Auxins are plant growth hormone regulators which are generally produced in the growing apices of the stems and roots, from where they are translocated to the regions of their action by vascular bundles.

The meaning of the following terms related to auxins are as follows:

a) auxin precursors: Substances that are responsible for production of auxins are known as auxin precursors. For example, the amino acid tryptophan is an auxin precursor, biosynthesis of auxin requires this amino acid.

b) anti-auxins: Substances that inhibit auxin action by competing for the same receptor. It does not cause any growth response. An example of anti-auxin is PCIB (p-chloro phenoxyisobutyric acid).

c) synthetic auxins: Some chemical compounds that have been synthesized in the laboratories and function as auxins are called synthetic auxins. They are not synthesized naturally by the plant parts. For example: Phenoxyacetic acid.