What is Auxin?

The ‘auxin’ word is taken from the Greek word ‘auxein’, which means to grow. Auxins was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth-regulating properties. Auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.

Auxin is a powerful growth hormone naturally produced by plants. Auxins are found in the shoot and root tip of the plants to promote cell division, stem and root growth. It drastically changes the plant orientation by promoting cell division to one side of the plant, which is in response to sunlight and gravity.


Auxin Structure 

Auxin molecules are normally derived from the amino acid tryptophan. These types of amino acids have a six-sided carbon ring, which is attached to the five-sided carbon ring. The difference between the auxin molecule and the tryptophan is based on what is attached to the ring. 

To create a common auxin IAA molecule, two enzymes are needed to act on tryptophan first, an aminotransferase removes nitrogen and hydrogen from the side-chain attached to the 5-sided ring. After that decarboxylase enzyme removes the carboxyl group, leaving COOH. A chloride ion attached to the six-sided ring and IAA is born. Most auxins are some derivation of this molecule.


Auxin Function

Some of the function of auxin in plants are given below -  

  • Auxin hormones have a wide range of uses in plants. Molecules of auxins are found in all tissues of the plants. They are mainly concentrated in the meristems, which are growth centres of growth. 

  • Growth centres release the auxin molecules and it is then distributed towards the roots. In this way, plants can coordinate its size and the growth and development of different tissues based on the gradient of the auxin concentration.

  • Auxin affects many cellular processes at different molecular levels. Auxin molecules affect cytoplasmic streaming such as the movement of fluids within a cell and even the activity of various enzymes. 

  • Auxins directly control the growth, development, and proliferation of the cells within the plant. The gradient of the auxins directly processes flower initiation, fruit development, and even tuber and bulb formation. 

  • The level of auxins also affects phototropism on a daily basis, it also helps the plant to follow the sun and gain most of the energy from it. 

  • Apical dominance is another feature of the auxin gradients in many plants. It is formed when the single meristem is growing faster and more efficiently. Auxin released from this type of meristem inhibits new shoots from budding off below it. When the stem is cut off many new shoots will arise below it, when the auxin gradient has been disrupted, the system must create a new leading shoot. The primary physiological effect of auxin in plants is to stimulate the elongation of cells in the shoot, it can be observed in phototropic curvature. 

Physiological Role of Auxins 

Some of the important physiological roles of auxins are -

  • Apical dominance - It is the growing apical bud, which inhibits the growth of lateral buds.

  • Root initiation - Auxins helps to initiate rooting after stem cutting.

  • Flowering  - Auxins helps to promote flowering in pineapple.

  • Abscission - Auxins promotes the abscission of older leaves and fruits.

  • Auxins help in cell division.

Uses of Auxins 

Auxins induce parthenocarpy, in tomatoes. They are widely used as herbicides such as  2, 4-D, which is widely used to kill dicotyledonous weeds. But it does not affect the mature monocotyledonous plants. Auxin is also used to prepare weed-free lawns by gardeners having large gardens. Auxins also control xylem differentiation and it also helps in the cell division.


Conclusion 

After going through complete information about auxin, what auxin is, structure, function and uses, and also after discussing the physiological roles of the auxin, it is clear that auxin is not considered a hormone, as its characteristics doesn't exactly fit with the hormone definition. There are two types of auxin, one which is naturally found in plants and other synthesized by humans. Auxins greatly support cell elongation in the dark side of the plant compared to the other side of the plants.

FAQs (Frequently Asked Questions)

1. Why is Auxin Not a Hormone?

Answer. Auxin is not a hormone because it may act at low concentrations and can be transported but it is not produced in a specific tissue. It is also too pleiotropic to be considered as a hormone. In fact, auxin appears to be a signal that triggers the preset system rather than a hormone with a specific function.

2. What Do You Mean By Natural Auxin?

Answer. Generally, there are two types of hormones natural and synthesized. Natural auxins are the ones which are present in the plant naturally. Some of the examples of the natural auxins are IAA (Indole 3 acetic acid) and IBA (indole 3 butyric acid). Auxin which is synthesized artificially is known as synthesized auxins. Some of the examples of synthesized auxins are 2,4-D.

3. Is Auxin Toxic To Humans?

Answer. Indole-3-acetic acid is the main auxin produced by plants and It plays an important role in the growth and development of the plants. The same hormone is present in humans also, where it is considered as uremic toxin deriving from tryptophan metabolism. So it is not toxic to the human body.

4. How Do Auxins Affect Plant Growth?

Answer. Auxins play an important role in stem elongation and also they inhibit the growth of lateral buds. Auxins are found in the stem, buds and root tips. Auxins also produce curing in the plant stem tip towards the light known as phototropism. It also plays an important role in maintaining apical dominance.