34.9. Auxin

In his later years, the great evolutionist Charles Darwin became increasingly devoted to the study of plants. In 1881, he and his son Francis published a book called The Power of Movement in Plants, in which they reported their systematic experiments concerning the way in which growing plants bend toward light, a phenomenon known as phototropism.

After conducting a series of experiments shown in figure 34.12, they observed that plants grew toward light (panel 1). If the tip of the seedling was covered, the plant didn’t bend toward the light (panel 2). A control experiment showed that the cap was not interfering with the directional growth pattern (panel 3). Another control showed that covering the lower portions of the plant did not block the directional growth (panel 4). The Darwins hypothesized that when plant shoots were illuminated from one side, an “influence” that arose in the uppermost part of the shoot was then transmitted downward, causing the shoot to bend. Later, several botanists conducted a series of experiments that demonstrated that the substance causing the shoots to bend was a chemical we call auxin.

Figure 34.12. The Darwins' experiment with phototropism.

From these experiments, the Darwins concluded that, in response to light, an "influence" that causes bending was transmitted from the tip of the seedling to the area below the tip, where bending usually occurs.

How auxin controls plant growth was discovered in 1926 by Frits Went, a Dutch plant physiologist, in the course of studies for his doctoral dissertation. From his experiments, described in figure 34.13, Went was able to show that the substance that flowed into agar from the tips of the light-grown grass seedlings (steps 1 and 2) enhanced cell elongation (shown in step 3). This chemical messenger caused the tissues on the side of the seedling into which it flowed to grow more than those on the opposite side (step 4). Control experiments indicated that the effects were not due to properties of the agar (steps 1a and 2a). He named the substance that he had discovered auxin, from the Greek word auxin, meaning “to increase.”

Figure 34.13. How Went demonstrated the effects of auxin on plant growth.

The experiment showed how a chemical at the tip of the seedling caused the shoot to elongate and to bend. Steps 1a and 2a show the control experiment.

Went’s experiments provided a basis for understanding the responses the Darwins had obtained some 45 years earlier: Grass seedlings bend toward the light because the side of the shoot that is in the shade has more auxin; therefore, its cells elongate more than those on the lighted side, bending the plant toward the light as in the enlargement in figure 34.14. Later experiments showed that auxin in normal plants migrates away from the illuminated side toward the dark side in response to light and thus causes the plant to bend toward the light.

Figure 34.14. Auxin causes cells to elongate.

Plant cells that are on the shaded side have more auxin and grow faster, elongating more, than cells on the lighted side, causing the plant to bend toward light.

Auxin appears to act by increasing the stretchability of the plant cell wall within minutes of its application. Researchers speculate that the covalent bonds linking the polysaccharides of the cell wall to one another change extensively in response to auxin, allowing the cells to take up water and thus enlarge.

Synthetic auxins are routinely used to control weeds. When applied as herbicides, they are used in higher concentrations than those at which auxin normally occurs in plants. One of the most important of the synthetic auxins used in this way is 2,4-dichlorophenoxyacetic acid, usually known as 2,4-D. It kills weeds in lawns without harming the grass because 2,4-D affects only broad-leaved dicots. When treated, the weeds literally “grow to death,” rapidly reducing ATP production so that no energy remains for transport or other essential functions.

Closely related to 2,4-D is the herbicide 2,4,5-trichloro-phenoxyacetic acid (2,4,5-T), which is widely used to kill woody seedlings and weeds. Notorious as the Agent Orange of the Vietnam War, 2,4,5-T is easily contaminated with a by-product of its manufacture, dioxin. Dioxin is harmful to people because it is an endocrine disrupter, a chemical that interferes with the course of human development. The growing release of endocrine disrupters as byproducts of modern chemical manufacturing is a subject of great environmental concern.

Key Learning Outcome 34.9. The primary growth-promoting hormone of plants is auxin, which increases the plasticity of plant cell walls, allowing growth in specific directions.