Any of a class of plant growth substances whose principal effects are brought about by their ability to promote the elongation of shoots and roots when present in low concentrations. The most widely occurring natural auxin is *indole acetic acid
. Other naturally occurring auxins are also based on the indole ring (e.g. indole acetonitrile and indole pyruvic acid). However the indole group is not essential for auxin activity as is shown by the auxin activity of certain synthetic compounds, e.g. *naphthaleneacetic acid (NAA)
and 2,4-dichlorophenoxyacetic acid (*2,4-D
). Auxins are usually synthesized in meri-stematic regions and transport is generally from apex to base, i.e. basipetal. The concentration of auxin is very important in determining the nature of the growth response and the optimum auxin concentration differs for different organs (see diagram). The inhibition of growth at higher auxin concentrations may be due to the auxin-promoted synthesis of *ethylene
, which inhibits cell elongation. Such inhibitory effects have been exploited in the production of herbicides based on 2,4-D and *2,4,5-T
. Auxins also promote root initiation, and certain synthetic auxins (e.g. NAA and indole butyric acid or IBA) are widely used as rooting compounds. Abscission has been correlated with low levels of auxin in the organ concerned and auxins have thus been used to prevent premature fruit drop. Other phenomena in which auxins have been implicated include *apical dominance
, phototropism (see Avena curvature test), and epinasty. Many effects are brought about by the combined action of auxin with other growth substances. For example, the stimulation of cambial activity, the induction of parthenocarpy, and the enhancement of internode elongation are all more effectively promoted by a combination of auxin and gibberellin than by either substance alone. Similarly appropriate concentrations of auxin and cytokinin are needed in culture media to promote cell division in tissue explants. Depending on the relative concentration of each, root meristems (high auxin: low cytokinin) or shoot meristems (low auxin: high cytokinin) may be initiated.
The effects of auxin concentration on the growth of different organs.