Summary

The cellular pattern and distribution of membranes have been analyzed during cytokinesis in pollen mother cells of Tradescantia and compared with those of actin microfilaments (MFs) and microtubules (MTs). Membranes have been stained with DiOC₆(3) and MFs with rhodamine-labeled phalloidin (RP); analysis has been carried out on the confocal laser scanning microscope. MTs have been visualized as birefringent elements in the polarized light microscope. The results show that when the interzone first appears in mid anaphase it contains an even distribution of membranes. However, by late anaphase these elements have been cleared away, leaving the interzone largely devoid of DiOC₆(3)-positive material. MTs are found throughout this zone, while MFs appear in two non-overlapping sets on both sides of the cell equator. Thereafter membrane elements reappear in the interzone, but only along the equatorial line of the forming cell plate. Presumably these equatorial elements are composed of endoplasmic reticulum and Golgi vesicles, since the larger organelles, including amyloplasts and mitochondria, are excluded from the phragmoplast. MFs, like MTs, arrange preferentially normal to the cell plate, forming a dense array on both sides, but being absent from the zone occupied by the membranes. By contrast, the parallel set of MTs, while excluding larger organelles from the phragmoplast, intermingle with the membrane elements in the cell equator. As cytokinesis proceeds membranes continue to concentrate on the cell plate as indicated by its marked increase in staining with DiOC₆(3). From a consideration of spatial and temporal organization of the phragmoplast components it is reasonable to suggest that both cytoskeletal components participate in the aggregation of vesicles that give rise to the cell plate. Membranes, on the other hand, through the provision of surface binding sites and/or through the regulation of the cytoplasmic calcium ion concentration, might be involved in the assembly and stabilization of the cytoskeleton.