Auxins: Overview, Discovery, Types of auxins, Bioassay, Auxin Functions
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Auxins: Overview, Discovery, Types of auxins, Bioassay, Auxin Functions

ByManoj Ahirwar

Here we are going to discuss “Auxins: Overview, Discovery, Types of auxins, Bioassay, Auxin Functions” Among the growth hormones Auxin was the first to be discovered. Auxins are weakly acidic plant growth hormones capable of promoting cell elongation in shoots. Auxin (GK. auxeine = to grow). Have an unsaturated ring structure.

Auxins help in the elongation of roots and shoots in the plants. However, the concentration of Auxin for the root and shoot development is different. The optimum concentration in stem is 10 is ppm is and the optimum concentration for roots is 0.0001 ppm. higher concentration of auxins inhibits the growth in plants. Auxins also work as an inhibitor when its concentration is less than 100 ppm.

Discovery of Auxins

1. Julius Von Sachs Experiment

Julius Von Sachs was the first person who indicates the presence of organ forming substances in plants.

2. Charles Darwin Experiment

The first plant growth hormone came from the work of Darwin and Darwin. “Power of movements in plants” is a book of Darwin, in which Darwin and his son found that bending moments of coleoptile of grass (Canary Grass) was due to exposure of coleoptile tip to unilateral light. The botanical name of Canary grass is Phalasis canariensis.

Auxins: Charles Darwin Experiment
Auxins: Charles Darwin Experiment

3. Boysen Jensen Experiment

Boysen Jensen during their work from 1910-1913 found that the tip produces a chemical. That chemical later named “Auxin”

Auxins: Boysen Jensen Experiment
Auxins: Boysen Jensen Experiment- Placed Gelatin
Auxins: Boysen Jensen Experiment
Auxins: Boysen Jensen- placement of Mica Plate

4. Paal Experiment

Paal removed the coleoptile tip and replaced it asymmetrically. Paal was trying to find a curvature during his work from 1914 -1919.

Auxin Plant Hormone: Paal Experiment
Auxin Plant Hormone: Paal Experiment

5. F.W. Went Experiment

WENT collected Auxins from coleoptile tip of Avena in 1928.WENT also developed test for bioassay of Auxins

F.W. Went Experiment
F.W. Went Experiment

F.W. Went also performed an experiment to measure the quantity of Auxin. He choosen Oat coleoptile for this. He placed two Agar blocks equally on a cutted coleoptile tip and reached to the result that 65% Auxins are produced in part of coleoptile which is exposed for sunlight whereas only 35% auxins produced on the side of coleoptile where light is not exposed.

F.W. Went Experiment to measure quantity of Auxins
F.W. Went Experiment to measure quantity of Auxins

Auxins Movement

Auxins Show polar moment which is moved from Apex to base in the plant stem. This type of movement is called basipetal movement. But in case of root, the movement of auxins is acropetal. In the acropetal movement, auxins moved from root tip towards the shoot. By diffusion process, auxins move slowly from Cell to cell and not through the vascular tissues.

Types of Auxins

There are two major categories of auxins
1. Natural auxins and 2. synthetic auxins

1. Natural Auxins

Naturally occurring auxins in plants called natural Auxins. These are phytohormones. Best known natural occurring auxin is indole 3 acetic acid or heteroauxin. Indole 3 acetic acid found in all types of Plants and Fungi. Amino acid Tryptophan is responsible for the synthesis of natural occurring auxins

Natural Auxins: Indole 3 Acetic Acid
Natural Auxins: Indole 3 Acetic Acid

Kogl and Haagen-Smith first isolated naturally occurring auxin from human urine in 1931. isolated auxins were identified as Auxenotriolic acid (C18 H32 O5). Later Kogl and Haagen-Smith and Erxleben obtained another auxin in 1934. This isolated was called Auxin B (Auxenolonic Acid), the chemical formula of Auxenolonic Acid is C18 H30 O4. Auxenolonic Acid was isolated from common germ oil which was extracted from germinating corn seeds.

