The pattern of colony development and the formation of the uredinium of Uromyces viciae-fabae on Vicia faba

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Abstract
Low-temperature scanning electron microscopy of frozen-hydrated specimens and transmission electron microscopy of cytochemically stained thin sections and enzymically digested samples have been used to study the pattern of colony development from the initial infection in the substomatal chamber of the leaf to the production of urediniospores within uredinia. Rapid immobilization of leaf samples by freezing enables them to be fractured, revealing information on internal structural relationships between intercellular hyphae and host cells. Extracellular matrix substances, which are well preserved by low-temperature scanning electron microscopy, play an important role in the cohesion of hyphae during the early stages of uredinium formation. Cytochemical staining indicates the presence of two such matrix materials: (i) colloidal iron-negative matrix surrounding intercellular hyphae, forming a pad between adjacent hyphal cells and (ii) a colloidal iron-positive matrix between hyphal and host cell walls. A positive reaction with colloidal iron indicates acidic mucopolysaccharides. Enzymic digestion with pronase and the staining reaction with silicotungstic acid – chromic acid suggests that a glycoprotein is present in the matrix surrounding intercellular hyphae. Possible roles for the extracellular matrix, including hyphal cohesion, cell–cell communication, and protection against desiccation, are discussed.