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The Phragmoplast

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Plant cells are enclosed by a rigid cell wall, making it difficult for cells to divide by contracting from the outside. Hence, plant cells divide by forming a new cell wall between the two daughter nuclei in an “inside out” fashion. A specialized structure in plants called the phragmoplast guides the new cell wall formation.

The plane of cell division, which is the future site of cell wall formation, is marked by a preprophase band made up of microtubules and actin filaments that forms during the G1 phase of the cell cycle. The preprophase band assembles at the cell cortex and disappears at the beginning of the M phase.

During mitosis, chromosomes separate and start moving to opposite poles. At the poles, the nuclear envelope forms around the separated chromosomes and the spindles start to disappear. The remaining microtubules of the mitotic spindle form the phragmoplast.

The phragmoplast microtubules are polar, with plus ends near the equatorial plane, and minus ends near the poles.

Golgi bodies secrete small vesicles that are carried by motor proteins along the microtubules to the cell center. These vesicles are filled with polysaccharides and glycoproteins, two important components required for new cell wall synthesis. The fusion of vesicles forms a disc-like structure called an early cell plate.

The phragmoplast microtubules are successfully regenerated at the free margins of the cell plate, enabling the phragmoplast to extend the plate laterally. The new cell plate joins with the mother cell walls at the site that was marked by the preprophase band before the M phase. The plasma membrane fuses with the cell plate and a new cell wall is formed, separating the two daughter cells.

Overview

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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.

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