, targeting fungi in the roots of 36 plant species that co-occur in a subtropical forest. In total, 580 fungal operational taxonomic units were detected, of which 132 and 58 were probably ectomycorrhizal and arbuscular mycorrhizal, respectively. As expected, the composition of fungal symbionts differed between fagaceous (ectomycorrhizal) and non-fagaceous (possibly arbuscular mycorrhizal) plants. However, non-fagaceous plants were associated with not only arbuscular mycorrhizal fungi but also several clades of ectomycorrhizal (e.g., Russula) and root-endophytic ascomycete fungi. Many of the ectomycorrhizal and root-endophytic fungi were detected from both fagaceous and non-fagaceous plants in the community. Interestingly, ectomycorrhizal and arbuscular mycorrhizal fungi were concurrently detected from tiny root fragments of non-fagaceous plants. The plant–fungal associations in the forest were spatially structured, and non-fagaceous plant roots hosted ectomycorrhizal fungi more often in the proximity of ectomycorrhizal plant roots. Overall, this study suggests that belowground plant–fungal symbiosis in subtropical forests is complex in that it includes “non-typical” plant–fungal combinations (e.g., ectomycorrhizal fungi on possibly arbuscular mycorrhizal plants) that do not fall within the conventional classification of mycorrhizal symbioses, and in that associations with multiple functional (or phylogenetic) groups of fungi are ubiquitous among plants. Moreover, ectomycorrhizal fungal symbionts of fagaceous plants may “invade” the roots of neighboring non-fagaceous plants, potentially influencing the interactions between non-fagaceous plants and their arbuscular-mycorrhizal fungal symbionts at a fine spatial scale.     

Fungi associated with hair roots of Rhododendron lochiae (Ericaceae) in an Australian tropical cloud forest revealed by culturing and culture-independent molecular methods

The culturable fungal assemblage associated with hair roots of Rhododendron lochiae (Ericaceae) from a tropical cloud forest in Queensland, Australia was investigated using rDNA internal transcribed spacer (ITS) restriction fragment length polymorphisms (RFLPs) and sequence analysis, and the abilities of the fungi to form ericoid mycorrhizas were tested. DNA was further extracted directly from hair roots and partial fungal ITS products compared with those from the cultured isolate assemblage using denaturing gradient gel electrophoresis (DGGE). A range of ericoid mycorrhizal and non-mycorrhizal fungi was identified using both approaches, with ericoid mycorrhizal fungi found to be taxonomically similar to those associated with Ericaceae in temperate habitats worldwide. Both approaches identified several unique fungi and, although most of the abundant RFLP types identified in the cultured fungal assemblage were also present in DGGE profiles of DNA extracted directly from roots, one the most commonly isolated RFLP types, a putative Xylariaceae taxon, was absent. The data suggest that a combination of culturing and culture-independent approaches may be more efficacious than either method individually.     

Formation of structures resembling ericoid mycorrhizas by the root endophytic fungus Heteroconium chaetospira within roots of Rhododendron obtusum var. kaempferi

A resynthesis study was conducted to clarify the relationship between the root endophyte, Heteroconium chaetospira and the ericaceous plant, Rhododendron obtusum var. kaempferi. The host plant roots were recovered 2 months after inoculation, and the infection process and colonization pattern of the fungus were observed under a microscope. The hyphae of H. chaetospira developed structures resembling ericoid mycorrhizas, such as hyphal coils within the host epidermal cells. These structures were morphologically the same as previously reported ericoid mycorrhizal structures. The frequencies of hyphal coils within the epidermal cells of host roots ranged from 13 to 20%. H. chaetospira did not promote or reduce host plant growth. This is the first reported study that H. chaetospira is able to form structures resembling mycorrhizas within the roots of ericaceous plants.     

