• Auxins are a family of plant hormones with diverse roles in plant morphogenesis including phototropism (bending toward light), cell expansion, root formation, and bud development.
  • Auxins were the first plant hormones to be discovered and studied. Charles Darwin found that coleoptiles (the sheaths around the leaves of young grass plants) would bend toward light. By shading various parts of coleoptiles, Darwin found that the source of the bend response was located in the tips of the coleoptiles.
  • Other workers expanded upon Darwin’s work and the discovery of the first auxin, indole-3-acetic acid (IAA), is credited to the Dutch botanist Fritz Went who worked in the 1920s and ’30s.
  • It is now known that IAA is produced in the growing tip of a plant shoot and diffuses downward through the stem. Providing the tip of the plant (the apical meristem) is intact, IAA suppresses the development of axillary buds and branching growth below the tip.
  • Which is why cutting off the main stem (topping) of a plant increases bushiness by allowing the axillary buds below the tip to be released from their IAA-induced dormancy to begin growing.
  • Although IAA is the principle auxin produced by plants, there are a few others. One of particular note is indole-3-butyric acid (IBA), the active ingredient in most rooting powders and cloning aids used to stimulate root development in cuttings.
  • As is usually the case with plant hormones, auxins interact with other hormones in complex ways. For example, the auxin-cytokinin ratio has diverse regulatory effects on plant growth and development.
  • Along with the naturally occurring auxins, there are numerous synthetic auxins that are structural analogs of IAA.
  • One of the more notorious synthetic auxins is 2,4-dichlorophenoxyacetic acid, more commonly known as 2,4-D. 2,4-D was one of the compounds (in combination with 2,4,5-T) in Agent Orange, used by the United States as a defoliant during the Vietnam War.
  • 2,4-D is still used today as an herbicide. Compared to grasses, broad-leafed plants such as dandelions are much more susceptible to large doses, so it is an effective weed killer for use on lawns.