Visualization and characterization of the extracellular matrix of Bipolaris sorokiniana

Extracellular matrix (ECM) surrounding conidia and germlings of B. sorokini ana was studied using light microscopy (LM), scanning (SEM) and transmissio n electron microscopy (TEM). Conidial ECM surrounding dry-inoculated, unger minated conidia was fluid-like and observed only using a cryo-preparation t echnique, suggesting that the material was water soluble. ECM enveloping ge rmlings appeared fibrillar in LM, TEM and SEM but amorphous in cryo-SEM, in dicating 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 pres ence of fungal fimbriae. The strong resemblance between the extending organ ization of fibrillar thread-like ECM structures and fimbriae suggested that fimbriae constitute a basic structural component of the ECM and serve as t he aggregation centre for the other ECM components. Histochemical labelling revealed significant differences between ECM surrounding the fungus at dif ferent morphological stages. The germ tube ECM was labelled for both protei ns and polysaccharides whereas germling ECM consisted of two layers: an inn er rich in proteins and an outer composed mainly of polysaccharides. Furthe rmore, the newly released ECM localized on germ tubes and hyphal tips showe d affinity for microspheres carrying any type of surface properties while h yphal ECM had affinity only for negatively charged microspheres. This toget her suggests that ECM after its release is subjected to structural changes.