19.6: Seedlings grow and properly orient themselves under the direction of auxins.

Auxins are a small group of naturally occurring hormones (and a larger group of synthetic variants that chemists can produce) that play several important roles in stimulating and regulating a plant’s growth and development. They are found primarily in shoot tips and immature plant tissue, such as young leaves. The primary role of auxins in plant growth is to stimulate the expression of genes that promote cell division, stem elongation, the formation of roots, and the formation of vascular cambium. Auxins also influence a plant’s orientation —its growth in response to directional light and gravity.

The chief effects of auxins are of four types (FIGURE 19-12).

1. Stimulating shoot elongation. Auxins enhance the effect of gibberellins in shoot elongation. As we saw in Section 18-14, primary growth in plants causes them to get taller as apical meristems—clusters of active, dividing cells—at the ends of roots and shoots or branches repeatedly divide. This causes plants to grow “up.” There are numerous other buds, called lateral buds, lower on the stem that also are capable of rapid cell division and growth, but typically these lateral buds remain dormant.

When the top meristem of a shoot is cut off, however, lateral buds that are lower on the plant increase their growth. The inhibition of the lateral buds by the apical meristem is called apical dominance and is caused by the large amount of auxin produced by the apical bud. Cutting off the top meristem removes the inhibiting source of auxin, which is why the lateral buds then grow. This is the basis for “pruning” and makes it possible to control the branching pattern and shape of plants.

2. Controlling seedling orientation. Charles Darwin and his son Francis were the first to document that seedlings exposed to light bend in whichever direction the light is coming from, whether up, down, or sideways. Immediately after emerging from the seed, the shoot grows as if it senses which way is up. Whether a shoot grows down into the earth or up toward the light, though, actually depends on (1) where the auxins are located and (2) how the auxins influence the cells in those specific locations. Like gibberellins, auxins are produced near the growing tips of shoots and branches, but unlike gibberellins, they don’t remain there. Auxin molecules move in two directions within a cell: they are pulled downward by gravity, and they move laterally away from light. These two movements distribute the auxin molecules unevenly within the plant. In regions of higher auxin concentration, cells in the stem elongate more rapidly than those in regions of lower auxin concentration (FIGURE 19-13). This difference in growth occurs because auxins increase the flexibility of the usually rigid cell wall (so that it can grow) and increase its permeability (so that it can take on more water and expand). The overall effect on growth is best seen when a plant is tipped onto its side. First, the auxins flow downward to the bottom side of the now-horizontal shoot. The bottom side then elongates more rapidly than the top side, causing the shoot to bend upward, against gravity.

3. Stimulating root branching. Auxins induce the formation of roots. In fact, you can buy auxin powders that can be dusted onto the bottom of plant stem cuttings. The powder causes the stems to send out numerous roots (transforming some stem cells into root cells), so that the cuttings can be planted and will form new plants.

4. Promoting fruit development. Auxins produced within an embryo have several roles in the development of fruit. First, they promote maturation of the ovary wall, and then they promote several steps in the full development of the fruit. It is even possible to trick plants into producing fruits by applying auxins to unfertilized flowers. Because there has been no fertilization, such fruits are always seedless (for example, seedless tomatoes). Auxins applied to fruiting plants can also prevent the fruits from prematurely dropping from the tree or vine. These fruits are easier and less expensive to harvest, because they can all be picked at one time.

Too much of a good thing can be terrible, and this is particularly true when it comes to plant hormones. Auxins, for example, are among the chief growth-stimulating hormones in plants, but in extreme concentrations they are deadly. While this is bad news if you are a plant, it can be good news if you are a human trying to kill weeds (FIGURE 19-14). When synthetic auxins are sprayed on plants in higher concentrations than any plant would normally experience, they cause the plants to begin growing uncontrollably. Like a mismanaged start-up company, the plants devote so much of their energy budget to growth that they quickly find themselves without sufficient energy for essential metabolic maintenance functions. And then they die. One type of synthetic auxin, called 2,4-D, is a particularly useful weed killer because it kills only eudicots—such as dandelions—while not harming monocots, including grasses and cereals such as the valuable crop plants corn, rice, wheat, and barley. (The reason for this difference is not fully understood.)

The plant-destroying properties of synthetic auxins have even found military uses. During the 1960s, the United States sprayed the synthetic, auxin-based herbicide called Agent Orange on more than 3,000 villages in Vietnam, in an attempt to reduce the brush under which opposing military forces could hide. But the manufacture of Agent Orange creates a highly toxic by-product called dioxin. Agent Orange contaminated with dioxin was unexpectedly responsible for causing birth defects, leukemia, liver diseases, and other disorders. Its use has since been stopped.