Adhesion of conidia and germlings of the plant pathogenic fungus Bipolaris sorokiniana to solid surfaces

Soon after coming in contact with its host, the plant pathogenic fungus Bipolaris sorokiniana produces an extracellular material that appears to be important for adhering conidia and germlings to the host surface. To further understand this step of the infection, the adhesion of B. sorokiniana to artificial solid surfaces was examined. On a hydrophobic (polystyrene) surface adhesion occurred in two stages, the first by conidia and the second by germlings. Conidial adhesion occurred shortly (0-1 h) after hydration. The conidia were easily detached by increasing the shear force and including detergents in the washing buffer. As conidia were hydrophobic, these observations indicate that conidial adhesion to polystyrene is due to weak, hydrophobic interaction. The second stage of adhesion was accompanied by conidial germination and occurred 1-2 h after hydration and contact with the surface. Concomitant with the delayed adhesion, the fungus produced an extracellular matrix (ECM). The adhesion of germlings was firm and surface-unspecific since they adhered to both hydrophobic and hydrophilic (glass) surfaces, Except for strong bases, hydrochloric acid and broad-specificity proteases (including Pronase E), none of the hydrolytic enzymes, electrolyte solutions, ionic and hydrophobic detergents and organic solvents removed germlings from the solid surfaces. The adhesion of germlings incubated in the presence of the protein glycosylation inhibitor tunicamycin or the lectins Con A (Concanavalin A) and GNA (from Galanthus nivalis) was significantly reduced, which indicates the involvement of surface glycoproteins in this process. The surface proteins of germlings were labelled with I-125, extracted and analysed by two-dimensional gel electrophoresis. This revealed about 40 surface proteins over a wide pH range (4-10) with molecular masses between 10 and 100 kDa

Visualization and characterization of the extracellular matrix of Bipolaris sorokiniana

Extracellular matrix (ECM) surrounding conidia and germlings of B. sorokiniana was studied using light microscopy (LM), scanning (SEM) and transmission electron microscopy (TEM). Conidial ECM surrounding dry-inoculated, ungerminated conidia was fluid-like and observed only using a cryo-preparation technique, suggesting that the material was water soluble. ECM enveloping germlings appeared fibrillar in LM, TEM and SEM but amorphous in cryo-SEM, indicating that the structure of the ECM is dependent on the water content of the matrix. Fibrillar ECM formed thread-like structures that extended over long distances on the substrate or towards neighbouring conidia and hyphae. TEM of germlings negatively stained with uranyl acetate revealed the presence of fungal fimbriae. The strong resemblance between the extending organization of fibrillar thread-like ECM structures and fimbriae suggested that fimbriae constitute a basic structural component of the ECM and serve as the aggregation centre for the other ECM components. Histochemical labelling revealed significant differences between ECM surrounding the fungus at different morphological stages. The germ tube ECM was labelled for both proteins and polysaccharides whereas germling ECM consisted of two layers: an inner rich in proteins and an outer composed mainly of polysaccharides. Furthermore, the newly released ECM localized on germ tubes and hyphal tips showed affinity for microspheres carrying any type of surface properties while hyphal ECM had affinity only for negatively charged microspheres. This together suggests that ECM after its release is subjected to structural changes

Extracellular matrix, esterase and the phytotoxin prehelminthosporol in infection of barley leaves by Bipolaris sorokiniana

Light microscopy and cryo-scanning electron microscopy showed that hyphae of Bipolaris sorokiniana adhered to the wax surfaces of barley leaves by means of an extensive extracellular matrix (ECM). Prehelminthosporol, the major non-host specific phytotoxin formed by B. sorokiniana was immunolocalized in large amounts in the ECM surrounding the hyphae. Similarly, esterase activity involved in degradation of the cuticular wax surface was found in the ECM. Therefore, it appears that the ECM is not only important for adhesion of the fungus to its host, but also functions as a sink of phytotoxins and lytic enzymes important for infection of the host plant

Infection events in the fungus-nematode system

Infection of vermiform nematodes by nematophagous fungi generally follows the same route whether the nematode is captured in adhesive traps of predatory fungi or mediated by conidia of endoparasites. A successful infection always results in complete digestion of the nematode corpus. The total time for the infection process varies between fungal species and also between species of nematodes being parasitized. The zoosporic phycomycete Catenaria anguillulae usually completes its life cycle in a rhabditoid nematode in 24 hr, whereas the hyphomycetous endoparasite Meria (Drechmeria) coniospora takes up to 72 hr for the same process. The infection of nematodes by nematophagous fungi usually follows a general sequence of events starting with recognition between the two types of organisms. We will use the term recognition in a broad sense including attraction phenomena as well as interactions on a molecular level involved in adhesion.1-2 The recognition step is followed by penetration of the nematode cuticle, possibly a toxification step, and finally the animal is digested by trophic hyphae of the fungus. The different infection events, some of which may not necessarily be present in all fungal-nematode interactions, will be discussed in this chapter