7.16
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…
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.
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Q1: What is the phragmoplast and why do plant cells need it?
The phragmoplast is a specialized structure that guides new cell wall formation in plant cells. Because plant cells have rigid cell walls, they cannot divide by contracting from the outside like animal cells. Instead, the phragmoplast enables an inside-out division process by directing the assembly of a new cell wall between daughter nuclei, ensuring proper cytokinesis in plants.
Q2: How does the preprophase band mark the plane of cell division?
The preprophase band is a ring of microtubules and actin filaments that assembles at the cell cortex during the G1 phase and marks the future site of cell wall formation. This structure determines the plane of cell division before mitosis begins. The preprophase band disappears at the start of M phase, but the location it marked guides where the phragmoplast will eventually form the new cell wall.
Q3: What is the structure and composition of the mature phragmoplast?
The mature phragmoplast is donut-shaped with three distinct regions: an outer leading region where new microtubules assemble continuously, a middle transition zone where secretory vesicles form a tubular network, and an inner lagging region where microtubules depolymerize. This architecture enables the phragmoplast to expand outward while depositing cell wall material between daughter cells.
Q4: How do Golgi vesicles contribute to cell plate formation?
Golgi bodies secrete small vesicles filled with polysaccharides and glycoproteins, which are transported along phragmoplast microtubules by motor proteins to the cell center. These vesicles fuse to form an early cell plate and continue accumulating to build the tubular network. The vesicle membranes become the plasma membrane of daughter cells, while their contents form the new cell wall.
Q5: What role does microtubule nucleation play in phragmoplast expansion?
Microtubule nucleation in the phragmoplast's leading region is initiated by the γ-tubulin ring complex (γ-TuRC), which is recruited to existing microtubules by the augmin protein complex. Continuous microtubule assembly at the leading edge, combined with depolymerization at the lagging edge, drives the phragmoplast's outward expansion and enables the growing cell plate to reach the parental cell membrane.
Q6: How does the phragmoplast differ from the contractile ring in animal cells?
Unlike the stationary midbody and contractile ring in animal cells, the phragmoplast expands centrifugally outward to form the new cell wall. The phragmoplast's dynamic growth, driven by microtubule polymerization and vesicle accumulation, allows it to extend laterally until it joins with the mother cell walls, whereas animal cell cytokinesis relies on inward contraction.
Q7: What happens when phragmoplast function fails during plant cell division?
Failure in phragmoplast functions can result in multinucleated cells and embryonic lethal phenotypes that compromise plant development. Without proper phragmoplast-guided cell wall formation, daughter cells cannot separate correctly, leading to abnormal cell division outcomes and reduced viability during plant growth and development.