Epiphytic and terrestrial mycorrhizas in a lower montane Costa Rican cloud forest

The epiphyte community is the most diverse plant community in neotropical cloud forests and its collective biomass can exceed that of the terrestrial shrubs and herbs. However, little is known about the role of mycorrhizas in this community. We assessed the mycorrhizal status of epiphytic (Araceae, Clusiaceae, Ericaceae, and Piperaceae) and terrestrial (Clusiaceae, Ericaceae) plants in a lower montane cloud forest in Costa Rica. Arbuscular mycorrhizas were observed in taxa from Araceae and Clusiaceae; ericoid mycorrhizas were observed in ericaceous plants. This is the first report of intracellular hyphal coils characteristic of ericoid mycorrhizas in roots of Cavendishia melastomoides, Disterigma humboldtii, and Gaultheria erecta. Ericaceous roots were also covered by an intermittent hyphal mantle that penetrated between epidermal cells. Mantles, observed uniquely on ericaceous roots, were more abundant on terrestrial than on epiphytic roots. Mantle abundance was negatively correlated with gravimetric soil water content for epiphytic samples. Dark septate endophytic (DSE) fungi colonized roots of all four families. For the common epiphyte D. humboldtii, DSE structures were most abundant on samples collected from exposed microsites in the canopy. The presence of mycorrhizas in all epiphytes except Peperomia sp. suggests that inoculum levels and environmental conditions in the canopy of tropical cloud forests are generally conducive to the formation of mycorrhizas. These may impact nutrient and water dynamics in arboreal ecosystems.     

Anatomical study on the interaction between the root endophytic fungus <Emphasis Type="Italic">Heteroconium chaetospira</Emphasis> and Chinese cabbage

Chinese cabbage roots colonized by the dematiaceous fungal taxon Heteroconium chaetospira were previously found to become highly resistant to clubroot and Verticillium yellows. The dematiaceous fungus possesses an endophytic nature, but no detailed anatomical studies on endophyte–host plant interactions have so far been provided. Light and electron microscopy revealed that hyphae of H. chaetospira were abundant on and inside the root epidermal cells by 3 weeks following inoculation. The penetration pegs easily breached into epidermal cells, and the infection hyphae penetrated into cortical cells. Some appressorium-like swollen structures formed from intracellular hyphae, but no visible degradation of the host cell walls was evident where the hyphae contacted. No visible signs of host reactions and no invagination of the host plasma membrane around the hyphae were seen in the host cells. By 8 weeks following inoculation, masses of closely packed fungal cells had been formed in some cells of the epidermis and cortical layers, but further hyphal ingress was halted, mostly in the inner cortical cell layer. Thus, root vascular cylinders remained intact.     

In vitro interactions between ectomycorrhizal fungi and ericaceous plants

In view of the close association between ericaceous shrubs and ectomycorrhizal trees in forest ecosystems, the interaction between ectomycorrhizal basidiomycetes and the hair roots of four typical ericoid mycorrhizal hosts was investigated in vitro. Seedlings of Vaccinium myrtillus, V. vitis-idaea, V. macrocarpon and Calluna vulgaris were inoculated with each of four ectomycorrhizal basidiomycetes from different phylogenetic groups (Laccaria bicolor, Lactarius musteus, Suillus variegatus and Tomentellopsis submollis) in a low carbon and nutrient agar-cellophane culture system. Two ericoid mycorrhizal Helotiales ascomycetes (Meliniomyces bicolor in the Rhizoscyphus ericae aggregate and a mycobiont out of the Rhizoscyphus ericae aggregate) were included for comparison. Interactions between fungi and hair roots ranged from neutral to surface attachment, and the formation of intracellular hyphal coils. Root and shoot responses to inoculation were different between the host/fungus combinations. The ectomycorrhizal fungus L. bicolor formed extensive intracellular colonization, spreading cell-to-cell with multiple hyphal entry points and intracellular hyphal coils with single entry points in C. vulgaris and V. macrocarpon epidermal cells respectively, however, no significant effects on plant growth were detected. Meliniomyces bicolor formed intracellular hyphal coils in the epidermal cells of V. myrtillus and V. macrocarpon but not the other host spp. The M. bicolor isolate stimulate V. myrtillus root length about 2.5 times. Interestingly, although the unknown ascomycete strain out of the Rhizoscyphus ericae aggregate formed intracellular hyphal coils in epidermal cells of all host plants, it suppressed the growth of C. vulgaris, V. myrtillus, and V. vitis-idaea but not to V. macrocarpon. Further and more detailed experimentation under more ecological realistic conditions for a longer period of time is needed.     

