No abstract

A wide range of ascomycetous microfungi inhabits roots without forming the anatomical features typical of mycorrhizas or causing overt signs of pathogenesis. The most-studied taxa have darkly pigmented hyphal walls and are referred to as "dark septate endophytes" (DSE). We provide a dichotomous key and annotated descriptions for a cross-section of the most common dark septate endophytes. The term DSE is sometimes used to imply taxonomic and physiological similarity even though a diverse range of root endophytic taxa form pigmented hyphae. Among these, Phialocephala fortinii Wang & Wilcox is a well-known representative; it is widespread, easily observed in roots, and readily grown in culture and with plants. Nevertheless, the basis of its symbiotic relationship with plants remains ambiguous. It may be a weak pathogen, a saprotroph on senescent root tissues, or a mutualist. More detailed studies of interactions between identified taxa of microfungal endophytes and host plants are necessary to elucidate the functional basis of these symbioses; it may be necessary to look beyond the paradigms of traditional mycorrhizal and pathogenic associations to understand the ecological roles of these fungi. Reports of cryptic speciation in Phialocephala fortinii emphasize the need for accurate identification of isolates of microfungal endophytes used in experiments.Key words: dark septate endophytes (DSE), Phialocephala fortinii, mycorrhiza, fungus–host interactions, fungi.

A taxonomically diverse suite of fungi interacts with bryophytes as pathogens, parasites, saprobes, and commensals. Necrotrophic pathogens such as Tephrocybe palustris (Peck) Donk and Nectria mnii Döbbeler form patches of moribund gametophytes in otherwise healthy mats of mosses. These pathogens exhibit different methods of host cell disruption; N. mnii appears to displace the host cell protoplast with intracellular hyphae, while T. palustris causes host protoplast degeneration. Host responses to infection by bryopathogens are also variable. Host–pathogen relationships can be highly evolved, as in Bryophytomyces sphagni (Navashin) Cif., in which fungal propagules replace the bryophyte spores, and exploit the explosive dispersal mechanisms of the Sphagnum host. Bryophilous parasites tend to exhibit high tissue or cellular specificity with varying host specificity. For example, Octospora similis (Kirchstein) Benkert infects the rhizoids of species of Bryum, and Discinella schimperi (Navashin) Redhead specifically colonizes the mucilage producing cells of stems of Sphagnum squarrosum Crome. Eocronartium muscicola (Pers.) Fitzp. demonstrates a highly evolved host–parasite relationship in which the basidiocarp displaces the sporophyte and is fed directly by the gametophyte through specialized transfer tissues. Fungi such as Oidiodendron maius Barron are capable of decomposing moss cell walls that are generally resistant to decomposition because of their polyphenolic component. Mycorrhizal fungi, including Glomus, Suillus, and Endogone, have not been observed to form functional, nutrient-exchanging mycorrhizal interfaces with bryophytes, rather, they function as saprobes on moribund and senescent gametophytes. Finally, endophytic fungi may provide bryophyte hosts with greater tolerance to extreme pH or promote vegetative growth. In vivo observation of bryophyte–fungus interactions has provided insight into the types of interactions that occur; however to further understand the physiology, anatomy, and etiology of these interactions, it is necessary to culture bryophilous fungi in vitro and create artificial axenic systems for study.

Fossil ectomycorrhizae were found recently among permineralized plant remains in the middle Eocene Princeton chert of British Columbia. The ectomycorrhizae are associated with roots of Pinus and have a Hartig net that extends to the endodermis, a pseudoparenchymatous mantle, and contiguous extramatrical hyphae that are simple-septate. The mycorrhizal rootlets lack root hairs and dichotomize repeatedly to form large, coralloid clusters. Reproductive structures are absent. Based on the morphological characteristics, and the identity of the host, the closely related basidiomycete genera Rhizopogon and Suillus are suggested as comparable extant mycorrhizal fungi. These exquisitely preserved specimens represent the first unequivocal occurrence of fossil ectomycorrhizae and demonstrate that such associations were well-established at least 50 million years ago.

Resynthesis studies were done to determine the ecological role of Leptodontidium orchidicola, a dematiaceous hyphomycete commonly associated with roots of plants growing in cool and humus-rich soils. Results were compared with those of Phialocephala fortinii, another common root endophyte with similar cultural and vegetative characteristics. In axenic culture with Salix glauca seedlings, L. orchidicola caused a marked increase in host root Length but also invaded the stele, causing extensive cellular lysis. Phialocephala fortinii formed a Hartig net and a thin, patchy mantle. In pot monocultures with Potentilla fruticosa, Dryas octopetala, S. glauca, and Picea glauca seedlings, the effects of four L. orchidicola strains on host dry weight were strain-and host-specific; the effects of Phialocephala fortinii were also host-specific. Leptodontidium orchidicola formed a range of symbiotic associations that could be considered mycorrhizal to parasitic, whereas the effects of Phialocephala fortinii suggest amensal, parasitic, or neutral association. In pot combination cultures, the Phialocephala fortinii -Potentilla fruticosa symbiosis resulted in a significant increase in shoot weight in contrast with the results of the same symbiosis in monoculture resynthesis. The resynthesis experiments demonstrated that the effects of both L. orchidicola and Phialocephala fortinii also vary according to cultural conditions. RQumC : Les auteurs ont conduit des Ctudes de resynthkse pour diterminer le rBle Ccologique du Leptodontidiurn orchidicola, un hyphomyckte dCmatiC communCment associC avec les racines des plantes poussant dans des sols froids, et riches en humus. 11s ont compare les rCsultats avec ceux obtenus avec le Phialocephala fortinii, un autre endophyte racinaire commun ayant des caractCristiques culturales et vCgCtatives similaires. En culture axinique avec des plantules de Salix glauca, le L. orchidicola provoque une forte augmentation de l'allongement des racines de I'hBte mais envahi Cgalement la stkle et cause une importante lyse cellulaire. Le Phialocephala fortinii forme un rCseau de Hartig et un mince manteau discontinu. En monoculture avec des plants de Potentilla fruticosa, Dryas octopetala, S. glauca et Picea glauca, les effets de quatre souches du L. orchidicola sur le poids sec des plants sont spCcifiques a la souche et a l'hBte; les effets du P. fortinii sont Cgalement spkcifiques l'h6te. Le L. orchidicola forme un ensemble d'associations qui peuvent &tre considCrCes comme allant de mycorhizienne a parasitaire, alors que les effets du Phialocephala fortinii suggkrent des relations amensales, parasitaires ou neutres. CombinCs en pots, la symbiose Phialocephala fortinii -Potentilla fruticosa conduit une augmentation significative du poids de la tige, contrairement a la resynthkse en monoculture de la m&me symbiose. Les experiences de resynthkse dCmontrent que les effets du L. orchidicola aussi bien que du Phialocephala fortinii varient selon les conditions culturales.

