Auxins: History, Bioassay, Function, Uses, Topics

What Are Auxins?

Auxins are a class of phytohormones that, among others, exert a key role in controlling various phenomena in plant development, such as cell elongation, the formation of roots, and mechanisms of response relating to light and gravity. Auxin activity and potency information are very important for both basic knowledge of the biology of plants and practical applications in agriculture and horticulture. The bioassays of auxins, standardized methods which quantify their biological activity by observation of their action on plant growth, are briefly expatiated upon in the following article. In line with this, it goes on to discuss more about the working principles of auxin bioassays, standard methods employed, and their meaning in plant science.

What Is a Bioassay?

A bioassay is an analytical procedure that measures the potency or effect of a substance by its effect on living organisms, including plants. The type of response measured may be based on this approach: either quantal or quantitative, based on a binary or continuous measured response. In the case of auxins, bioassays are used to test for the growth-promoting effects of these hormones on plants.

Principle Of Auxin Bioassay

It is founded on the principle of auxin bioassays: plant tissues are treated with a known concentration, and the ensuing growth response is measured. In many instances, this potency of the auxin is expressed in comparison to a standard reference to quantify its effectiveness. Various parameters can be used as a measure of the response, such as changes in the growth rate, curvature, or root development.

Standard Methods of Auxin Bioassay

Several methods that are standardized because of their universality are used in estimating the activity of auxins. The most prominent of these is the Avena curvature test, which estimates the degree of curvature of oat (Avena sativa) coleoptiles responsive to applied auxins.

Avena Curvature Test (Oat Coleoptile Test)

• Procedure: In this test, oat seedlings are cut at the tip to remove the apical meristem, which is the source of auxin. Auxin solutions of different concentrations are applied to the cut surfaces and left for a certain specified period. Later, the degree of curvature of the coleoptile is measured.

• Interpretation: The curvature is proportional to the concentration of applied auxin. The greater the degree of curvature, the higher the concentration of auxin that exerts growth-promoting effects.

Root Growth Bioassay

• Root Growth Bioassay: This technique involves treating the base of plant cuttings with auxin and quantifying root growth over time. The number and length of the roots are measured as a function of auxin activity.

Seed Germination Bioassay

• Seed Germination Bioassay: This is a bioassay whereby the seeds get treated with auxin solutions and see the rate of germination and its speed. This technique shows that auxin is essential for seedling establishment.

Importance Of The Auxin Bioassays

The auxin bioassays are important for various uses:

Limitations of Auxin Bioassays

Even though auxin bioassays have been very useful, they also have several limitations:

Auxin Bioassay NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Standard Method Auxin Bioassay

• Importance of Auxin Bioassays

Practice Questions for NEET

Q1. The bioassay of auxin was done on the germinating oat, Avena. Which scientist conducted the experiment?

1. Louis Pasteur

2. Paul Ehrlich

3. F.W. Went

4. Edward Jenner

Correct answer: 3) F.W. Went

Explanation:

The bioassay of auxin was conducted by F.W. Went using germinating oats (Avena). He devised an experiment to study the effect of auxin on plant growth, which became known as the Avena curvature test. In his experiment, Went placed agar blocks containing auxin on one side of decapitated oat coleoptiles, causing them to bend towards the opposite side due to differential cell elongation. This demonstrated the role of auxin in promoting growth and cell elongation.

Hence, the correct option is 3) F.W. Went.

Q2. The precursor for the synthesis of auxin is

1. Acetyl Co-A

2. Tryptophan

3. Trypsin

4. Malic hydrazide

Correct answer: 2) Tryptophan

Explanation:

The precursor molecule essential for the synthesis of auxins, which are vital plant hormones, is tryptophan. These hormones are derived from the amino acid through specific biochemical pathways. For instance, indole-3-acetic acid (IAA), a well-known auxin, is produced by the transformation of tryptophan into intermediary substances like indole-3-pyruvic acid and other indole compounds. The significance of auxins in plant biology is profound, as they are responsible for controlling various growth and developmental processes. These include:

1. Facilitating cell elongation, which is a key aspect of plant growth.
2. Maintaining apical dominance, ensuring the main shoot grows more than the side shoots.
3. Inducing root initiation, which is crucial for the plant's anchorage and nutrient absorption.
4. Regulating tropic responses such as phototropism, the bending towards light, and gravitropism, the growth in response to gravity.

Hence, the correct answer is option 2. Tryptophan

Q3. Which of the following is not a synthetic auxin?

1. 1-naphthaleneacetic acid

2. 2,4-dichlorophenoxyacetic acid (2,4-D)

3. 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)

4. Indole-3-butyric acid

Correct answer: 4) Indole-3-butyric acid

Explanation:

Synthetic auxins include 1,4-naphthaleneacetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). They are synthetic plant growth regulators that replicate the actions of auxin, a naturally occurring hormone.

However, plants contain indole-3-butyric acid (IBA), a naturally occurring auxin. Since it is found naturally in plant tissues, it is not a synthetic auxin, even though it is utilized as a rooting agent.

Hence, the correct answer is option 4)Indole-3-butyric acid

Also Read:

• MCQs on Auxin Bioassay

• Ethylene

• Phases of Growth in Plants

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