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.     

Anatomy and ultrastructure of mycorrhizal associations of neotropical Ericaceae

Ericaceae are obligatory associated with symbiotic fungi forming several, distinctive categories of mycorrhizas. While ericoid, arbutoid, and monotropoid mycorrhizas are known since many years from ericads of the northern hemisphere and the ericoid mycorrhiza also from Australia, a further mycorrhizal category with hyphal sheath, Hartig net, and intracellular colonization was described by us recently and termed cavendishioid mycorrhiza because it was found on Cavendishia nobilis, a species belonging to the Andean clade (Vaccinioideae) of Ericaceae. As the previous findings indicated a correlation between the mycorrhizal category and the systematic position of Ericaceae, we tested the hypothesis that other ericads of the Andean clade might also form cavendishioid mycorrhizas, while ericads occurring in the same area but not belonging to the Andean clade might not. Mycorrhizas of 20 different ericaceous species, 15 belonging to the Andean clade and 5 to other Vaccinioideae or Ericoideae, were sampled in the tropical mountain rain forest area of South Ecuador and investigated by light and electron microscopy. All the 15 members of the Andean clade ericads displayed a hyphal sheath, as well as inter- and intracellular colonization by hyphae as was found on Cavendishia previously. The five species not belonging to the Andean clade ericads displayed only intracellular colonization by hyphae and hence were typical ericoid mycorrhizal. Ultrastructural studies revealed Sebacinales and ascomycetes as mycorrhiza formers in both associations even within one single cell. The results thus support the hypothesis that the Andean clade of Ericaceae forms mycorrhizas distinct from the arbutoid category and most likely presents an independent evolutionary line in the Ericaceae derived from the ericoid mycorrhizas, justifying the new term “cavendishioid mycorrhiza”.     

Ericoid mycorrhizal colonization and associated fungal communities along a wetland gradient in the Acadian forest of Eastern Canada

Wetlands provide numerous ecosystem services, and ericaceous plants are important components of these habitats. However, the ecology of fungi associated with ericaceous roots in these habitats is poorly known. To investigate fungi associated with ericaceous roots in wetlands, ericoid mycorrhizal colonization was quantified, and fungal communities were characterized on the roots of Gaultheria hispidula and Kalmia angustifolia along two upland – forested wetland transects in spring and fall. Ericoid mycorrhizal colonization was significantly higher in the wetlands for both plant species. Both upland and wetland habitats supported distinct assemblages of ericaceous root associated fungi including habitat specific members of the genus Serendipita. Habitat was a stronger driver of ericoid mycorrhizal colonization and ericaceous root associated community composition than host or sampling season, with differences related to soil water content, soil nutrient content, or both. Our results indicate that ericaceous plant roots in forested wetlands are heavily colonized by habitat specific symbionts.     

Structural changes to a mycothallus along a latitudinal transect through the maritime and sub-Antarctic

Previous studies have shown the leafy liverwort Cephaloziella varians to associate consistently with fungi, typically the ericoid mycorrhizal symbiont Rhizoscyphus ericae, across a wide latitudinal gradient in the maritime and sub-Antarctic. Hitherto, however, there are no quantitative data on the intensity of colonisation of C. varians by fungal structures in the natural environment and how colonisation might vary with changing environmental conditions. A study is hence reported showing that the frequency of colonisation by fungal structures of C. varians alters along a latitudinal transect from South Georgia (54° S, 38° W) to Moutonnée Valley on Alexander Island (71° S, 68° W). The percentage of stem length colonised by dark septate (DS) hyphae increased significantly along the transect, from 30% at South Georgia to 97% at Moutonnée Valley. In contrast, the percentage of stem length colonised by hyaline hyphae decreased significantly, from 85% at South Georgia to 13% at Moutonnée Valley, and that colonised by hyphal coils similarly decreased from 71% at the former location to 15% at the latter. The frequencies of DS hyphae were negatively associated with mean annual and seasonal air temperatures, whereas those of hyaline septate hyphae and hyphal coils were positively associated with air temperatures. Coils at northerly locations were more convoluted than those at southerly locations. The data indicate that hyphal coils, usually associated with nutrient exchange between partners in ericoid mycorrhizas, do form in C. varians tissues in the maritime and sub-Antarctic, but that the frequency of these structures diminishes in colder habitats.     

