Ultrastructural Cytochemistry of the Host-Fungus Interfaces in the Endornycorrhizal Association Glomus mosseae / Allium cepa

Abstract

The structure of the host-fungus interface in the VA mycorrhizal association Glomus mosseae / Allium cepa has been investigated cytochemically (Thiery test; differential DMSO and EDTA extraction; phosphotungstic acid staining) and using different fixation procedures (glutaraldehyde and OsO4 post-fixation; simultaneous glutaraldehyde — OSO4 fixation). The continous, highly invaginated host plasmalemma surrounding the intracellular fungal structures retains its affinity for phosphotungstic acid and continues to produce polysaccharide fibrils, but in the presence of the endophyte it progressively loses the capacity to organise these into well-structured wall material. At the point of cell penetration and in the primary arbuscule branches the fungal wall, which is stained by the Thiery reaction and phosphotungstic acid but unaffected by DMSO or EDTA, is surrounded by a collar of host wall material which has the same cytochemical properties as the primary walls of the plant cell. The fine arbuscule branches are surrounded by a layer of condensed matrical fibrils. When the arbuscule branches senesce and collapse, they become encased by the polysaccharide fibrillar material derived from the host. The appearance of the interfacial matrix separating the host plasmalemma from the fungal wall is always the same, no matter which fixation procedure is used. It contains membranous vesicles and scattered polysaccharide fibrils, both of which are elaborated by the host plasmalemma. The importance of the host and endophyte plasmalemma formations is emphasised. The vesicles arising from the host plasmalemma are often in contact with the walls of the endophytic hyphae. In the fungus, these bulbous or tubular configurations of the plasmalemma can become very numerous and can occupy the whole hyphal width. Although these structures could be involved in host-fungus exchanges, their eventual participation in wall synthesis is also orooosed.