Auksin

Auksin, more commonly spelled auxin, is a class of plant hormones that regulate growth and development. The name derives from Greek auxein, to grow. The most important natural auxin is indole-3-acetic acid (IAA); others include indole-3-butyric acid (IBA) and phenylacetic acid (PAA). Synthetic auxins such as 2,4-dichlorophenoxyacetic acid (2,4-D), naphthaleneacetic acid (NAA) and picloram are widely used in research and agriculture.

Auksins are produced mainly in apical buds and young leaves and are transported through the plant in a polar manner, creating concentration gradients that provide positional information. Transport involves PIN family efflux carriers and AUX/LAX influx carriers. Biosynthesis and degradation are regulated, enabling rapid changes in hormone levels in response to environmental cues.

In the cell, auxin perception uses receptor–signaling components such as TIR1/AFB F-box proteins. Auxin binding promotes the ubiquitin ligase–mediated degradation of Aux/IAA repressor proteins, releasing ARF transcription factors to regulate target genes. This leads to changes in gene expression that drive growth responses.

Physiological roles include promoting cell elongation via the acid growth mechanism, maintaining apical dominance, regulating tropisms (growth toward or away from stimuli), initiating and patterning roots, and influencing fruit development and abscission. In horticulture and tissue culture, auxins are used to induce rooting and to control callus formation; in agriculture, synthetic auxins are used as herbicides for broadleaf weed control.

See also: Auxin transport, IAA, TIR1, plant hormone.