Plant hormones are chemical messengers that are produced in one part of the plant and have a physiological effect on a target tissue that may be distant from the site of production. When hormones reach the target tissue they can: (1) have a direct effect on the target tissue causing a rapid metabolic response; (2) involve the use of a second messenger within target cells; and/or (3) affect transcription of nuclear deoxyribonucleic acid (DNA). Unlike animals, plants have no specialized organs designed solely for hormone synthesis and secretion . Leaves, stem tips, root tips, flowers, seeds, and fruits all produce hormones. Most plant hormones are functional at very low concentrations.
Auxins, cytokinins, gibberellins, abscisic acid, and ethylene are the best known plant hormones. All are in some way involved in regulating plant growth and development. Some promote growth by stimulating cell enlargement or division while others inhibit growth by inducing dormancy or promoting senescence. Recently brassinolides, jasmolates, and salicylic acid have been shown to have hormonal function.
Principles of Hormone Function
Often two or more hormones work synergistically. In a classic 1957 experiment, Skoog and Miller provided evidence that auxins and cytokinins work together in the differentiation of plant organs. Using tobacco tissue culture, they showed that when a tissue culture medium contains low concentrations of auxin and optimal cytokinin levels, then formation of shoots is favored. In contrast, when the culture medium is supplied with optimal concentrations of auxin combined with low concentrations of cytokinins, root formation is favored.
Hormones sometimes work antagonistically. Apical dominance is a process in which lateral buds of stems remain dormant as long as the stem apex remains intact. It has been shown that auxin produced in the stem apex is responsible for maintaining lateral bud dormancy by causing cells in the lateral buds to produce another hormone, ethylene, which is a growth inhibitor. During early spring, rapidly growing root tips will generate a high concentration of cytokinin that counteracts the effect of ethylene on the lateral buds of the stem. The lateral buds released from dormancy by cytokinins can then begin growth on their own.
Auxins were the first class of plant hormones to be identified. Many auxins, both natural and synthetic, are now known and all have similar effects on plant growth and development. The most widely studied naturally occurring auxin is indol-3-acetic acid (IAA), which is chemically related to the amino acid tryptophan. IAA can be synthesized from tryptophan in intact cells but other synthetic pathways are available. Because auxins can have an effect in very low concentrations, plants regulate synthesis and disassembly of auxin very precisely. Auxins are produced in young shoots and always travel downward in the plant from shoot to root. This polar movement of auxin is not well understood but requires calcium ions (Ca 2 ) and most likely involves special carriers in cell membranes. Naturally occurring auxins promote cell enlargement, are important in tropisms, prevent abscission , promote fruit development, and are involved in apical dominance. Synthetic auxins such as naphthalene acetic acid are used as rooting hormones. Other synthetic auxins include 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) that are used as weed killers.
The effect of IAA on cell enlargement has been well studied. IAA stimulates special pumps in the cell membrane of target cells to release H ions into the cell wall, resulting in a pH drop to approximately 5.0 in the cell wall. Enzymes that are pH-dependent then break down important structural bonds between cellulose microfibrils causing an increase in cell wall plasticity . As the cell wall becomes more plastic, water is able to flow in and the cell enlarges. Auxin also may have an effect on transcription of nuclear DNA that can contribute to cell enlargement.
Calcium acts as a second messenger in processes involving auxin. Auxin stimulates the release of Ca2 from the vacuole and endoplasmic reticulum in target tissues which affects Ca-dependent enzymes, including kinases , phophatases, and phospholipases.
|Plant Hormones: Roles|
|Auxins||Involved in differentiation of vascular tissue, control cellular elongation, prevention of abscission, involved in apical dominance and various tropisms, stimulate the release of ethylene, enhance fruit development|
|Cytokinins||Affect cell division, delay senescence, activate dormant buds|
|Gibberellins||Initiate mobilization of storage materials in seeds during germination, cause elongation of stems, stimulate bolting in biennials, stimulate pollen tube growth|
|Abscisic Acid||Maintains dormancy in seeds and buds, stimulates the closing of stomata|
|Ethylene||Causes ripening of climacteric fruits, promotes abscission, causes formation of aerenchyma tissue in submerged stems, determines sex in cucurbits|
|Jasmonates||Involved in response to environmental stresses, control germination of seeds|
|Brassinolides||Promote of elongation, stimulate flowering, promote cell division, can affect tropic curvature|
|Salicylic Acid||Activates genes involved with plant's defense mechanisms|
Auxins are involved in tropisms, which are growth responses to directional environmental stimuli such as light, gravity, and touch. In phototropism, unidirectional light will cause auxin to move toward the darkened side of the organ and stimulate enlargement of cells on the darkened side. This causes the organ to bend toward the light. This effect is often seen in potted plants growing in windowsills.
Other Plant Hormones
Cytokinins (for example, zeatin, isopentenyl adenine) have an effect on cell division. As previously mentioned, cytokinins work synergistically with auxin in the control of tissue and organ differentiation. Cytokinins are produced in root tips and may be transported in the xylem toward the shoot.
Gibberellins are a very large class of compounds, all with a similar chemical makeup. There have been as many as eighty-four gibberellins identified (named GA1 through GA84), but GA3, called gibberellic acid, has been the most studied. Gibberellins promote cell elongation, overcome genetic dwarfism, stimulate bolting in biennials, and are involved in seed germination. During the germination of grass seeds the imbibition (intake) of water stimulates the production of gibberellins by the embryo that diffuse throughout the seed. A protein -rich layer just internal to the seed coat, the aleurone layer, responds to gibberellins by synthesizing hydrolytic enzymes that aid in mobilization of stored food in the endosperm for use by the embryo.
Abscisic acid (ABA), is a growth inhibitor that, despite its name, is probably not involved in leaf or fruit abscission. One role of ABA is the stimulation of stomatal closure. When ABA binds to receptors on guard cell membranes, chloride ion channels open, letting chloride ions move out of the guard cells . The resulting depolarization of the membrane stimulates the movement of potassium ions (K ) ions out of guard cells, which then lose water, causing the stomata to close.
Ethylene is the only plant hormone that is a gas. Ethylene is also considered a growth inhibitor as it may have a role in causing bud dormancy, and it is involved with leaf abscission, causes fruit ripening, may determine sex in cucurbits (melon family), and stimulates formation of aerenchyma (gas transport tissue) in submerged roots and stems.
Brassinolides are plant steroids (many animal hormones are steroids) that may be involved in the light-induced expression of genes.
Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.
Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.