Ultrastructural Organization of the Non-Differentiated Uredospore Germling ofUromyces PhaseoliVarietyTypica

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Abstract
The ultrastructure of Uromyces phaseoli var. typica bean rust uredospore germ tubes was studied to ascertain the position and organization of the cellular components. Specimen fixation by freeze-substitution allowed us to document a more artifact-free cytoplasm compared to room temperature fixation as well as to elucidate new structures. Cytoplasmic microtubules, primarily oriented with the longitudinal axis of the hypha, were most abundant near the hypha-substrate interface. The protofilament nature of microtubules was visualized in sectioned cells. Microfilaments of at least nine morphologies were associated with various organelles or were free in the cytoplasm. The cluster of apical vesicles and microvesicles was studied with respect to position in the hyphal apex. The majority of the apical vesicles were positioned nearer the substrate as opposed to being centrally located in the dome of the hyphal tip. The ultrastructure and position within the germ tube of Golgi cisternae, endoplasmic reticulum, vacuoles, multivesicular bodies, and mitochondria were determined. The area within the nucleoplasm, subjacent to the spindle pole body, contained 5.2-7.8 nm diam microfilaments. A new organelle, termed filamentous distalsome, was observed in the basipetal-most region of the cytoplasm. It consisted of 12 nm diam microfilaments attached to membrane-bounded cylindrical cisternae (ca. 30 nm diam × 0.65 μm long).