Function of the Septal Pore Apparatus in Rhizoctonia Solani during Protoplasmic Streaming

Protoplasmic streaming is regarded as an important means of food transport and translocation of cytoplasm in the filamentous fungi (1-3). In addition, streaming may play a vital role in the migration of nuclei through fungous thalli (1, 4). In the septate fungi, cross-walls possessing a single minute central pore, 0.1 /~ diameter, are situated at intervals in the mycelium. Complex septal structure is typical of many Basidiomycetes (4-6) and strongly suggests a potential physical barrier to protoplasmic streaming and nuclear migration in the mycelium of these organisms (5). Nevertheless these phenomena have been reported in fungi possessing an intricate septal pore apparatus (1, 4, 6). Such an apparatus, described earlier (6), is found in Rhizoctonia solani Kuehn, a widespread plant pathogenic fungus. This report introduces evidence for a mechanism in R. solani which permits the septal pore to accommodate the passage of objects as large as 0.5 t~ diameter, through an increase in the diameter of the pore. For electron microscopy, the mycelium was fixed