The response of ectomycorrhizal fungal inoculum to long-term increases in nitrogen supply

The inoculum of ectomycorrhizal (EM) fungi was examined in a 16 y long nitrogen fertilization experiment maintained in a temperate oak savanna. To measure EM fungal inoculum, bur oak seedlings were grown in three types of bioassays: (i) intact soil cores that measure inoculum such as spores, mycelia and mycorrhizal roots; (ii) resistant propagule bioassays that measure inoculum types resistant to soil drying; and (iii) previously mycorrhizal root bioassays that measure the ability of EM fungi to colonize new roots from mycorrhizal roots. Colonization of bur oak seedlings was characterized by morphotyping and where necessary by restriction analysis and internal transcribed spacer (ITS) sequencing. Fourteen morphotypes were found in intact soil core bioassays with species of Cortinarius, Cenococcum and Russula abundant. Five morphotypes were found in resistant propagule bioassays with Cenococcum, a thelephoroid morphotype and a Wilcoxina-like ascomycete abundant and frequent. In intact soil core bioassays total percent root colonization and number of morphotypes were not affected by N supply in 2000 and 2001. However the composition of EM fungi colonizing oak seedling roots was different with increased N supply such that Russula spp. (primarily Russula aff. amoenolens) were most abundant at the highest level of N supply. Dominant Russula spp. did not colonize any roots in resistant propagule bioassays but did colonize oak seedling roots from previously mycorrhizal roots. Results suggest that in this savanna N supply can influence the kinds of inoculum propagules present and thereby might affect the dynamics of ectomycorrhizal communities by differentially influencing reproductive and colonization strategies.     

Diversity and classification of mycorrhizal associations

Most mycorrhizas are 'balanced' mutualistic associations in which the fungus and plant exchange commodities required for their growth and survival. Myco-heterotrophic plants have 'exploitative' mycorrhizas where transfer processes apparently benefit only plants. Exploitative associations are symbiotic (in the broad sense), but are not mutualistic. A new definition of mycorrhizas that encompasses all types of these associations while excluding other plant-fungus interactions is provided. This definition recognises the importance of nutrient transfer at an interface resulting from synchronised plant-fungus development. The diversity of interactions between mycorrhizal fungi and plants is considered. Mycorrhizal fungi also function as endophytes, necrotrophs and antagonists of host or non-host plants, with roles that vary during the lifespan of their associations. It is recommended that mycorrhizal associations are defined and classified primarily by anatomical criteria regulated by the host plant. A revised classification scheme for types and categories of mycorrhizal associations defined by these criteria is proposed. The main categories of vesicular-arbuscular mycorrhizal associations (VAM) are 'linear' or 'coiling', and of ectomycorrhizal associations (ECM) are 'epidermal' or 'cortical'. Subcategories of coiling VAM and epidermal ECM occur in certain host plants. Fungus-controlled features result in 'morphotypes' within categories of VAM and ECM. Arbutoid and monotropoid associations should be considered subcategories of epidermal ECM and ectendomycorrhizas should be relegated to an ECM morphotype. Both arbuscules and vesicles define mycorrhizas formed by glomeromycotan fungi. A new classification scheme for categories, subcategories and morphotypes of mycorrhizal associations is provided.     

Fungal associations of Danish Calluna vulgaris roots with special reference to ericoid mycorrhiza

Fungi were isolated from young, serial-washed roots of Calluna sampled from a Danish heathland, Hjelm Hede. Of the 626 isolates, those that were dark, sterile and septate were divided into 13 morphological groups based on their appearance in culture on malt agar. Mycorrhizal synthesis in vitro showed that several groups formed typical ericoid mycorrhiza with seedlings of Calluna; these ericoid mycorrhizal fungi were morphologically similar to Hymenoscyphus ericae. The identities of the other dark, septate fungi are uncertain. Oidiodendron spp. were isolated in a very low frequency; these fungi also formed typical ericoid mycorrhiza. The Calluna root system on Hjelm Hede demonstrated a high morphological diversity among the associated dark, septate fungi suggesting that more than one fungus could coexist in the same host root system.