First synthesis of ericoid mycorrhizas in the Epacridaceae under axenic conditions

The first axenic synthesis of morphologically typical ericoid mycorrhizas of the Epacridaceae has been achieved in micropropagated Epacris impressa Labill. with eight fungi isolated from roots of two epacrid species, E. impressa and Astroloma pinifolium (R.Br.) Benth. Mycorrhizal synthesis has also been achieved between E. impressa and both Hymenoscyphus ericae (Read) Korf and Kernan and Oidiodendron griseum Robak, recognized endophytes of Ericaceae, suggesting that the endophytes of the Epacridaceae and Ericaceae are capable of cross-infection. Infection rate of epidermal cells on hair roots varied from 3–77% infection and the density of hyphal coils varied widely. This synthesis makes possible studies of the roles of these endophytes in the Epacridaceae and comparison with their roles in the Ericaceae.     

Co-occurrence of three fungal root symbionts in Gaultheria poeppiggi DC in central Argentina

The roots of Gaultheria poeppiggi (Ericaceae) were examined for fungal symbiont colonization. Typical structures of ericoid mycorrhizas (hyphae and intracellular coil hyphae complexes), dark septate fungal endophytes (hyphae and sclerotia), and arbuscular mycorrhizas (hyphae, coils, vesicles and arbuscules) were found in the roots of all the individuals examined. The evolutionarily derived position of Gaultheria within the Ericales may suggest that G. poeppiggi recently acquired the ability to form arbuscular mycorrhizas rather than having retained it from ancestral lines.     

Tricholoma matsutake– an assessment of in situ and in vitro infection by observing cleared and stained whole roots

 Structures present within field-collected Tricholoma matsutake/Pinus densiflora ectomycorrhizas and in vitro infections of P. densiflora roots by T. matsutake were observed by clearing, bleaching and staining whole lateral roots and mycorrhizas. Field mycorrhizas were characterized by a lack of root hairs, by the presence of a sparse discontinuous mantle composed of irregularly darkly staining hyphae over the root surface, primarily behind the root cap, and by the presence of Hartig net mycelium within the root cortex. Hartig net 'palmettis' were classified into three basic structures, each with distinctive morphologies. Aerial hyphae, bearing terminal swellings, were observed emanating from the mantle. Cleared, bleached and stained in vitro-infected roots possessed multibranched hyphal structures within the host root cortex and aerial hyphae bearing terminal swellings were observed arising from the mycelium colonizing the root surface. T. matsutake on P. densiflora conforms to the accepted morphology of an ectomycorrhiza. This staining protocol is particularly suited to the study of Matsutake mycorrhizal roots and gives rapid, clear, high-contrast images using standard light microscopy while conserving spatial relationships between hyphal elements and host tissues. Accepted: 26 August 1999     

Decomposition of organic matter by the ericoid mycorrhizal endophytes of Formosan rhododendron (Rhododendron formosanum Hemsl.)

