18.17: The Phragmoplast
Cell division is essential for organismal growth and development. In animal cells, the central spindle and its associated proteins form the midbody, a structure that has an essential role in cytokinesis. In plants, the central spindle, along with the microtubules, actin, and other cell components, matures into the phragmoplast, which is necessary for cytokinesis. Unlike the stationary midbody, the phragmoplast expands centrifugally, eventually leading to the formation of the new cell wall.
The mature phragmoplast is donut-shaped and contains an outer leading region, a middle transition zone, and an inner lagging region. In the leading region, new microtubules are assembled continuously. Microtubule nucleation is initiated by the γ-tubulin ring complex (γ-TuRC), which contains six subunits—GCP1 to GCP6. However, γ-TuRC cannot directly interact with the microtubules; therefore, augmin, a microtubule-interacting protein complex, mediates the recruitment of γ-TuRC to the microtubules. The microtubules present in the phragmoplast act as tracks for transporting the secretory vesicles produced by the nearby Golgi body.
In the middle transition zone, the vesicles initially transition into tubule-like structures and then interconnect to form an intertwined tubular network. The continuous addition of secretory vesicles results in the outward growth of the tubular network until it meets with the parental cell membrane. The vesicle membranes are used to form the plasma membrane of the daughter cells, while the biomolecules present in the vesicles are used to build the cell plate present between the two daughter cells’ membranes.
In the lagging region, the microtubules depolymerize, leaving behind the mature portion of the cell plate. The fully formed cell plate subsequently develops into the cell wall that separates the two daughter cells. The polymerization of microtubules at the leading end and the depolymerization of the microtubules at the lagging region cause the outward expansion of the phragmoplast and the cell plate until the latter joins with the parent cell wall. Failure in phragmoplast functions can result in multinucleated cells and embryonic lethal phenotypes.