Cytochalasin is known to inhibit cytoplasmic streaming rapidly in characean cells without disassembling their actin bundles. Lower cytochalasin concentrations than those needed for streaming inhibition are now shown to disrupt bundle assembly and, over longer periods, assembled bundles. After local wounding, cytochalasin limited bundle regeneration to the production of polygons and straight, discontinuous bundles that rarely connected to bundles outside the wound. The regenerated bundles supported only scattered organelle movements, whereas long, oriented bundles of control cells were connected to those outside the wound and supported bulk endoplasmic streaming. Unwounded Chara plants cultured for up to 2 weeks in 1 microM-cytochalasin maintained normal bundle orientation and rapid cytoplasmic streaming, but the mean number of bundles per file of chloroplasts fell from 5.2 in controls to 2.0 in growing cells and 3.4 in nongrowing cells. These structural effects seem more likely than the streaming inhibition to reflect cytochalasin's in vitro effect of blocking extension at the barbed but not the pointed end of F-actin. In particular, cytochalasin inhibited the extension into the wound of bundles in which only the barbed ends of filaments would be exposed. However, short lengths of isolated bundles grew within the wound and bundle growth in the intact cell continued, albeit in modified form. It is suggested that these examples of continuing bundle growth involve cytochalasin-resistant mechanisms that are not wholly dependent on barbed-end filament growth.

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