Natural Auxins Structures
Natural Auxins Structures

Natural auxins are synthesized in shoot Apices, leaf primordia, developing seeds, developing buds, developing embryos. In other words, we can say that natural auxins are synthesized in physiological active parts of plants.

2. Synthetic Auxins

Synthetic auxins are synthetic compounds which cause various physiological responses. Physiological responses are common to indole 3 acetic acid. These are some Names of synthetic auxins

  1. 2, 4-D (2,4-dichlorophenoxy acetic acid)
  2. 2,4,5-T (2,4,5-trichlorophenoxy acetic acid)
  3. IBA (Indole 3 Butyric acid)
  4. NAA (naphthalene acetic acid)
  5. PAA (Phenyl acetic acid)
  6. IPA (indole 3 Propionic Acid)

Note – IBA (Indole 3 Butyric acid) is both natural and synthetic auxin. 2, 4-D (2,4-dichlorophenoxy acetic acid) is weedicide.

Synthetic Auxins structures
Synthetic Auxins structures

Antiauxins

Antiauxins are those compounds which inhibit the action or effects of auxin.

  1. PCIB – p- chlorophenoxy isobutyric acid
  2. TIBA – 2,3,5-tri iodobenzoic acid

Bioassay of Auxins

Bioassay is the testing of biological activity of a substance by using living material.

  1. Avena coleoptile curvature test- In 1928 Avena coleoptile curvature test is carried out by F.W. Went. He demonstrated the effect of organs in plants by performing some experiment with the Oat coleoptile.
  2. Pea Stem curvature test– In 1934, Pea Stem curvature test is also carried out by F.W. Went.
  3. Root growth inhibition test ( cress root inhibition test)

Functions of Auxins

Functions of Auxins
  • Cell elongation-auxins promote cell elongation during the growth of stems and roots. auxins also help in enlargement of the cells of many fruits. auxins stimulate elongation of cells in all directions. Cell enlargement occurs by solubilization of carbohydrates, loosening of microfibrils, synthesis of more wall material, increased permeability of the membrane, increased rate of respiration.
  • Apical dominance- auxins inhibit the growth of lateral Buds and promote the growth of apical buds. In the case of many plants, the apical bud grows and the lower axillary words are surprised if we remove the ethical word then lower buds grows.
  • Control of abscission layer– abscission of fruits and leaves is promoted by abscisic acid. Auxins play opposite role of abscisic acid. Inhibits the formation of abscission layer in leaves and fruits so that it inhibits the obsession. Premature fall of fruits can be controlled by spraying auxins. Generally, premature fruits fall occurs in Apple, pear and citrus plants
  • Weed control- weed control is another very important effect of auxins. At present in the agriculture field, 2,4- D auxin is very popular for weed control. By the way of 2,4- D auxin broad-leaved weeds can be destroyed. But it does not affect mature monocotyledons plants
  • Root differentiation- plantation is very important to keep the environment balanced. Nowadays we are cutting so many plants only for our benefits. That’s why it is our duty to do plantation as possible as. Some plants stems can generate roots. After root formation, we can plant it as a new plant. Auxin promotes root formation in cutting stems or branches. If we dip the lower cut and of cutting in a dilute solution of oxygen a large number of roots are developed in a short time from the stem. IBA is perfect for root formation in cutting stems. These cuttings can be successful plant after plantation.
  • Parthenocarpy- production of seedless fruits is called parthenocarpy. Parthenocarpic fruits formed without fertilization. That’s why these fruits cannot be used to produce a new generation that species. Auxins of induce parthenocarpy in banana, oranges and grapes. IAA (indole 3 acetic acid) is the oxygen which is used to produce parthenocarpic fruits at commercial. Auxins are applied in Lanonin paste on stigma for inducing parthenocarpy to produce parthenocarpic fruits.
  • Control of lodging- when the crop is ripe and if there is heavy rain accompanied by strong winds the plants bend due to this The inflorescence gets submerged in water undertakers. If we use a dilute solution of any oxygen which is available and spread on young plants there is a possibility of bending of plants is reduced. By spraying auxins stem becomes stronger.
  • Flowering- flowering is important to step to produce a new generation for any species. Fruits cannot be formed without flowering in flowering plants. Auxins promote flowering in pineapple. Auxins also Delay the flowering in lettuce. Auxins are also used to increase cotton production in the cotton plant. NAA is generally used for flowering.
  • Differentiation of vascular tissues- differentiation of xylem and phloem cells can be induced by the use of auxins. Differentiation is the process by which tissues get a distinct form from each other to do particular specified function Promotes the differentiation of xylem and phloem in intact plants as well as in callus produced in vitro.
  • Sex expression- sex expression is the production of sexual organs. Auxins increase the number of female flowers in cucurbits. NAA induces the formation of hermaphrodite or female flowers in a male inflorescence during the period of Fluorescence differentiation in Maize plant. thus we can say that Auxins supports femaleness in plants.
  • Healing- auxins induced cell divisions around the injured area. Traumatic Acid or traumatin is the chemical which was formerly named for this.
  • Nodule formation- nodules formation are specific are a characteristic feature of legumes plants. Auxins Stimulate nodule formation in legumes. Legumes contain symbiotic bacteria which produce nitrogen compounds. These nitrogen compounds help the plant to grow to compete with other plants. After the death of plants, the nitrogen gets fixed and released, making it available to other plants. This helps to fertilize the soil.
  • Respiration– auxins may increase the rate of respiration indirectly through increased supply of ADP rapidly utilising the ATP in the expanding cells according to French and Beevers. French and Beevers discovered that in 1953.

