Ultrastructural and cytochemical changes in the wall of a vesicular–arbuscular mycorrhizal fungus during symbiosis

During the various stages of the symbiotic association of the vesicular–arbuscular mycorrhizal (VAM) fungus Glomus fasciculatus (Thaxter) Gerd, et Trappe with the roots of Vitis vinifera L. cv. Barbera, the hyphal wall undergoes marked modifications as follows: (i) a gradual thinning out, from the extraradical hyphae to the intraradical hyphae; (ii) changes in its ultrastructural architecture, in that the wall consists of stacked lamellae in both the extraradical and coiled hyphae occurring in the outer layers of the cortical parenchyma, whereas it is monostratified in the intercellular and arbuscular hyphae occurring in the inner parenchymal layers; and (iii) different responses to cytochemical reactions. The protein and polysaccharide components, though consistently present in the fungal wall at all stages, vary in their staining characteristics after the PATAg and Swift procedure and in their responses to alkali extraction, suggesting a gradual simplification of the wall components from the extraradical hyphae to the intraradical ones.