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 surfaceunspecific 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, 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.
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.
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