Auxin: Discovery and Functions, Cholodny Went Theory, Practice Problems and FAQs

You all know about hormones. They are the chemical messengers which are produced in one part of the body, from there they travel to other parts of the body and help control the various functions like growth, development, metabolism etc. You know that plants also possess hormones. But do you know which plant hormone was first isolated from human urine? Yes, it is auxin and is present in the tips of plant stems. 

You have observed the movement of plants towards light. Have you ever wondered how and why this happens?

This is because of the presence of auxin hormone. The bending of plants towards light is termed as phototropism. On the other hand, bending of plant roots towards gravity is termed as gravitropism or geotropism. Apart from tropism, auxin is responsible for various other functions also. Let’s take a deep dive into the details of auxin hormone and its major functions in this article.

Table of contents:

• Auxin
• Discovery of auxin
• Cholodny Went theory
• Functions of auxin
• Practice Problems
• FAQs

Auxin

Auxin is an indole derived phytohormone which is weakly acidic. It has an unsaturated ring structure. The word auxin is derived from a word ‘Auxein’ which is Greek. It means ‘to grow’. Auxins belong to a group of hormones that have indole structure. Indole has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered ring.

                 Fig: Structure of auxin

Types of auxins

There are two types of auxins as follows:

• Natural auxin
• Synthetic auxin

Natural auxin

Natural auxins produced naturally by plants. They are normally synthesised in shoot apices, leaf primordia and developing seeds from the amino acid tryptophan. Auxins move in the tissues by cell to cell diffusion and not through the vascular tissues. Examples of natural auxins are as follows:

• Indole 3-acetic acid (IAA)
• Indole 3-butyric acid (IBA)
• 4-chloro-indole acetic acid
• Phenyl acetic acid

Synthetic auxin

Synthetic auxins are synthesised artificially. Examples are as follows:

• Naphthalene acetic acid (NAA)
• 2, 4-dichlorophenoxyacetic acid (2, 4-D)
• 2, 4, 5-trichlorophenoxy acetic acid (2, 4, 5-T) 

Discovery of Auxin

The existence of auxin was first described by Charles Darwin and Francis Darwin. 

Darwin’s experiment

Presence of a growth stimulator in the tip of the stem was discovered by Charles Darwin and his son Francis Darwin. This experiment was performed using canary grass (Phalaris canariensis). 

                    Fig: Canary grass

Hypothesis

The coleoptile tip of the grass senses light. The function of the coleoptile is to protect the emerging young shoot and leaves during germination.

                          Fig: Coleoptile tip

Experiment

For this particular experiment, three observed cases in different perturbations and one control case were taken.

• Control case: The control is observed in intact seedling in which the shoot bends towards the light.
• Case I: Here the tip of the coleoptile is cut because of this the shoot tip does not bend towards light.
• Case II: Here an opaque cap is placed on the tip because of this, light does not reach the coleoptile tip and therefore, the shoot tip does not bend towards the light.
• Case III: Here a transparent cap is placed on the tip. The light reaches to the coleoptile tip and therefore, the shoot tip bends towards the light. 

                                            Fig: Experiment of Charles Darwin

Conclusions

The coleoptile tip contains a signal that senses the light. This signal is responsible for the ‘bending’ of the coleoptile. 

Went’s experiment

In 1926, F.W. Went performed this experiment using oats (Avena sativa).

                 Fig: Avena sativa

This test is known as the Avena curvature test. Here, ‘Avena’ refers to oat plant and ‘curvature’ refers to bending of plant.

                                           Fig: Avena curvature

Hypothesis

Presence of a growth promoting factor (named as auxin) at the tip of the plant caused its bending. Went isolated auxin mainly from the tips of coleoptiles of oat seedlings. He named it as ‘auxin’ which means ‘to grow’ in Greek.

Experiment

Avena curvature test is a simple bioassay test for auxin-type growth regulators. Etiolated oat coleoptiles are used for this test. This test is conducted in perspex trays under diffuse daylight.

For this experiment first, the cut coleoptile tips are placed on agar block. This auxin containing agar block is now placed on a decapitated coleoptile. On another decapitated coleoptile they placed an agar block without auxin to serve as control. Within a few hours the coleoptile with auxin agar blocks bends onto the side opposite to which the agar block is placed. No curvature is observed in the control. Here the curvature occurs due to rapid growth of the side of the coleoptile on which the agar block is placed. 

