ABSTRACTThe distribution of the actin cytoskeleton was examined in hyphal tips of the plant pathogenic fungusSclerotium rolfsii.Actin was visualized with light microscopy using immunofluorescence techniques and with ultrastructural immunocytochemistry. Computer-aided, three-dimensional reconstruction analysis of ultrathin serial sections and immunoblot analysis techniques were also employed in this study. Immunofluorescence data indicated that the majority of actin was localized in brightly fluorescent plaques that were principally localized in the first 10 to 12μm of the hyphal tip. Focusing through the hyphal cell revealed that most plaques were positioned in the peripheral cytoplasmic regions. Actin microfilaments were not resolved at the light microscopic level; however, coarse actin fibers oriented parallel to the long hyphal axis were observed in subapical regions and in association with septa. A diffuse, amorphous cytoplasmic staining was present in the apical regions of the hypha except for that area occupied by the Spitzenkörper. Ultrastructural immunocytochemistry and three-dimensional reconstruction analysis suggested that the fibrillar coating of filasomes contained actin and that filasomes represented the ultrastructural equivalent of actin plaques. Immunoblot analysis of total protein extracts separated by one-dimensional sodium dodecyl sulfate Polyacrylamide gel electrophoresis identified two actin related proteins, one with a relative molecular mass of 43,000 and the other migrating with a slightly higher molecular mass of 47,500.