The Dividing Cell: Cytokinesis

The section comprises of pages on the Phragmoplast, Cortical Division Zone, Cell Plate and Secretory compartments found at the division plane. Contribution / Reviewing of this section is by

Steven Backues1 & Sebastian Bednarek1 and Daniel Van Damme2 & Danny Geelen3

1. Department of Biochemistry, University of Wisconsin, Madison 53706, Wisconsin. USA.
2. VIB Department of Plant Systems Biology, Ghent University Technologiepark 927, 9052 Gent, Belgium.
3. Faculty of Bioscience Engineering, Department of Plant Production, Coupure links 653, 9000 Ghent, Belgium.

The last step in cell division, after the chromosomes have been separated during mitosis, is cytokinesis, the physical separation of the cell contents in order to form two new daughter cells. In plant cells, cytokinesis is accomplished by the de novo synthesis of a new plasma membrane and cell wall. This new cell boundary must be impermeable enough to allow each daughter cell its own identity, and yet traversed by plasmodesmata to allow cell-cell communication. It must be precisely positioned in order to properly partition the cytoplasm, organelles and other cellular contents. Its formation is a complex and fascinating process requiring the coordination of massive amounts of membrane trafficking, membrane remodeling, cytoskeletal dynamics and cell wall deposition.  
Years of study by light and electron microscopy have defined the structures necessary for plant cytokinesis, and the stages through which it progresses. In somatic cell cytokinesis, which is focused upon in this section, the first structure to form is the phragmoplast, an array of microtubules that guides and supports the formation of the cell plate, which will mature into the new plasma membrane while the new cell wall is synthesized in its lumen. The cell plate initiates in the center of the cell within the phragmoplast midzone, and expands outward until it fuses with the existing plasma membrane at the cortical division zone. Cell plate formation and maturation also requires the involvement of many secretory compartments that are found in close proximity to the division plane.
More recently, the use of fluorescent-protein fusions (FP-fusions) has allowed these structures and their dynamics to be visualized in bright colors by live cell imaging, further illuminating their complicated interplay. The labeling of specific proteins with fluorescent fusion tags has helped to define the molecular players in this fascinating game, and shed light on how this essential process unfolds. Here we describe some of those FP-fusions, their localizations and their possible roles in plant cytokinesis.

Future Directions
FP fusion proteins have proven themselves very useful in studying the localization and dynamics of proteins involved in plant cytokinesis. However, much more work is needed to clarify the actual role of many of these proteins in the process of cell plate formation. In some cases it is not even clear whether proteins are actually localized to the cell plate itself or to other organelles that cluster at the division plane, such as ER or MVB. This distinction requires greater spatial resolution than provided by light microscopy, so localization should be verified by additional methods such as electron microscopy.
FP fusions have also contributed to our understanding of the general process of cytokinesis, and in particular, the dynamics involved in the maturation of the phragmoplast and cell plate. In the near future, co-localization studies of proteins fused to different FPs should give further insight into both the spatial and temporal distribution of the different pathways and processes necessary for cytokinesis. The continued discovery of new cell plate targeted probes should also contribute to our understanding of the timing and organization of membrane domain transitions between morphologically distinct cell plate regions. In addition, correlation of the dynamic information gained from FP localization studies with the high-resolution models of cell plate formation derived from electron tomography will be essential in our attempt to form an accurate picture of the complex and essential process of plant cytokinesis.