The Phragmoplast

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.