auxin A hormone that promotes longitudinal growth in the cells of higher plants. Typically, low concentrations of auxin promote growth whereas high concentrations inhibit it. Auxins are produced at the growing points of stems and roots, and promote growth by increasing the rate of cell elongation rather than that of cell division. They are involved in the curvature of parts of the plant towards light (phototropism) or gravity (geotropism), and the initiation of cambium activity in association with cytokinins; and they may control fruit growth or leaf fall, and inhibition of lateral-bud development in favour of apical buds. A natural example of an auxin is indole-3-acetic acid (IAA), which has been isolated from fungi and bacteria, and from the endosperm of corn, as well as from urine and saliva in humans. Auxins have also been synthesized, and are widely used to regulate growth in a variety of plants of agricultural and horticultural importance. Some of these may have differential effects on different plants. For example 2, 4-dichlorophenoxyacetic acid (2, 4-D) is toxic to dicotyledons (see DICOTYLEDONEAE) but not to monocotyledons (see MONOCOTYLEDONEAE), and is used to control weeds in cereal crops and lawns.

auxin Any of a group of plant growth substances responsible for such processes as the promotion of growth by cell enlargement, the maintenance of apical dominance, and the initiation of root formation in cuttings. Auxins are also involved in suppressing the abscission of leaves, fruit, or other plant organs and in the development of flowers and fruits. Naturally occurring auxins, principally indoleacetic acid (IAA), are synthesized in actively growing regions of the plant, from where they are transported to other parts of the plant. IAA is stored in the plant in an inactive form, conjugated (attached) to various compounds, such as myo-inositol. Synthetic auxins include 2,4-D, which is used as a weedkiller, and indolebutyric acid and naphthaleneacetic acid, which are sold in preparations of ‘rooting hormones’.

auxin Plant hormone produced mainly in the growing tips of plant stems. Auxins accelerate plant growth by stimulating cell division and enlargement, and by interacting with other hormones. Actions include the elongation of cells in geotropism and phototropism (by increasing the elasticity of cell walls, allowing the cells to take up more water), fruit drop and leaf fall. See also gibberellin

aux·in / ˈôksin/ • n. a plant hormone that causes the elongation of cells in shoots and is involved in regulating plant growth.