Overview of Plant Cytokinesis

Cell division is a process that distributes the duplicated genome evenly to two prospective daughter cells and splits one mother cell into two daughter cells. Cell division is divided into two stages, namely karyokinesis and cytokinesis. Although the processes observed during karyokinesis (breakdown of the nuclear envelope, condensation of the chromosomes, alignment of the chromosomes on the equatorial plane, formation of the spindle and the movements of the chromosomes) are conserved in almost all eukaryotes, those of cytokinesis vary among different groups of organisms. Cytokinesis in animals and fungi is achieved by constriction of the cell membrane from the outside to the inside (Field et al. 1999); however, in plants it is achieved by the creation of cell walls from the inside to the outside during anaphase. The newly synthesized cell wall is called the cell plate.

The formation and development of the cell plates occurs in a plant-specific apparatus called the phragmoplast (J├╝rgens 2005). The phragmoplast is composed mainly of microtubules, whose plus ends are arranged head-to-head on the equatorial plane and the minus ends are directed towards each of the two daughter nuclei. The cell plate components are considered to be transported along the microtubules by vesicles derived from the Golgi body. However, it has recently been reported that endocytic delivery of cell surface material contributes significantly to the formation of the cell plate (Dhonukshe et al. 2006). Vesicles containing materials for the construction of new plasma membranes and cell walls accumulate at the cell plate and fuse to the pre-existing immature cell plate, resulting in expansion of the cell plate.

As the cell plate expands, vesicles are initially fused to each other to generate a fusion tube-generated network (FTN) at the edge of the cell plate (Samuels et al. 1995). The structure of the FTN gradually changes from that of a tubulovesicular network, through a tubular network and a fenestrated sheet, to the mature cell plate as the fusion of vesicles progresses. Finally, the cell plate reaches the parental plasma membrane and fuses with it. These changes in vesicular and tubular structures proceed from the edge to the center of the cell plate, coupled with the centrifugal expansion of microtubules from the center to the periphery.

The centrifugal expansion of the phragmoplast depends on the dynamic reconstitution of microtubules. In the early stage of phragmoplast development, microtubules form a cylinder-like structure between the two daughter nuclei. Upon expansion of the phragmoplast, this cylinder of microtubules becomes wider, eventually becoming a barrel-like structure. Such structural alteration of the phragmoplast is achieved by the depolymerization of micro-tubules and polymerization of tubulins in the inner and peripheral regions of the phragmoplast, respectively. The exponentially increasing turnover of microtubules has to support the transport of a vast number of vesicles to the mid-zone at the leading edge of the phragmoplast in order to provide sufficient material to supply the areas of the cell plate that are growing exponentially.

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