Auxin promotes growth by acting on cell walls

The expansion of plant cells is what causes plant growth. Thus the cell wall plays key roles in controlling the rate and direction of growth of a plant cell. Auxin acts on cell walls to regulate this process.

CELL WALL ARCHITECTURE DIRECTS CELL EXPANSION. The expansion of a plant cell is driven primarily by the uptake of water, which enters the cytoplasm of the cell and accumulates in its central vacuole. As the vacuole expands, the cell grows rapidly, with the vacuole often making up more than 90 percent of the volume of a mature cell. The vacuole presses the cytoplasm against the cell wall as it expands, and the wall resists this force.

The principal strengthening component of the plant cell wall is cellulose, a large polymer of glucose. In the wall, cellulose molecules tend to associate in parallel with one another. Bundles of approximately 250 cellulose molecules make up microfibrils that are visible under an electron microscope (Figure 38.13). What makes the cell wall rigid is a net-

Plant cell

The parallel microfibrils of cellulose associate in a crisscross pattern.

Cellulose microfibrils encircle the cell in a specific orientation and constrain cell expansion.

Plant cell

Cellulose Cell Wall

The parallel microfibrils of cellulose associate in a crisscross pattern.

Cell wall

38.13 Cellulose in the Cell Wall The plant cell wall is a network of cellulose microfibrils linked by other polysaccharides.

Cell wall

Cellulose microfibrils encircle the cell in a specific orientation and constrain cell expansion.

Orientation Plant Cell Expansion

The cell expands only in the direction of least resistance.

38.14 Plant Cells Expand The orientation of cellulose microfibrils in the plant's cell wall determines the direction of cell expansion.

38.13 Cellulose in the Cell Wall The plant cell wall is a network of cellulose microfibrils linked by other polysaccharides.

The cell expands only in the direction of least resistance.

38.14 Plant Cells Expand The orientation of cellulose microfibrils in the plant's cell wall determines the direction of cell expansion.

work of cellulose microfibrils connected by bridges of other, smaller polysaccharides. The orientation of the majority of cellulose microfibrils determines the direction of cell expansion (Figure 38.14).

For the cell to expand, its wall must loosen and be stretched. If the wall were only stretched, however, it would become thinner. Cell expansion involves more than stretching. New polysaccharides are deposited throughout the wall, and new cellulose microfibrils are deposited at the inner surface of the wall, maintaining its thickness. As a consequence of this pattern of cellulose deposition, the microfibrils in the outermost part of the wall are the oldest, and those in the innermost part the youngest. How do these properties of cell walls relate to the action of auxin on plant cell expansion?

auxin loosens the cell wall. Experiments with segments of oat coleoptiles have shown that plant cell walls recover incompletely from being stretched (Figure 38.15). Reversible stretching is called elasticity, and irreversible stretching is called plasticity. Treating the coleoptile segments with auxin before they were stretched significantly increased their plasticity; in other words, it loosened the cell walls. This result suggested that auxin-induced cell expansion might result from just such a loosening effect.

Auxin acts by causing the release of a "wall-loosening factor" from the cytoplasm. Studies in the 1970s indicated that the wall-loosening factor was sometimes simply hydrogen ions (protons, H+). Acidifying the growth medium (that is, adding H+) caused segments of stems or coleoptiles to grow as rapidly as segments treated with auxin. Furthermore, treating coleoptile segments with auxin caused acidification of the growth medium. Auxin increases the activity of proton pumps in the plasma membrane, increasing the H+ con-

Question: How does auxin affect cell walls?

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  • rufino
    How does auxin make more plasma membrane?
    8 years ago

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