The Auxins

The Auxins

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The best known plant hormones are auxins, substances related to growth regulation. Of the auxins, the best known is the IAA - indolylacetic acid.
EIA in plants is not only produced in coleoptile (Coleoptile is called the first portion of the plant that appears on the surface of the soil. It develops according to light.

If its intensity is constant, the plant will develop vertically, if lit laterally the coleoptile will grow towards the light, bending). Its production also occurs in seed embryos, in pollen tubes, and even in developing ovarian wall cells. In the adult plant, it is producing in the apical buds, mainly the stem ones.

IAA transport is polar, that is, it occurs only at the production sites to the action sites through special parenchymal cells. EIA acts in very small quantities, in the order of millionths of mg, stimulating growth.

An optimal dose to stimulate stem growth can inhibit root growth.

  • The root and stem of the same plant react differently to the same hormone:
    The optimal dose for root growth is less than the optimal dose for stem growth. The root, then, is more sensitive to EIA than stem;
  • The optimal dose for stem growth is inhibitory for root growth and also inhibits lateral bud growth.

Auxins effect

In apical dominance

Auxins act on the genes of plant cells, stimulating the synthesis of enzymes that promote softening of the cell wall, enabling the distension of cells. The body shape of many plants, especially those of the perennial group is defined by hormonal action. The apical bud, which acts on the longitudinal growth of the stem, produces auxin on the surface to inhibit the lateral buds, leaving them dormant. By eliminating the apical bud, growth will be promoted by lateral buds activated by the absence of auxin. The vegetable will then have a canopy shape: little height and more branches.

In growth under the light

Coleoptils submitted to unilateral illumination showed growth in the opposite direction of light. O AIA It moves from the illuminated to the unenlightened side, exerting its effect there. The curvature of the coleoptile will be greater the longer the illumination time, as more EIA ends up reaching the opposite side.
If a beetle is evenly lit, it will grow in a straight line, as will if it is left in the dark.


Geotropism is a response of plant organs to the force of gravity. This response results in growth of the shoot in the opposite direction to the force of gravity (negative geotropism) and root growth in the direction of gravitational force (positive geotropism). Stem geotropism seems to be in accordance with the Cholodny-Went theory.

When the plant is placed in a horizontal position, the accumulation of auxins in the lower part of the stem causes a greater growth of this part, occurring curvature in a direction opposite to the force of gravity, causing the stem to move upwards. In the horizontal root there is a greater stretching in the upper compared to the lower, causing root curvature in the direction of gravitational force. There is little evidence that an asymmetric distribution of natural IAA occurs in horizontally placed roots.

Other effects of auxins

  • The application of auxins on the stem surface promotes the formation of adventitious roots, which is useful in vegetative propagation by cuttings.
  • The level of auxins in the ovary tissues rises appreciably at the time of fertilization, promoting fruit development.
  • THE synthetic auxin 2,4-D (2,4-diclofenoxyacetic acid) is used as a herbicide and acts only on eudicotyledonous plants.


In the wild, it is common for ovaries to develop without seed formation. This is the case with bananas. Auxin exists in the ovary wall and also in the pollen tubes that ensures the growth of the fruit.
Artificially, it is possible produce parthenocarpic fruits by applying auxins directly to the ovaries, previously removing the stamens to avoid pollination. This is done to obtain grapes, watermelons, and seedless tomatoes.


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