Conclusions

Elongation of cells causes bending of the coleoptile. The amount of auxin present in the agar block is considered directly proportional to the degree of curvature or bending. 

                               Fig: Chemical signal at the tip

Action of auxin on tips

Light stimulates auxin to move towards the shady region.

                           Fig: Effect of light on distribution of auxin hormone

In the shady region, the concentration of auxin is more as compared to the light illuminated region and this creates a concentration gradient. The cells in the shady region elongate faster. This causes bending of the tip towards the light.

                              Fig: Elongation of cells due to the activity of auxins

Cholodny Went Theory

The Cholodny Went theory was proposed in 1927. This theory describes the tropic movements in monocotyledons due to unequal distribution of hormone, auxin. It describes phototropism i.e. the tendency of shoots to bend towards light and gravitropism i.e. the tendency of roots to grow downwards. In connection with phototropism, the theory proposes the following:

• Auxin is produced in the coleoptile tip.
• Auxin moves into the un-irradiated side once the coleoptile tip detects the asymmetric illumination.
• Auxin travels down the coleoptile, causing auxin asymmetry in the lower areas.
• The coleoptile bends toward the light source due to the increased auxin concentration on the unirradiated side.

Functions of auxins

The effects of auxins on plants are classified under tropic movements and developmental effects.

                                                                 Fig: Functions of auxins

Tropic movements

Tropic movements are of two types as follows:

• Phototropism
• Gravitropism

             GIF: Tropic movements

Phototropism

It is a phenomenon of bending shoots of plants towards light. Roots grow away from light.

Gravitropism

It is the phenomenon of coordinated growth of plants in response to gravity. It is also called geotropism. 

Positive gravitropism

It occurs towards gravity. Auxin suppresses growth in the roots. High concentration of auxin seen at the lower side. Cell elongation takes place at the opposite side i.e., upper side. Hence the root tip bends downwards.

             GIF: Growth of root tips

                                                         Fig: Positive gravitropism

Negative gravitropism

It occurs opposite to gravity. Auxin promotes growth in the shoots. High concentration of auxin seen at the lower side. Cell elongation takes place on the lower side. Hence the shoot bend upwards.

                                                            Fig: Negative gravitropism

                                GIF: Growth of shoot tip

Developmental effects

Auxins perform the various developmental effects, such as root initiation, flower initiation, parthenocarpy, prevention of abscission, apical dominance, feminising effect, cell division, and cell enlargement.

                                                           Fig: Functions of auxins

Root initiation

Auxin promotes growth of lateral and adventitious roots. It also initiates rooting in stem cuttings. It is commonly used in plant propagation methods. NAA and IBA are commonly used for this. 

                         Fig: Root initiation

Flower initiation

Auxin promotes flowering. For example, foliar spray of dilute solutions of 2, 4-D and NAA causes flowering in pineapple and litchi. But flowering is inhibited by spraying high concentrations of auxins. This technique is used in lettuce where flowering is inhibited by this method, as the leaves are edible here not flowers. 

                                                      Fig: Flowering in plants

Prevention of abscission

Auxins delay abscission i.e. it delays the falling of leaves, flowers and fruits. Abscission commonly occurs due to the formation of an abscission zone below a leaf or fruit. This abscission zone cuts off the supply of nutrients and water to the leaf and fruits. This helps in leaf and fruit falling. But when the concentration of auxin is more, formation of the abscission zone is prevented. 

Vascular differentiation

Auxins help in differentiation of xylem and phloem. It also helps in cell elongation. Formation of xylem strengthens the basal part of the stem. This can prevent the fall of crop plants during the windy season. Application of naphthalene acetamide is effective in this case. 

                                                Fig: Vascular differentiation

Feminising effect

Auxin is shown to have a feminising effect in plants such as Opuntia stenopetala and Cannabis sativa.

                                                     Fig: Feminising effect

Apical dominance

Apical dominance is an adaptive feature that is seen in plants. Here the auxin produced in the apical buds inhibits the growth of lateral buds. Growth of plants is usually vertical. We can observe the dominance of the main stem over the other lateral stem in normal conditions. Lateral buds start developing into branches when the apical bud is removed. 

                                                            Fig: Apical dominance

For example, the Araucaria heterophylla plant shows strong apical dominance. Its main stem is predominant over its lateral stems. In Weeping larch, apical dominance is absent. It shows lateral growth.

