Botanists recognize five major groups of phytohormones (plant hormones): auxins, gibberellins, ethylene, cytokinins, and abscisic acid.
Auxins
Auxins are hormones involved in cell elongation, apical dominance, and rooting. The auxin, indoleacetic acid or IIA, is produced in actively growing shoot and root tips and in developing fruits. IIA triggers an increase in the plasticity, or stretchability of cells walls allowing elongation to occur. Developing seeds also produce IIA, which stimulates the development of fleshy fruits. For example, if the seeds are carefully removed from a tiny strawberry that is just beginning to develop, the fruit will never get any larger. However, if the seeds are removed from a tiny strawberry and then this seedless strawberry is treated with IIA, the fruit will enlarge to a normal size.
Auxins, produced in the apical meristem of a stem, causes the stem to grow toward light and away from the pull of gravity. If the source of light is to one side of the stem, however, auxin will move away from the light to the shaded side of the stem. The higher concentration of auxin in the shaded side of the stem will stimulate cells in that side to elongate more than cells in the lighted side. This will cause the stem to bend toward the light.
In addition to controlling cell elongation, auxin can also cause cell division in meristematic regions to stop or start. As a stem grows in length, cells in its apical meristem periodically produce other meristematic regions along the side of the plant known as lateral buds. Side branches off the main stem will grow from these lateral buds.
However, side branches begin to form only after the main growing tip (apical meristem) is some distance from them. Why? It has been discovered that high concentrations of auxin inhibit lateral buds while low concentrations of auxin stimulate growth in lateral buds. As the apical meristem grows away from the lateral buds, the concentration of auxin drops and they “awaken” and begin to grow side branches.
Gardeners, florists, orchardists, and those who grow plants in their homes will often purposefully pinch off the apical meristem of a plant to keep it short and bushy.
Gibberellins
In the 1920s, Japanese biologists studying certain rice plants made an important discovery. The rice plants they were working with seemed to be afflicted with a “disease” that made the plants grow so tall and spindly that they could no longer be supported by their stems, and so they fell over. The scientists found that the rice plants’ extraordinary growth was caused by a substance produced by a fungus Gibberella fujkuroi. They named the growth-producing substance gibberellin after the fungus that produced it.
Gibberellin was later found to be produced in small quantities by plants themselves. This hormone primarily stimulates elongation growth.
Like auxins, gibberellins have important commercial applications. Almost all seedless grapes are sprayed with gibberellins to increase the size of the fruit. Beer makers also use gibberellins to increase the alcohol content of beer by increasing the amount of sugar produced in the malting process. Gibberellins are also used to treat seeds of some food crops because they cause the seeds to break dormancy resulting in uniform germination of the seeds.
Ethylene
Ethylene is the only plant hormone that is a gas at room temperature. This hormone is responsible for the ripening of fruits. The saying, “One bad apple spoils the whole barrel” has its basis in ethylene gas. One rotting apple produces ethylene gas, which stimulates nearby apples to ripen and eventually spoil.
There is less damage to fruit when it is picked and shipped green. Once at their destination, green fruit of bananas, melons, and tomatoes are ripened by exposing them to ethylene gas. Oranges, lemons, and grapefruits are often green when they are ripe. Although the green fruit tastes good, consumers will not buy them. The application of ethylene gas to green citrus fruits causes the development of the yellow and orange colors people associate with these fruits.
Cytokinins
Cytokinins promote cell division in plants. The first of the cytokinins to be discovered was zeatin, which was isolated from corn kernels in 1964. Since then, three others have been identified, including kinetin. When coupled with other hormones, cytokinins appear to promote cell division in meristematic tissues. Cytokinins also appear to play a role in preventing senescence (aging) in plants. If picked leaves are coated with cytokinins, they stay fresh longer. Cytokinins are also used to promote lateral bud growth of flower crops.
Abscisic Acid
Abscisic acid or ABA, generally inhibits other hormones, such as IIA. ABA helps to bring about dormancy in a plant’s buds and maintains dormancy in seeds. ABA also causes the closure of a plant’s stomata in response to drought conditions. Unfortunately, it is too costly to synthesize ABA for agricultural use.
Today many plant hormones can be synthesized in the laboratory, increasing the quantity of hormones available for commercial applications.
Dennis Holley - Looking back, it's clear that I have been a science nerd my entire life. A simple, almost toy-quality microscope my poor parents scraped ...