Ericoid mycorrhizas are associated with a number of host plants in the Ericaceae in high-elevation regions of Taiwan. The ability of these microorganisms to thrive in harsh environmental conditions in the regions implies their capability of decomposing plant organic matter (raw humus). The objective of this study was to investigate the decomposition characteristics of three ericoid mycorrhizal endophytes isolated from the roots of Formosan rhododendron (Rhododendron formosanum Hemsl.). Molecular analysis indicated that strains Rf9 and Rf32 belong to the genus Cryptosporiopsis while strain Rf28 is a member of the genus Phialocephala. Mycorrhizal synthesis experiment showed that the roots of synthesized seedlings produced hyphal coils, a characteristic of ericoid mycorrhiza. Decomposition ability analysis revealed that strains Rf28 and Rf32 had the highest rates of decomposition of organic matter (up to 10.4% after 70 days) while the value for strain Rf9 was about 6.8%. Consistently, these strains secreted extracellular oxidases when cultured on tannic acid medium. Enzyme assay revealed that strains Rf28 and Rf32 secreted peroxidase, laccase, tyrosinase, and cellulase, but strain Rf9 secreted mainly peroxidase and tyrosinase. Apparently, the differences in secreted hydrolytic enzymes among the three endophytes are related to their ability to decompose organic matter. In the mycorrhizal synthesis experiment, all inoculated seedlings survived in the organic matter substrate for 70 days and exhibited a stronger vigor than the control. This study demonstrated that these three isolated endophytes, Rf9, Rf28, and Rf32, are ericoid mycorrhizal fungi, capable of forming ericoid mycorrhiza with Formosan rhododendron. Meanwhile, all three endophytes can secrete hydrolytic enzymes to decompose organic matter for growth, presumably a prerequisite for the adaptation of Formosan rhododendron to the harsh environments of high elevation.     

Colonization process of the root endophytic fungus Heteroconium chaetospira in roots of Chinese cabbage

Dark septate endophytic fungi (DSE) may have an important functional relationship with host plants, but these functions and the colonization process remain unknown. We made microscopic observations of the growth of an endophytic hyphomycete in Chinese cabbage roots to understand its colonization process. This hyphomycete was Heteroconium chaetospira, a suspected DSE. Three weeks post inoculation, some hyphae became irregularly lobed and formed microsclerotia within host epidermal cells of healthy plants. In stunted plants, hyphae formed closely packed masses of fungal cells within host epidermal cells, but conidiophores rarely broke through the cell walls to produce conidia. Received: December 7, 2000 / Accepted: November 20, 2001     

Intracellular colonization of Rhododendron and Vaccinium roots by Cenococcum geophilum, Geomyces pannorum and Meliniomyces variabilis

Four in vitro experiments were set up to verify the colonization potential of ectomycorrhizal (EcM) Cenococcum geophilum FR. (strain CGE-4), saprotrophic Geomyces pannorum (LINK) SIGLER & CARMICHAEL (GPA-1) and a frequent root-associated, potentially ericoid mycorrhiza (ErM)-forming Meliniomyces variabilis Hambleton & Sigler (MVA-1) in roots of Rhododendron and Vaccinium. A typical ErM fungus, Rhizoscyphus ericae (Read) Zhuang & Korf (RER-1), was included for comparison. All fungal strains intracellularly colonized rooted Vaccinium microcuttings: GPA-1 occasionally produced hyphal loops similar to ErM, MVA-1 and RER-1 exhibited a typical ErM colonization pattern. CGE-4 hyphae grew vigorously on and around newly formed roots and rarely penetrated turgescent rhizodermal cells forming intracellular loose loops. Rooting of Rhododendron sp. microcuttings was not promoted by any fungal strain except CGE-4, which also promoted the most vigorous growth of Rhododendron ponticum L. seedlings. The widespread EcM fungus C. geophilum has a potential to colonize non-EcM roots and support their development which may influence overall growth of ericaceous plants. As shown for G. pannorum, structures resembling ErM may be formed by fungi that are to date not regarded as ericoid mycorrhizal.     

The interface between the arbuscular mycorrhizal fungus Glomus intraradices and root cells of Panax quinquefolius: a Paris-type mycorrhizal association

Two major types of arbucular mycorrhizal associations, the Arum-type and the Paris-type, have been identified based on morphological features. Although the Paris-type is the most common, it is the Arum-type that has been most intensively studied in terms of structure/function because of its prevalence in agronomically important plant species. In this study, the interface between the host cell cytoplasm and intracellular hyphae (extensive hyphal coils and arbusculate coils), which typify the Paris-type mycorrhiza, was studied. Using immunofluorescence techniques combined with laser scanning confocal microscopy, dramatic changes in the cytoskeleton in colonized cells were observed. Changes in the positioning of both host cell microtubules and actin filaments occurred in colonized plant cells. Both microtubules and actin filaments were associated with the hyphal coils and the arbusculate coils. An interfacial matrix, of host origin, was demonstrated between hyphal coils and arbusculate coils using various affinity techniques. It formed an apoplastic compartment consisting of cellulose and pectins between the fungus and host cell cytoplasm. There was less labelling adjacent to the fine branches of arbusculate coils compared to the hyphal coils. These observations show some similarities to those seen with Arum-type mycorrhizas.     

