Auxin

Auxins are a group of plant hormones that play an important role in the regulation of plant growth. The name of this class of hormone has been derived from the Greek word ‘auxein’, meaning ‘to grow’ or ‘increase’, as they induce cell division, differentiation and elongation in plants.

Where is it Produced

It is mostly produced in the apical meristem of tips of roots and shoots, young leaves and seeds

Discovery: How was Auxin Discovered

Auxin was one of the first major plant hormones to be discovered. In the early 1880s, Charles Darwin and his son Francis detected phototropic movement in the coleoptile of canary grass. He observed that there was some influencer at the tip of the coleoptile, which was responsible for the bending towards the light. Later, in 1928, Dutch botanist Frits Went isolated the hormone from the tip of the oat coleoptile which was responsible for phototropism, and named it as ‘Auxin’.

Chemical structure

Auxin can be eithernaturally produced by the plant, or created synthetically.

  • Natural auxin: Indole-3-acetic acid (IAA)
  • Synthetic auxin: Indole butyric acid (IBA), Naphthalene acetic acid (NAA), 2,4-Dichlorophenoxyacetic acid (2,4-D)

How Does Auxin Work

Auxin promotes cell growth and elongation by changing its distribution within the plant that alters the cell wall plasticity. For better understanding, let us take the example of phototropism in shoots.

  1. As stated, auxin is found on the apex of stem. When a unilateral light beam hits the shoot, most of the auxins migrate towards the shaded side, creating a gradient of the hormone across the stem. The molecules of hormone travel between the cells by diffusion.
  2. This phenomenon leads to the activation of a proton pump, which lowers the pH by secreting H+ ions into the cell wall.
  3. The decreased pH, in turn activates enzyme expansins that loosens the cellulose fibers within the cell wall by breaking the bonds between them.
  4. As a result, there is an influx of water within the cell that in turn increases the turgor pressure.
  5. Consequently, cells on the shaded side become elongated and cause the shoot tip to bend towards light.

Functions: What do Auxins do

  1. Cell elongation: It promotes cell elongation in shoots and coleoptiles.
  2. Cell division and differentiation: It helps in cell division, differentiation, and regeneration of vascular tissues.
  3. Apical dominance: Auxin exhibits a unique phenomenon called apical dominance, where the growth of apical meristem suppresses the growth of lateral buds.
  4. Root initiation on cuttings: Stimulate root initiation on stem cuttings and lateral root development in tissue culture.
  5. Fruit development: It stimulates fruit development along with gibberellins.
  6. Flowering: A high auxin concentration promotes flowering in some plants, such as litchi and pineapples.
  7. Delays senescence: It delays the senescence, that is, prevents the premature fall of leaves, flowers, fruits.
  8. Promotes Tropism: Auxin induces tropic movements, like phototropism, gravitropism, and thigmotropism.
  9. Callus Formation and Morphogenesis: Along with cytokinin, it induces callus formation in explant and stimulates morphogenesis.
  10. Secondary Growth: It promotes secondary growth and induces cell division in the vascular cambium.

Commercial Applications

  1. Auxin develops transgenic seedless variants of fruits without ferrtilization, such as tomato, strawberry, and eggplant. The hormone is also sprayed on these fruits to make them grow larger than usual.
  2. Synthetic auxin 2-4-D is used as a pesticide and herbicide to kill weeds (only dicots). It does not affect grasses (monocots).
  3. For asexual propagation, another synthetic auxin, NAA, is used to initiate root formation in stem cuttings and tissue culture. Using this, rooting takes less time for the new plant, thereby  greatly increasing its chance of survival.

Article was last reviewed on Monday, May 2, 2022

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