Auxins can be described as a combination of natural and synthetic plant hormones. They are important in controlling plant growth. Auxins were first isolated from human urine.

Auxin can be translated as “enlarge” or to “increase”. They stimulate cell division, differentiation and elongation.

Charles Darwin observed phototropism (bending of plants towards the light) in the coleoptile canary grass. He noticed that the coleoptile was tipped by an influencer, responsible for the bend towards the light.

Frits Went then isolated the substance and named it “Auxin”, which was responsible for phototropic movement in the oat coleoptile.

Kenneth Thimann elucidated and purified the structure of primary auxins (e.g. IAA (Indoleacetic Acid).

Naturally Occurring Auxins (IAA, IBA)

Artificially synthesized Auxins – 2,4-dichloro-phenoxy acetic Acid (2,4-D), Naphthalene Acetic Acid (NAA).

Mechanism of Action of Auxin

• Auxin is most commonly produced in the apical meristem of shoots, young leaves, and seeds.
• Auxin’s movement is either unidirectional or bipolar. It moves down from the site of its production.
• Polar transport creates an auxin concentration gradient that stimulates certain responses
• The plasma membrane’s auxin-specific transport proteins control the movement of auxin from the cell
• Signal transduction is the mechanism by which plant hormones work. They can elicit multiple cellular responses.
• Auxin binds with enzyme-linked receptors which promote catalysis.
• Auxin binds to receptors and initiates binding to repressor protein to ubiquitin. This results in the degradation of the repressor proteins and progression of auxin response genes, which leads to cell growth and development.

Auxin Function

1. Cell Expansion:Auxin promotes cell elongation in shoots and coleoptiles. Acidification can increase the plasticity of cells.
2. Cell division and differentiation:Auxin promotes the healing process. It aids in the differentiation of cells and regeneration of vascular tissues (phloem and xylem).
3. Morphogenesis and Callus Formation:Auxin and Cytokinin induce callus formation in explants and stimulate morphogenesis
4. Secondary Growth:Auxin stimulates secondary growth and incites cell division within the vascular cambium
5. Root Initiation from Cuttings: To initiate root formation in stem cuttings, NAA can be used
6. Apical dominance: When the growth of the apical meristem blocks the growth of the axillary buds it is called apical dominance. The shoot tip can be removed to encourage the growth of lateral flowers. This is used to encourage branching, e.g. This is used in tea plantations and hedge-making.
7. Parthenocarpy When auxin is applied on some flowers, it induces parthenocarpy (i.e. The ovary grows and becomes a seedless (unfertilized) fruit. This method is widely used to produce seedless tomatoes.
8. Fruit development:Auxin, which is made from seeds, stimulates fruit growth with gibberellins. It also delays senescence
9. It delays the senescence among flowers. Some plants are more feminine if they have high levels of auxin. It encourages the growth of litchi and pineapples.
10. Herbicides: Synthetic auxins, e.g. To kill weeds, 2,4,5-T and 2,4-D are common. It doesn’t affect grasses (monocotyledons).
11. Promotes Tropism: Auxin incites phototropism. Movement in response to light, gravity and touch.

Phototropic movement, which is bending towards the light, can be explained through cell elongation caused by auxins. Due to auxin migration, the auxin concentration is higher on the shaded side. This causes more cell elongation on the shaded side than on the exposed side.