What are plant hormones? Plant hormones are a lot like animal hormones. They are chemical messengers that control how a plant grows. They are generally organic molecules made in one part of the plant and then transferred to another part of the plant where they act.

For example, when a seedling emerges from the soil, it grows toward the light source. How does it know where the light is? The coleoptile is a protective sheath covering the shoot and seed leaf of grasses. Early experiments demonstrated that when this tip is cut off, the stem no longer bends toward the light.

If the tip is placed on agar, so that any chemicals present in the tip can leach into the agar, then contact with the agar alone causes the plant to bend. These and other experiments demonstrate that a chemical in the tip of the coleoptile is responsible for the bending of the stem. The chemical is indole-3-acetic acid, an auxin hormone. Auxins are compounds that induce cell elongation and other plant activities, and chemically resemble indole-3-acetic acid.

The directional growth of a plant in response to light, such as their bending toward it, is called phototropism. In some plants, exposing the coleoptile to light causes the phosphorylation of photoreceptors on the plasma membrane. The phosphorylation initiates a signal that causes auxin to accumulate on the shaded side. The higher concentration of auxin on the shaded side causes the shaded side to grow faster and the plant bends toward the light.

Plant stems grow up and plant roots grow down. They both display gravitropism—the directional growth in response to gravity. If the coleoptile is placed on its side, increased growth occurs on the lower side, causing the tip to curve upward. The increased growth results from an increase in auxin concentration on the lower side.

Scientists think specialized cells, called statocytes, are responsible for sensing gravity. Statocytes contain dense starch grains or other solid particles. Experiments, conducted on space flights, have suggested that the settling of these dense particles in response to gravity enables the plant to sense the downward force of gravity, and alter its auxin concentrations and growth accordingly.

Auxins are also involved in thigmotropism—the directional growth of a plant in response to touch. Thigmotropism is seen in climbing vines and in the curvature of plants around rocks and other solid objects.

Another important class of plant hormones is the gibberellins. Gibberellins have 19 or 20 carbons that form four– or five– carbon rings. Gibberellins are named after the fungus Gibberella fujikuroi, which causes rice plants to grow too fast and die. The Japanese scientist Eiichi Kurosawa demonstrated that this fungus causes the rapid growth of rice plants by secreting a chemical, now known as a gibberellin.

Later it was demonstrated that plants themselves make gibberellins. When a mutant plant with short stems is sprayed with gibberellin, it grows to its normal size.

Gibberellins affect most aspects of plant growth. In some plants, they break seed dormancy and promote germination. They also stimulate flowering and the growth of fruit. These effects are often interconnected. For example, the gibberellin produced in grape seeds stimulates the growth of the fruit. For this reason, seedless varieties of grapes are smaller than grapes with seeds. Today, nearly all of the seedless grapes we eat are sprayed with gibberellin to promote their growth.

The hormone ethylene causes fruit to ripen. Ethylene is unique in that it is the only hormone found in gaseous form. Bananas ripen faster when placed in a bag because the bag traps the ethylene gas given off by the bananas. In addition to inducing ripening, ethylene promotes the plant senescence, or aging, that generally occurs at the end of the growing season. One of the reasons that leaves drop off in the fall is because ethylene concentrations in the leaf tissue rise and auxin concentrations drop.

Unlike ethylene, which promotes plant senescence, the cytokinin hormones delay plant death. They stimulate cell division and are often added to stimulate shoot and bud formation in plants grown in tissue culture. Cytokinins are the only hormones known to be found in both plants and animals. They are derived from the base adenine—the same base found in adenosine triphosphate, or ATP.

The hormone abscisic acid, or ABA, is a plant-growth inhibitor. It induces dormancy and prevents seeds from germinating. ABA is a 15-carbon compound produced in chloroplasts and other plastids. Water-stress induces the production of ABA, which causes the stomata to close.

Many different plant hormones work together to control plant growth and development. Now that we’ve seen how plant hormones control growth, let’s see how growth is controlled by a plant’s biological clock.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education