Hormones & the Control of Plant Growth

Auxins Produced in the stem, root tip and buds Promotes elongation Causes plant to grow tall and straight. Bends stem toward light (phototropism). Downward root growth away from light. Prevents leaves from falling. Auxins promote stem elongation, inhibit growth of lateral buds (maintains apical dominance). They are produced in the stem, buds, and root tips. Example: Indole Acetic Acid (IA). Auxin is a plant hormone produced in the stem tip that promotes cell elongation. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism. Auxin also plays a role in maintaining apical dominance. Most plants have lateral (sometimes called axillaryAuxin also plays a role in maintaining apical dominance. Most plants have lateral (sometimes called axillary) buds located at nodesAuxin also plays a role in maintaining apical dominance. Most plants have lateral (sometimes called axillary) buds located at nodes (where leaves attach to the stem). Buds are embryonic meristemsAuxin also plays a role in maintaining apical dominance. Most plants have lateral (sometimes called axillary) buds located at nodes (where leaves attach to the stem). Buds are embryonic meristems maintained in a dormant state. Auxin maintains this dormancy. As long as sufficient auxin is produced by the apical meristem, the lateral buds remain dormant. If the apex of the shoot is removed (by a browsing animal or a scientist), the auxin is no longer produced. This will cause the lateral buds to break their dormancy and begin to grow. In effect, the plant becomes bushier. When a gardener trims a hedge, they are applying apical dominance.

Auxins - (represented by the purple dots in the picture below)

Gibberellins Promotes seed germination. Promotes stem and root growth. Promotes leaf growth. Promotes flower Development. Increase fruit size Gibberellins promote stem elongation. They are not produced in stem tip. Gibberellic acid was the first of this class of hormone to be discovered. Seedless grapes are usually smaller than seeded grapes because the developing seeds produce another hormone, gibberellin, that promotes the fruits increase in size and sugar content. By spraying seedless grapes with gibberellins artificially, the farmer can produce large, marketable grapes.

Cytokinins Promotes division and differentiation of cells. Found in growing areas of plant (meristems at shoot tip) Promotes seed germination. Promotes flowering. Prevents aging. Cytokinins promote cell division. They are produced in growing areas, such as meristems at tip of the shoot 67. Plants That Don't Wilt In March a team of researchers working in England and the Czech Republic announced the creation of plants that stay green and lush for more than seven months after being cut and placed in water. Peter Meyer, a molecular biologist at the University of Leeds, and his colleagues identified a gene they labeled Sho (for shooting), which controls production of cytokinins, hormones that delay aging in plants.

Ethylene Ripens fruit, prompts leaves to change colour, and petals to die off. Higher temperatures trigger the production of ethylene. The gas is able to travel from cell to cell and plant to plant. Ethylene is a gas produced by ripe fruits. Why does one bad apple spoil the whole bunch? Ethylene is used to ripen crops at the same time. Sprayed on a field it will cause all fruits to ripen at the same time so they can be harvested. Later research showed that wounding and high temperatures trigger plants to produce ethylene. Researchers later discovered that plants produce ethylene in many tissues in response to cues beyond the stress from heat and injury. It is made during certain developmental conditions to signal seeds to germinate, prompt leaves to change colors, and trigger flower petals to die. Because the gas diffuses easily it can travel within the plant from cell to cell as well as to neighboring plants, serving as a warning signal that danger is near and that it is time to activate the appropriate defense responses. Special receptors in plant cells bind to the ethylene. The first known plant genes involved in this process, ETR1 and CTR1, were identified in 1993; they keep the fruit ripening genes from activating until ethylene is made. Once that happens, ETR1 and CTR1 turn off, which allows a cascade that ultimately turns on other genes that make various enzymes: pectinases to break down cell walls and soften the fruit; amylases to convert carbohydrates into simple sugars; and hydrolases to degrade the chlorophyll content of the fruit resulting in color change. Such changes invite animals to consume the fruit and disperse the mature undigested seeds via their defecation.

Abscisic Acid Inhibits growth. Induces dormancy. Causes leaves to fall. Abscisic Acid promotes seed dormancy by inhibiting cell growth. It is also involved in opening and closing of stomata as leaves wilt.

Tropisms Plants also change their growth pattern in response to external stimuli Changes in the environment around a plant affect its growth. These responses are called tropisms and are controlled by hormones.

Positive tropism = growth with stimulus Negative tropism = growth away from stimulus

Phototropism stems grow towards light (positive) roots grow away from light (negative);

Gravitropism roots grow with gravity; stems grow against gravity

Thigmotropism growth in response to touch (e.g. vines)