Ericoid mycorrhizal fungi are common root associates of a Mediterranean ectomycorrhizal plant (Quercus ilex)

Mycorrhiza samples of neighbouring Quercus ilex and Erica arborea plants collected in a postcutting habitat were processed to see whether plants differing in mycorrhizal status harbour the same root endophytes. Three experiments were performed in parallel: (i) isolation, identification and molecular characterization of fungi from surface-sterilized roots of both plant species; (ii) re-inoculation of fungal isolates on axenic E. arborea and Q. ilex seedlings; (iii) direct inoculation of field-collected Q. ilex ectomycorrhizas onto E. arborea seedlings. About 70 and 150 fungal isolates were obtained from roots of Q. ilex and E. arborea, respectively. Among them, Oidiodendron species and five cultural morphotypes of sterile isolates formed typical ericoid mycorrhizas on E. arborea in vitro. Fungi with such mycorrhizal ability were derived from both host plants. Isolates belonging to one of these morphotypes (sd9) also exhibited an unusual pattern of colonization, with an additional extracellular hyphal net. Ericoid mycorrhizas were also readily obtained by direct inoculation of E. arborea seedlings with Q. ilex ectomycorrhizal tips. Polymerase chain-restriction fragment length polymorphism and random amplified polymorphic DNA analyses of the shared sterile morphotypes demonstrate, in the case of sd9, the occurrence of the same genet on the two host plants. These results indicate that ericoid mycorrhizal fungi associate with ectomycorrhizal roots, and the ecological significance of this finding is discussed.     

The role of the motile tubular vacuole system in mycorrhizal fungi

Mycorrhizal fungi, to be effective for the plant, must be able to transfer mineral nutrient elements from sites of uptake at hyphal tips across various distances to the exchange region in the mycorrhiza. Vacuoles are likely to be important in this transport, since they contain elements of nutritional significance in abundance. In tip cells of hyphae of most fungi –- known to include three ectomycorrhizal basidiomycetes, an ericoid mycobiont, and two arbuscular mycorrhizal fungi –- the vacuoles form a motile tubular reticulum. The vacuoles are most active in hyphal tips, but non-motile vacuoles at a distance from the tip can be induced to become motile by environmental changes. Neither the tubular vacuolar reticulum nor its contents are properly preserved by conventional fixation and embedding. Vacuolar tubules are readily shown in vivo with fluorescent tracers, throughout the extramatrical mycelium and in outer hyphae of the sheath in eucalypt mycorrhizas synthesised with Pisolithus sp., but they have proved harder to label in field-collected ectomycorrhizas and ericoid mycorrhizas. Freeze-substitution does preserve the structure of vacuoles and vacuolar tubules, and careful anhydrous techniques allow them to be microanalysed, indicating high content of K and P in vacuoles of hyphal tips, and also in sheath and Hartig net of ectomycorrhizas. Vacuoles contain polyphosphate in diffuse, non-granular form. Polyphosphate is present right up to the tip region of hyphae as well as in sheath and Hartig net: thus important mineral nutrient elements are present at both ends of the long hyphal transport pathway. Exactly what happens in between, however, remains to be elucidated.     

Ultrastructural investigations of Arbutus unedo-Laccaria amethystea mycorrhiza synthesized in vitro

Summary Anatomy and ultrastructure of the arbutoid mycorrhiza of Arbutus unedo-Laccaria amethystea from axenic culture are described. In comparison to non-inoculated roots, the rhizodermal cells of mycorrhizas are of greater volume, their nuclei are enlarged and show an irregular shape, plasmalemma and cytoplasm with mitochondria, plastids, endoplasmic reticulum and dictyosomes are increased. Several ontogenetical states are documented. The arbutoid mycorrhiza as a connecting link between ectomycorrhiza and ericoid mycorrhiza is discussed.     

