Auxin is a plant hormone involved in many aspects of plant growth and development. One function of auxin is to trigger cell elongation in shoots. Auxin's action in shoots can be deduced from Arabidopsis thaliana plants that do not make auxin, because these plants are short, and supplying them with the hormone reverses this phenotype. Auxin also mediates phototropism, in which cells elongate more on one side of a shoot, causing the plant to bend toward light.
Plant cell elongation requires two elements: high turgor pressure inside the plant cell and a loosening of the cell wall so that the turgor pressure and additional incoming water can force the cell to expand. In this tutorial, we focus on how auxin loosens the cell wall—a process described as the acid growth hypothesis.
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, it presses the cytoplasm against the cell wall. However, the cell wall is tough and resists this force. This is the basis of turgor pressure.
The cell wall is an extensively cross-linked network of polysaccharides and proteins, dominated by cellulose microfibrils. If the cell is to expand, cellulose and other polysaccharides must loosen their grip on each other, allowing the wall to "give" under turgor pressure. Once the cell wall is stretched, new cellulose microfibrils will be laid down to maintain the strength of the wall.
The plant hormone auxin plays an important role in the expansion of the cell wall. When a plant grows in the direction of light, for instance, it does so because auxin made at the tip of the plant is transported to the cells on the dark side of the plant. Auxin triggers the cells on the dark side to elongate.
Scientists wanted to know how auxin loosens the cell wall. Studies in the 1970s showed that acidifying the medium in which shoot segments were growing—that is, adding hydrogen ions (also called protons)—caused the segments to grow just as rapidly as shoots treated with auxin.
Additionally, treating shoot segments with auxin increased the hydrogen ion concentration in the growth medium. These results suggested that auxin might loosen the cell wall by causing the release of hydrogen ions from cells, thereby decreasing the pH in the cell walls.
Consider an experiment in which you add auxin to shoots. Half of the shoots tested are also incubated in a buffer that prevents the cell walls from becoming acidic. What do you think will happen to each set of shoots?
When a buffer is used to prevent the wall from becoming acidic, auxin-induced cell expansion is blocked. The buffer-treated shoots do not elongate.
How might this so-called "acid growth" process occur in nature? Auxin has two roles in this process. In one, auxin enters cells and acts with a protein in the cell to stabilize a proton pump. The pump is then inserted into the plasma membrane. The proton pump uses ATP as an energy source to pump protons from the cell into the cell wall. The pH of the cell wall is thus reduced, or acidified.
Auxin also enters the nucleus and, by a multistep process, turns on the expression of genes. One such gene codes for the proton pump that is so important in acidifying the cell wall. Auxin, therefore, both increases the synthesis of the proton pumps and helps guide their insertion into the plasma membrane.
How does acidifying the cell wall actually loosen it up? It turns out that the lower pH activates enzymes called expansins in the cell wall. Expansins disrupt interactions between cell wall polymers. With a high turgor pressure inside the cell, the cell can now expand, because the cellulose microfibrils are no longer tightly tethered to each other. In the acid growth hypothesis, auxin causes acidity in the cell wall that activates expansins.
The acid growth hypothesis explains auxin-induced cell expansion. The hypothesis holds that protons (H+) are pumped from the cytoplasm into the cell wall. The lower pH of the wall then activates cell wall enzymes called expansins that disrupt interactions between cell wall polymers. The disruption loosens the cell wall, making it easier to stretch.
Keep in mind that cells can expand only if they take up more water. Water increases a cell's turgor pressure and, therefore, drives cell expansion.