Ultrastructural characterization of an extracellular fibrillar sheath on cells of Ascocalyx abietina, the scleroderris canker agent of conifers

Abstract

Morphology, ultrastructure, and some aspects of the chemical composition of a fibrillar sheath surrounding cells of the fungus Ascocalyx abietina (Lagerberg.) Schlaepfer-Bernhard were studied using electron microscopy and gold-labeled ligands. Although consistently present around all cells, the fibrillar matrix was found to vary greatly in morphology within the same isolate, depending apparently on age and (or) physiological conditions of the cells. Around cells considered younger, the sheath appeared always to be constituted of fibrillar masses that varied in size and shape but were delineated by a well-defined border. In contrast, cells expected to be older were generally bordered by a regular and uniform matrix composed of numerous intertwined fine fibrils, some being associated with small osmiophilic knobs. The presence of RNA in the denser layers of the sheath was revealed through gold complexes with either RNase A or RNase B. Continuity of portions of the sheath with similar material surrounding endocells or intact cells through gaps in the wall was frequently observed. This observation was considered as one of the possible explanations for the presence of RNA in the sheath. Association of sugars such as β-glucopyranosides and especially sialic acid with the extracellular matrix is, most probably, relevant to specific biological functions such as attachment to host cells, protection against unfavourable physical conditions and transport of cationic compounds. Peculiarities of this sheath produced by A. abietina contribute, therefore, to distinguish it from those described in other fungi.