Structure and Development of the Endophyte in the Parasitic Angiosperm <Emphasis Type="Italic">Cuscuta japonica</Emphasis>

The endophyte, that is, the haustorial part within the tissues of the host plant Impatiens balsamina, of the parasitic angiosperm Cuscuta japonica was studied with light and electron microscopy. The endophyte consisted mainly of vacuolated parenchymatous axial cells and elongate, superficial (epidermal) cells. Then the elongate, epidermal cells separated from each other and transformed into filamentous cells, called searching hyphae. The hyphae grew independently either intercellularly or intracellularly in the host parenchyma. The apical end of the hyphal cells was characterized by conspicuous, large nuclei with enlarged nucleoli and very dense cytoplasm with abundant organelles, suggesting that the hyphal cells penetrating host tissue were metabolically very active. Numerous osmiophilic particles and chloroplasts were noted in the hyphae. The osmiophilic particles were assumed to be associated with elongation of the growing hyphe. Plasmodemata connections between the searching hyphal cells of the parasite and the host parenchyma cells were not detected. Hyphal cells that reached the host xylem differentiated into water-conducting xylic hyphae by thickening of the secondary walls. A xylem bridge connecting the parasite and the host was confirmed from serial sections. Some hyphal cells that reached the host phloem differentiated into nutrient-conducting phloic hyphae. Phloic hyphae had a thin layer of peripheral cytoplasm with typical features of sieve-tube members in autotrophic angiosperms, i.e., parallel arrays of smooth endoplasmic reticulum, mitochondria, and plastids with starch granules. Interspecific open connections via the sieve pores of the host sieve elements and plasmodesmata of the parasite phloic hyphae were very rarely observed, indicating that the symplastic translocation of assimilate to the parasite from the host occurred.     

A Possible Role for the Thick-walled Epidermal Cells in the Mycorrhizal Hair Roots of Lysinema ciliatum R. Br. and other Epacridaceae

Hair roots ofLysinema ciliatum R. Br. and some other Epacridaceaehave thick-walled cells in the epidermis. These are preferentiallycolonized with mycorrhizal fungi. Individual epidermal cellscontaining hyphal coils separate at the middle lamella and arereleased into the soil. Other colonized cells remain attachedto the roots, usually in groups, surrounded by bare exodermis,where epidermal cells have either collapsed or been sloughedoff. It is suggested that these colonized thick walled cellscan serve to prolong the mycorrhizal association and to infectnew hair roots as these emerge. The thick wall has a very specializedstructure and composition and could have a number of roles,either acting as a substrate or protective coat or in controllingwater status and uptake. Young hair-roots are surrounded bya mucilage sheath that is similar in appearance to that in Ericaceaeand apparently produced by root cap cells, not the epidermis. Lysinema ciliatum R. Br.; ericoid mycorrhiza; hair root; root cap; cortex; epidermis; exodermis     

Paris-type mycorrhizas in Smilax aspera L. growing in a Mediterranean scherophyllous wood

Paris- type mycorrhiza is described in Smilax aspera L., an evergreen climbing plant of Mediterranean sclerophyllous woods. Wild plants were sampled from a protected area inside the Regional Natural Park Migliarino-S.Rossore-Massaciuccoli, on the northwestern coast of Italy, near Pisa. Mycorrhizas formed by S. aspera were identified as a variation of Paris-type arbuscular mycorrhizas. Detailed observations on stained roots and on fresh root sections showed that, after forming the appressorium, the fungus colonized the root by penetrating individual cells, growing intracellularly from cell to cell, and producing many coils and terminal arbuscules. S. aspera seedlings inoculated with the arbuscular mycorrhizal fungi Glomus mosseae and G. viscosum, which are known to form Arum-type mycorrhizas in many plant species, produced the same Paris-type-like mycorrhizas found in nature. This confirms that the type of arbuscular mycorrhizal infection is largely governed by the plant host genotype. Plants of S. aspera inoculated with G. mosseae and G. viscosum had larger growth increments than uninoculated plants. Thus Paris-type mycorrhizas produce growth responses comparable to those of Arum-type mycorrhizas. Accepted: 11 January 2000