MCQs on auxin

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MCQs on auxin
  • When the content of Auxins decreases the abscission layer formed and leaf fall occurs.
  • Auxins were discovered by Went.
  • Apical dominance maintains in higher plants is due to the balance between auxins and cytokinins.
  • Cell elongation in plants is caused by the plant hormone auxins.
  • Indole 3 Acetic Acid called auxins was first isolated from the urine of human
  • Indole 3 acetic acid is an example of auxins
  • Parthenocarpy is induced by auxins
  • L. J Audus discovered the Avena curvature test to find out the concentration of auxins
  • NAA is a synthetic auxin
  • 2,4-D is not a naturally occurring plant hormone
  • Growing tips of plants contain the highest concentration of auxins.
  • Naturally occurring auxin is indole 3 acetic acid
  • The High concentration of synthetic auxin is generally used for weed control at commercial level
  • The primary precursor of IAA is Tryptophan.
  • Auxins inhibit the growth of lateral axillary buds
  • Phototropism in shoots is attributed to Auxins
  • Sex can be changed by the application of auxins in unisexual plants.
  • Both in callus and suspension cultures commonly used auxin is 2,4-D.
  • Auxins lowers the water potential of cells
  • Pollen extract stimulates the formation of fruits in the pollinated carpel. This effect is due to the presence of auxin plant hormone in Pollen extract.
  • Indole 3 acetic acid is the commonly used plant growth hormone in tea plantations.
  • 2,4-D is not found in plants
  • Apical dominance in plants is caused by a high concentration of auxins in the terminal buds.
  • Auxins are not used for fruit ripening. Ethylene hormone is responsible for fruit ripening in plants.
  • NAA prevents falling of fruits in plants.
  • The movement of auxin is largely Basipetal.
  • Indole butyric acid has proved very effective to induce rooting from the cut end of the stem.
  • Auxins are abundantly produced in meristematic regions of the shoots.
  • Phototropic curvature is the result of an uneven distribution of auxin plant hormone.
  • Auxin synthesis occurs in root and shoots tips in plants.
  • Differentiation of shoot is controlled by the ratio of –High Auxins: Cytokinins
  • Coleoptile test detects the presence of indole 3 acetic acid which is auxin.
  • During germination grows and root goes downward because of the presence of Auxins plant hormone.
  • Kogl, Erxalaben and Haagen-Smith first isolated Auxin B.
  • Bolting effect in plants is not influenced by auxins. Bolting effect is influenced by the gibberellins.
  • Abscission of fruits is prevented by IBA.
  • 2,4-D is used for causing defoliation of forest trees.
  • Presence of apical dominance causes an excurrent habit of growth in pinus Trees.
  • The movement of auxin is largely Basipetal.
  • Indole butyric acid has proved very effective to induce rooting from the cut end of the stem.
  • Auxins are abundantly produced in meristematic regions of the shoots.
  • Phototropic curvature is the result of an uneven distribution of auxin plant hormone.
  • Auxin synthesis occurs in root and shoots tips in plants.
  • Differentiation of shoot is controlled by the ratio of -High Auxins: Cytokinins
  • Coleoptile test detects the presence of indole 3 acetic acid which is auxin.

