Auxins are a class of plant growth substance (often called phytohormone or plant hormone). Auxins play an essential role in coordination of many growth and behavioral processes in the plant life cycle.
Overview
Auxins derive their name from the Greek word auxano (to grow). They were the first of the major plant hormones to be discovered and are a major coordinating signal in plant development. Their pattern of active transport through the plant is complex. They typically act in concert with (or opposition to) other plant hormones. For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds. Thus a plant can (as a whole) react on external conditions and adjust to them, without requiring a nervous system. On a molecular level, auxins have an aromatic ring and a carboxylic acid group (Taiz and Zeiger, 1998).
The most important member of the auxin family is indole-3-acetic acid (IAA). It generates the majority of auxin effects in intact plants, and is the most potent native auxin. However, molecules of IAA are chemically labile in aqueous solution, so IAA is not used commercially as a plant growth regulator.
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indole-3-acetic acid (IAA)
Indole-3-butyric acid (IBA)
4-chloroindole-3-acetic acid (4-CI-IAA)
2-phenylacetic acid (PAA)
2,4-Dichlorophenoxyacetic acid (2,4-D)
α-Naphthalene acetic acid (α-NAA)
2-Methoxy-3,6-dichlorobenzoic acid (dicamba)
4-Amino-3,5,6-trichloropicolinic acid (tordon or picloram)
2,4,5-Trichlorophenoxyacetic acid (2,4,5-T)
α-(p-Chlorophenoxy)isobutyric acid (PCIB, an antiauxin)
Auxins are often used to promote initiation of adventitious roots and are the active ingredient of the commercial preparations used in horticulture to root stem cuttings. They can also be used to promote uniform flowering, to promote fruit set, and to prevent premature fruit drop. Used in high doses, auxin stimulates the production of ethylene. Excess ethylene can inhibit elongation growth, cause leaves to fall (leaf abscission), and even kill the plant. Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) have been used as herbicides. Broad-leaf plants (dicots) such as dandelions are much more susceptible to auxins than narrow-leaf plants (monocots) like grass and cereal crops. These synthetic auxins were the active agents in Agent Orange, a defoliant used extensively by American forces in the Vietnam War. |