The differential effects of cytochalasin B on microfilament populations and cytoplasmic streaming

Abstract

Two distinguishable populations of microfilaments (mfs) can be identified in the radish root hair. Bundles of mfs are found throughout the cytoplasm, excluding the tip region of the hair. Single mfs occur only as a cortical array, specifically associated with the microtubules. Both mf populations are oriented parallel to the direction of streaming. Hairs grown in 5 μg/ml cytochalasin B (CB) exhibit site-specific differential responses to the drug in both their streaming pattern and sensitivity of their mfs. Cytochalasin B elicits the following responses: 1. cytoplasmic streaming is reduced in all regions of the hair; 2. small particles ( 1 μm in diameter) no longer stream but exhibit an oscillatory or rotational motion; 3. filament bundles show increasing sensitivity to CB along the length of the hair; 4. single mfs show decreasing sensitivity to CB along the hair length. The effects of CB on cytoplasmic streaming can be related to its effects on both mf populations, thus suggesting that although mf bundles are probably involved in streaming in the sub apical and basal regions of the hair, single mfs are most likely involved in generating the slower, more irregular streaming patterns exhibited in the hair tip and CB-treated hair base.