Birgit Nordbring-Hertz

Nematode-Induced Morphogenesis in the Predacious Fungus Arthrobotrys Oligospora

Living nematodes induced trap formation in Arthrobotrys oligospora more rapidly than did additions of morphogenetic peptides. In nematode-induced morphogenesis, excreted substances as peptides and amino acids were only partly responsible for the effect. Additional effects were due to volatile substances from nematodes or to direct contact between living nematodes and the hyphae. Ammonia was shown by gas chromatography to be excreted in nematode suspensions in amounts that could affect trap formation. It is proposed that living nematodes act primarily through another mechanism than peptide-induced morphogenesis.

Interactions Between Nematophagous Fungi and Plant-Parasitic Nematodes: Attraction, Induction of Trap Formation and Capture

The behaviour of five plant-parasitic nematodes was compared with that of a bacteria-feeding nematode. The attraction of the plant-parasitic nematodes to thirteen nematophagous fungi was tested. The two plant-parasitic nematodes also known to be mycophagous were attracted to all fungi. In the other nematodes there was no distinct attraction pattern. The ability of different fungi to attract tended to increase with increasing dependence of the fungi on nematodes for nutrients. The rapidity with which the nematodes induced trap formation depended on their motility and on the concentration of nutrients in the culture medium. The ability of a nematode to induce trap formation was not correlated with feeding behaviour (bacteria-feeding, mycophagous or plant-parasitic). All nematodes were rapidly captured when traps were present.

Fungal attachment to nematodes

The adhesion mechanisms in three nematophagous fungi are reviewed. In all these fungi the infection and subsequent digestion of nematodes is initiated by the firm adhesion of the fungus to the nematode surface. In Arthrobotrys oligospora the adhesive phase is restricted to special three-dimensional structures. Drechmeria coniospora conidia attach to the nematode cuticle by an adhesive bud while Catenaria anguillulae uses an adhesive phase of zoospore development for this purpose. In A. oligospora , the adhesion of nematodes to the traps is mediated by a layer of extracellular fibrillar polymers. The ultrastructure of this layer changes during adhesion, the fibrils become more dense and oriented in one direction. The surface layer consists mainly of carbohydrate-containing fibrils and a low-molecular-weight protein. Previous and present studies show that the low-molecular-weight protein of A. oligospora is a lectin. In contrast, the adhesive layer of D. coniospora does not seem to change during the adhesion process. The adhesin of C. anguillulae appears to consist mainly of protein. The studies suggest that adhesion of nematodes to A. oligospora involves a recognition event using a lectin-carbohydrate interaction which might trigger the reorganization of the surface polymer layer and release of enzymes, leading to the firm binding of the nematode. A similar process may take place also in other nematophagous fungi.

Attraction of Nematodes to Living Mycelium of Nematophagous Fungi

Methods were designed to detect attraction and repulsion of nematodes by fungi and determine the attraction intensity of different fungi. Of 23 fungi tested, 15 attracted the bacteria-feeding nematode Panagrellus redivivus. Of the 14 nematophagous fungi tested, ten attracted and one repelled nematodes, whereas three were neutral. Among nine non-nematophagous fungi, five attracted nematodes. In general, the attraction intensity increased with increasing dependence of the fungi on nematodes for nutrients.

NEMATOPHAGOUS FUNGI - PHYSIOLOGICAL-ASPECTS AND STRUCTURE-FUNCTION-RELATIONSHIPS

Publisher Summary The chapter discusses physiological, biochemical, and ultrastructural aspects of both predatory and endoparasitic nematophagous fungi and explains current views on the role of the fungi in natural ecosystems. Both predatory and endoparasitic fungi share the property of producing sophisticated morphological structures that enable them to attack living nematodes. Some predatory fungi form trapping structures only after induction by external stimuli (e.g. the presence of living nematodes), others develop traps spontaneously. Parasites of nematodes are found among all major fungal taxonomic groups. All devices that mediate fungal-nematode interactions are either specialized hyphal structures or differentiated cells—for example, conidia. These “special-purpose” structures are equipped to establish a firm contact with the prey and, subsequently, to mediate penetration of the cuticle. The various morphological adaptations employed in nematode-capturing are described. The complex nematode-fungus interaction is mainly descriptive in nature and is primarily based on various light- and electronmicroscopical methods as major tools. Light microscopy and video-enhanced contrast microscopy have been used to study interactions at the cellular level using living organisms, and have proved especially useful to investigate kinetics of the infection process.

