Auxins & Plant Growth


Auxins are an important class of plant hormones, that have diverse and complex biological effects. By definition, a hormone is a compound that is made in one part of an organism, but has its affect in another part.


Auxins were the first major class of plant hormones to be discovered. All auxins are morphogens, meaning they control growth and development during a plant's life.

Natural Auxins

Auxins are mostly produced in the root and shoot tips. Some naturally occurring auxins are indole-3-butyric acid (IBA), 4-chloro-indoleacetic acid, and phenylacetic acid (PAA).

Synthetic Auxins

Auxin analogs, similar to natural auxins, have been synthesized. One well-known example is 2,4-dichlorophenoxyacetic acid (2,4-D), which is used as an herbicide and was a component of the military defoliant Agent Orange.

Apical Dominance

As auxins are produced in growing shoot tips (meristems) and transported down toward the roots, they inhibit the growth of lateral buds. This prevents lateral shoots from competing with the top of the plant for light.

Cell Elongation

Auxin causes plant cells to elongate. This results in growth and extension of the plant in the region where auxin is present.


Light causes a reduction in auxin concentration. The side of a growing shoot exposed to the sun will have a lower auxin concentration than the opposite side. This causes the cells on the opposite side to elongate more, bending the shoot tip in the direction of the light.


  • Plant Physiology (4th Edition); Robert M. Devlin, Francis H. Witham; 1983

Who Can Help

  • Biology Online: Plant Auxins: Phototropism and Geotropism

About this Author

Philip McIntosh has more than 30 years of experience as an equipment engineer, scientific investigator and educator. He has been writing for 16 years, and his work has appeared in scientific journals, popular science magazines, trade journals and on science and technology websites. McIntosh holds a B.S. in botany and chemistry, and an M.A. in biological science.