35.16
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 what role does it play in plant cell division?
The phragmoplast is a specialized structure that forms during mitosis and cytokinesis in plant cells to facilitate cell plate formation. It consists of microtubules, endoplasmic reticulum, and Golgi-derived vesicles organized in a specific pattern. The phragmoplast guides vesicle delivery to the cell equator, enabling the construction of the new cell wall that separates daughter cells.
Q2: How do microtubules function within the phragmoplast?
Microtubules in the phragmoplast form a dynamic network that organizes and directs vesicle movement toward the cell division plane. These microtubules create a scaffold structure that maintains the spatial organization necessary for precise cell plate assembly. The microtubule arrangement ensures vesicles are delivered to the correct location during cytokinesis.
Q3: What is the relationship between the phragmoplast and cell plate formation?
The phragmoplast directly orchestrates cell plate formation by organizing vesicles at the cell equator. Golgi-derived vesicles accumulate within the phragmoplast structure and fuse together to create the cell plate, which eventually develops into the new cell wall. This process is essential for completing cytokinesis in plant cells.
Q4: How does the phragmoplast differ between plant and animal cells?
Plant cells form a phragmoplast to guide cell plate formation, while animal cells lack this structure entirely. Animal cells instead use a contractile ring composed of actin and myosin filaments to pinch the cell membrane during cytokinesis. This fundamental difference reflects the distinct cell division strategies required by plant and animal cell architecture.
Q5: What components make up the phragmoplast structure?
The phragmoplast comprises microtubules, endoplasmic reticulum, and Golgi-derived vesicles organized in a coordinated network. These components work together to create a functional platform for cell plate assembly. The precise arrangement of these elements ensures efficient delivery and fusion of vesicles at the division plane.
Q6: Why is the phragmoplast important for plant cell survival?
The phragmoplast is critical because it ensures proper cell wall formation between daughter cells, maintaining cellular integrity and function. Without the phragmoplast's organizing role, vesicles would not accumulate correctly, preventing cell plate formation and successful cytokinesis. This structure is therefore essential for plant growth and development.