Surface polymers of the nematode-trapping fungus Arthrobotrys oligospora

The nematophagous fungus Arthrobotrys oligospora captures nematodes using adhesive polymers present on special hyphae (traps) which form a three-dimensional network. To understand further the adhesion mechanisms, A. oligospora surface polymers were visualized by transmission electron microscopy and characterized by chemical methods. Both traps and hyphae were surrounded by a fibrillar layer of extracellular polymers which stained with ruthenium red. The polymer layer was resistant to most of the chemicals and enzymes tested. However, part of the layer was removed by sonication in a Tris-buffer or by extraction in a chaotropic salt solution (LiCl), and the structure of the polymers was modified by treatment with Pronase E. Chemical analysis showed that the crude extracts of surface polymers removed by sonication or LiCl solution contained neutral sugars, uronic acids and proteins. Gel chromatography of the extracts revealed that the major carbohydrate-containing polymer(s) had a molecular mass of at least 100 kDa, containing neutral sugars (75% by weight, including glucose, mannose and galactose), uronic acids (6%) and proteins (19%). There was more polymer in mycelium containing trap-bearing cells than in vegetative hyphae. SDS-PAGE of the extracted polymers showed that the trap-forming cells contained at least one protein, with a molecular mass of approx. 32 kDa, not present on vegetative hyphae. Examining the capture of nematodes by traps of A. oligospora in which the layer of surface polymers was modified, or removed by chemical or enzymic treatments, showed that both proteins and carbohydrate surface polymers were involved in the adhesion process.

An electron-microscopical analysis of capture and initial stages of penetration of nematodes by Arthrobotrys oligospora

A detailed analysis was made of the capture and subsequent penetration of nematodes by the nematophagous fungusArthrobotrys oligospora using different electron-microscopical techniques. Capture of nematodes by this fungus occurred on complex hyphal structures (traps) and was effectuated by an adhesive coating, present on these trap cells. The adhesive layer was largely fibrillar in nature and was absent on cells of normal hyphae. Following capture, penetration hyphae were formed at those sites where the trap cell wall was anchored to the nematode cuticle by the adhesive. New walls of these hyphae were formed underneath the original trap cell walls, which were partly hydrolysed to allow growth and development of the penetration tubes through the adhesive coating towards the cuticle. Our observations indicated that the cuticle of the nematode was subsequently penetrated by the penetration tubes by mechanical means. After penetration a large infection bulb was formed from which trophic hyphae arose. Cytochemical experiments indicated that the sites of penetration of the cuticle were intensely stained for acid phosphatase activity. At later stages of infection activity of this enzyme was present throughout the nematode contents; the enzyme was most probably secreted by complex membranous structures associated with the cytoplasmic membrane of the infection bulb and the trophic hyphae.

The Endoparasitic Nematophagous Fungus Meria coniospora Infects Nematodes Specifically at the Chemosensory Organs

SUMMARY: Conidia of the endoparasitic fungus Meria coniospora infected the bacterial-feeding nematode Panagrellus redivivus at specific sites, namely the mouth region and in male nematodes also in the tail. Plant-parasitic nematodes were also infected in other parts of the body. The specific infection sites in P. redivivus were the sensory organs, which are the sites of chemoreception. Blocking of chemoreceptors by adhered conidia of M. coniospora caused a complete loss of the ability of nematodes to be attracted to different sources of attractants. Inhibition of conidial adhesion by means of sugar haptens suggested a lectin-carbohydrate interaction. Out of 21 sugars only N-acetylneuraminic acid inhibited the adhesion, indicating the importance of sialic acids in the infection process. Reduction of conidial adhesion by treatment of nematodes with neuraminidase further supported this view.

Conidial traps — a new survival structure of the nematode-trapping fungus Arthrobotrys oligospora

Conidia of A. oligospora germinated directly into adhesive traps when applied close to cow faeces on water agar plates. With preincubation periods of 2–9 days before inoculation, around 90% of the conidia germinated as conidial traps close to the faeces and as normal germ-tubes further away from the faeces. Conidial traps appear to develop in response to diffusing substances from cow faeces, rather than in response to living nematodes. The conidial trap is considered a survival structure enabling the fungus to overcome fungistasis. Traps adhere to the surface of passing nematodes, thus facilitating the spread of the fungus, before penetration of the nematode cuticle and immobilization of the nematode take place.

Involvement of sialic acid in nematode chemotaxis and infection by an endoparasitic nematophagous fungus

SUMMARY: Conidia of the endoparasitic nematophagous fungus Meria coniospora adhere to the sensory organs of the nematode Panagrellus redivivus leading to a complete loss of the nematodes ability to be attracted to fungi. Sialic acids located on the head region of the nematode seem to be involved in this interaction. Treatment of the nematode with sialidase reduced attraction towards fungi, and treatment with the sialic acid-specific lectin, limulin, reduced both nematode attraction and conidial adhesion. Other lectins tested (WGA, SBL, HPL, Con A) gave no effect on either attraction or adhesion. Treatment of conidia with trypsin or glutaraldehyde reduced conidial adhesion, indicating the presence of a fungal sialic acid-specific carbohydrate-binding protein. The results show the importance of sialic acids in nematode chemotaxis and also in adhesion of conidia of M. coniospora. Sialic acid, therefore, seems to be a link between attraction and adhesion in this system.