By using fluorescently labeled phalloidin we have examined, at the light microscope level, the three-dimensional distribution and reorganization of actin-like microfilaments (mfs) during plant cell cycle and differentiation. At interphase, mfs are organized into three distinct yet interconnected arrays: fine peripheral networks close to the plasma membrane; large axially oriented cables in the subcortical region; a nuclear "basket" of mfs extending into the transvacuolar strands. All these arrays, beginning with the peripheral network, disappear at the onset of mitosis and reappear, beginning with the nuclear basket, after cytokinesis. During mitotic and cytokinetic events, mfs are associated with the spindle and phragmoplast. Actin staining in the spindle is localized between the chromosomes and the spindle poles and changes in a functionally specific manner. The nuclear region appears to be the center for mf organization and/or initiation. During differentiation from rapid cell division to cell elongation, mf arrays switch from an axial to a transverse orientation, thus paralleling the microtubules. This change in orientation reflects a shift in the direction of cytoplasmic streaming. These observations show for the first time that actin-like mfs form intricate and dynamic arrays in plant cells which may be involved in many as yet undescribed cell functions.

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