Phragmoplasts in Cytokinesis

The second phase of cell division is called cytokinesis. Most non-plant cells are analogous to water balloons. Cytokinesis and cell division are completed by constricting, then pinching off between the daughter nuclei (see related movie at left). In the case of plant cells, the box around the balloon makes cytokinesis conceptually more complex. A new cross-wall between the daughter nuclei requires coordinated deposition of new balloon and box material. This process involves yet another configuration of the cytoskeletal scaffold called the phragmoplast.

Here (44K gif) are images of a cell late in cell division. The daughter nuclei have already been separated by the spindle, and the phragmoplast has formed between them. The 3-D relationship between the expanding phragmoplast and forming cell plate are better appreciated in these animated GIFs, which have been created from real microscopic images.

The phragmoplast forms between the daughter nuclei - a logical location to construct the new cross-wall. It directs packets of new cross-wall materials which coalesce into a disk-shaped structure called a cell plate. The plate expands outward to fuse with the sidewalls, forming a new cross-wall between daughters. The new wall matures, then the cells can elongate as described above.

So the scaffolding in dividing cells separates the chromosomes and directs the construction of a new cross wall between the daughter nuclei. The scaffold's participation in cell division also means that it can play a critical role in not only elongative growth of the whole plant, but a more pronounced effect on the shape of the plant.