Auxins are a class of plant hormones (or plant growth regulators) derived from tryptophan. Auxins are found in the tips of shoots and roots and promote cell division, stem and root growth. They can also have a radical effect on plant orientation by promoting cell division on one side of the plant in response to sunlight and gravity, called phototropism. 

The main vegetable auxin is indole-3-acetic acid (IAA). Phenoxyacetic acid and 3-butyric indole acid are other compounds with auxin activity.

The major plant auxin is indole-3-acetic acid (IAA). A number of other compounds with auxin activity include phenoxyacetic acid and indole 3-butyric acid.

There are a variety of substances that are not known to occur in plants that have auxin activity. These include indolebutyric acid (IBA); naphthalene acetic acid (NAA); 2,4- dichloro-phenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). 

Synthetic auxins find widespread application in agriculture and horticulture. At high concentrations, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) are used as herbicides, particularly on broad-leaved plants, which are much more sensitive to them than monocots.