                                               Fig: Apical and lateral growth

Importance of apical dominance

The apical dominance is important in some plants because of the following reasons:

• It is an adaptive mechanism of some plants.
• Taller the plant, greater is the amount of light received for photosynthesis.
• Apical dominance aids to get dominance over other plants.
• It also aids in becoming fit than other plants.

Parthenocarpy

Parthenocarpy is the production of seedless fruits. Indole acetic acid is used for this phenomenon. Examples of seedless fruits include seedless tomatoes and watermelons.

                                Fig: Examples of parthenocarpic fruits

Weed killers

Auxins are used as weed killers for dicotyledons. They do not affect any mature monocots. 2, 4-dichlorophenoxy acetic acid and 2, 4, 5-trichloro phenoxy acetic acid are used to make the herbicide ‘Agent Orange’. The U.S. military used this to clear leaves or vegetation for military operations during the Vietnam war.

Metabolism

Application of auxins increases metabolism by mobilising the plant resources. 

Prevention of pre harvest fruit drop

Premature fruit drop can be prevented by spraying the dilute solution of 2, 4-D or NAA. 2, 4-D is used mainly for apples and oranges whereas NAA is commonly used for tomatoes. 

Practice Problems

Q 1. Which hormone is responsible for the induction of parthenocarpy in tomatoes?

a. Auxin
b. Cytokinin
c. Gibberellin
d. Abscisic acid

Answer: Parthenocarpy is the production of seedless fruits without undergoing fertilisation. Auxin is used for this phenomenon. In tomatoes, auxin induces the production of seedless fruits. Hence, the correct option is a. 

Q 2. Identify the incorrect statement about auxin.

a. It promotes flowering
b. It helps in seed germination
c. It inhibits senescence
d. It promotes apical dominance

Answer: Seed germination is not aided by auxin. Gibberellins aid in the germination of seeds. It aids in the conversion of starch to glucose by hydrolysis. This glucose is used in cellular respiration and gives the embryo energy. On the contrary, auxin promotes flowering and apical dominance. Auxins delay abscission i.e. they delay the falling of leaves, flowers and fruits. It prevents shedding of mature fruits and older leaves. Hence, the correct option is b.

Q 3. The term ‘positive phototropism’ refers to __________________.

a. Growth of plants in the direction of light 
b. Increased absorption of sunlight by plant cells
c. Growth of plants in the opposite direction to the light
d. All of the above

Answer: Phototropism is described as the phenomenon of bending plants towards light. It is mainly of two types like negative and positive. The growth of distinct portions of plants in the direction of sunlight is known as positive phototropism. Phototropism is usually positive in stems. Negative phototropism describes the growth of certain plant components in the opposite direction of sunlight. Phototropism is often negative in roots. Hence, the correct option is a.

Q 4. Which chemicals are used to make the herbicide ‘Agent Orange’?

a. 2, 4-dichlorophenoxy acetic acid
b. 2, 4, 5-trichloro phenoxy acetic acid
c. 4-chloro-indole 3-acetic acid
d. Both a and b

Answer: Auxins are used as weed killers for dicotyledons. They do not affect any mature monocots. 2, 4-dichlorophenoxy acetic acid and 2, 4, 5-trichloro phenoxy acetic acid were used to make the herbicide ‘Agent Orange’. This is a herbicide mixture that was used during the Vietnam war. The production of Agent Orange ended in 1970 and is no longer in use. Hence, the correct option is d.

FAQs

Q 1. What is the chemical name used for auxin?
Answer: Auxin is an indole derivative and therefore, the chemical name of auxin is Indole 3-acetic acid.

Q 2. How auxin promotes apical dominance?
Answer: Apical dominance is an adaptive feature that is seen in plants. The auxin produced in the apical buds inhibits the growth of lateral buds. Growth of plants is usually vertical. We can observe the dominance of the main stem over the other lateral stem.

Q 3. What happens if auxin is present in large concentrations in plants?
Answer: If auxin is present in large concentrations, it stimulates the production of ethylene. This will inhibit the growth of a plant.

Q 4. In which locations of a plant, auxin is synthesised?
Answer: Auxin mainly involved in the stem elongation. Therefore, it is synthesised in stem and shoot tips. Auxin is also produced in buds, stems and root tips.

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