Important FAQs related to Auxins

These are some important frequently asked questions during competitive exams. read carefully.

Important FAQs related to Auxins

What is the role of auxins in plants?

Auxins promote cell elongation, inhibits the growth of lateral Buds, promote the growth of apical buds, Inhibits the Premature fall of fruits, Weed control, promotes root formation in cutting stems, induces Parthenocarpy, increase cotton production, Delay the flowering in lettuce, induces differentiation of xylem and phloem cells, Auxins supports femaleness in plants, auxins induced cell divisions around the injured area. induces Nodule formation, increases the rate of respiration indirectly.

How are Auxins used commercially?

Auxins used commercially to promote the growth of plants, to reduce leaf and fruits falls, to controls weeds, to produce parthenocarpic fruits, for the formation of roots in cutting end of stems, to increase the production of cotton etc.

Who discovered auxin?

Auxins were discovered by Went in 1928 from coleoptile tip of Avena. but the Existence of first plant growth hormone was discovered by Darwin in 1881. Boysen- Jenson (1910, 1913) found that the tip produces a chemical which was later named as “Auxin”. Auxins was first collected by Went in 1928.

What are the 5 plant hormones?

These are 5 Plant Hormones
1. AUXIN, 2. GIBBERELLIN, 3. CYTOKININ, 4. ETHYLENE 5. ABSCISIC ACID.

What are the types of auxin?

Auxins are of two types
1. Natural Auxins- Indole 3 acetic acid
2. Synthetic Auxins- IBA, IPA, NAA, PAA etc.

Where auxin is synthesized?

Auxins are synthesized in meristematic regions of the shoots of plants.

How is auxin transported?

Auxins Show polar moment which is moved from Apex to base in the plant stem. This type of movement is called basipetal movement. But in case of root, the movement of auxins is acropetal. In the acropetal movement, auxins moved from root tip towards the shoot. By diffusion process, auxins move slowly from Cell to Cell.

What are synthetic auxins?

Synthetic auxins are those chemical compounds that are not occurs naturally in plants. these chemical compounds work as natural auxins (Indole 3 Acetic Acid). Synthetic auxins artificially made by the human such as I) 2, 4-D, II) 2,4,5-T III) IBA IV) NAA, V) PAA, VI) IPA.

What are auxins used for?

growth of plants, to reduce premature leaf and fruits falls, for weed control, to induce parthenocarpy, for the formation of roots in cutting end of branches, to increase the yield of cotton.

What is the main function of auxin?

Cell elongation, Parthenocarpy, Roots formation in cutting stems, weed controls are the main functions of Auxins.

What do you mean by natural auxin?

Natural Auxins are synthesized in tips of roots and shoots of plants. these are naturally occurring plant growth hormones. Indole 3 Acetic Acid is a very popular natural auxin.

Also Read-

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