Pure cultures of endophytic fungi were obtained from the mycorrhizae of some native Alberta orchids (Amerorchis rotundifolia, Calypso bulbosa, Coeloglossum viride, Corallorhiza maculata, Platanthera dilatata, P. hyperborea, P. obtusata). Isolates of Rhizoctonia constituted the largest group of endophytes, but few could be identified to species. One of these strains was identified as Rhizoctonia repens, a species reported to be a ubiquitous and common mycorrhizal fungus of orchids. Another strain produced the teleomorph stage and was identified as Ceratobasidium obscurum. Rhizoctonia anaticula Currah, sp.nov. is described based on five isolates bearing monilioid cells linked by prominent, narrow connections. A second group of isolates consisted of sterile, greenish black to grey fungi. After prolonged incubation, eight of these isolates formed conidia. Two isolates produced phialoconidia and were identified as Phialocephala fortinii. Five isolates, demonstrating sympodial development of conidia in the apical region of swollen or unswollen hyaline conidiogenous cells, are disposed in Leptodontidium orchidicola Sigler & Currah, sp.nov. The remaining conidial isolate was identified as Trichocladium opacum. Two isolates, resembling Rhizoctonia in cultural features, sporulated and are described as Trichosporiella multisporum Sigler & Currah, sp.nov.

Sixty-four species of filamentous fungi from five flare pits in northern and western Canada were tested for their ability to degrade crude oil using gas chromatographic analysis of residual hydrocarbons following incubation. Nine isolates were tested further using radiorespirometry to determine the extent of mineralization of model radiolabelled aliphatic and aromatic hydrocarbons dissolved in crude oil. Hydrocarbon biodegradation capability was observed in species representing six orders of the Ascomycota. Gas chromatography indicated that species capable of hydrocarbon degradation attacked compounds within the aliphatic fraction of crude oil, n-C12- n-C26; degradation of compounds within the aromatic fraction was not observed. Radiorespirometry, using n-[1-14C]hexadecane and [9-14C]phenanthrene, confirmed the gas chromatographic results and verified that aliphatic compounds were being mineralized, not simply transformed to intermediate metabolites. This study shows that filamentous fungi may play an integral role in the in situ biodegradation of aliphatic pollutants in flare pit soils.Key words: bioremediation, filamentous fungi, flare pits, hydrocarbon degradation, petroleum.

We investigated the myeorrhizal status of the dominant vascular plant species occurring in ten peatlands along a bog fen--marsh gradient in southern boreal Alberta in I997. All members of the Ericaceae were ericoid mycorrhizal, and members of the Salicaceae and Pinaccac were ectomycorrhizal. Also, some members of the Salicaceae and Betulaceae were simultaneously eeto-and vesicular-arbuscular mycov rhizal (VAM). Fruiting bodies of the known ectomycorrhizal fungal genera Cortinarius, Lactarius, and Russula were collected in late fall. Furthermore, the cosmopolitan eetomycorrhizal taxon Cenococcum geophifurn was associated with trees and shrubs in all fens and bogs. VA-mycorrhizal fungi were not found in any of the dominant herbaceous plant species in these peatlands; howevei; vesicles suggesting the presence of VAM fungi were found in Cafamagrostis canadensis in the riverine marsh and Rubus chamaemorus in the bog, Neither Carex species in fens and marshes, nor T'~pha latifotia in the lacustrine marsh were myeorrhizal; however, mierosclerotia, sclerotial plaques, septate, aseptate, and clamped hyphae were observed to grow on and within cortical cells of their roots. Many of these hyphae were dematiaceous and may belong to the Mycelium radicis atrovirens complex (MRA), partially consisting of the endophytic fungal genera Phialocephafa and Leptodontidium. Hyphae resembling Rhizoctonia were also observed, although definitive identifications were not attempted. The ecological significance of MRA genera remains largely unknown. Thus, the dominant vegetation in southern boreal bogs and fens is mycorrhizal, possibly enabling these planl species to proliferate in these nutrient-poor ecosystems by accessing otherwise unavailable nutrient pools. In contrast, marsh vegetation is generally non-mycorrhizal, possibly due to higher surface-water nutrient concentrations and fluctuating water levels.