Auxin Biosynthesis: Definition, Functions, & Uses

What Is Auxin?

Auxins are a class of chemical compounds responsible for regulating the developmental, growing, and long-lasting processes, including the reproductive process of plants. They are formed in nearly every part of the plant and transported through different parts to express their action. This competence of these hormones to act in synergy or alone implies that their functions can be synergistic or even antagonistic.

Auxin Mechanism Of Action

The main sites of auxin production are the apical meristems of shoots, young leaves, and seeds.

• Auxin transport is polar and goes only in one direction: basipetal from its site of production.

• In this way, the polar transport creates an auxin concentration gradient, which induces specific responses.

• The movements of auxin outside the cell are mediated by auxin-specific transport proteins in the plasma membrane.

• Plant hormones work through signal transduction and bring about more than one type of cellular response.

• Auxin binds to the enzyme-linked receptors, thereby catalyzing a reaction.

• In brief, when auxin binds to a receptor, a repressor protein for some genes binds to ubiquitin which causes degradation of the repressor protein, allowing the transcription of the auxin response genes leading to cellular growth and development.

Auxin Biosynthesis

Auxin is synthesized by tryptophan-dependent and tryptophan-independent pathways.

Tryptophan-Dependent Pathway (Major Pathway)

It is most dominant in higher plants. In auxin biosynthesis, tryptophan (Trp) is converted to indole-3-pyruvic acid and finally to indole-3-acetic acid. This pathway is considered to be the major route of auxin production in plants.

Tryptophan-Independent Pathway (Major Pathway)

It occurs in plastids. This pathway uses indole or indole-related intermediates but it is Less understood than the tryptophan-dependent pathway.

Regulation of Auxin Biosynthesis

Auxin biosynthesis is tightly regulated by:

• Auxin biosynthesis is a highly regulated process by development, tissue type, and environmental signals.

• Through these components, the genes involved in encoding the enzymes responsible for auxin biosynthesis are modulated, resulting in a rise or reduction of auxin.

Auxin Transport And Distribution

After being synthesised, auxin flows through the plant via two major pathways: the polar auxin transport system and the non-polar auxin transport system.

Polar Auxin Transport (PAT)

• PAT is known to contribute to the long-distance, basipetal, directional flow of auxin from primarily shoot apical meristem to root tips.

• This occurs via auxin influx and efflux carriers at the plasma membrane: AUX1/LAX PIN and ABCB, respectively.

• Leaning towards the non-polar auxin transport system, auxin moves through the phloem, allowing distribution to different plant parts.

Non-Polar Auxin Transport

• This type of transport occurs via phloem.

• It supports long-distance distribution to mature tissues.

Auxin Homeostasis And Metabolism

Plants maintain auxin homeostasis through the harmony of biosynthesis, transport, and metabolism. The levels of auxin are regulated by :

Conjugation

Auxin gets inactivated or stored when conjugated with amino acids, sugars or other compounds.

Oxidation

Auxin gets oxidized by enzymes called auxin oxidases and becomes inactivated.

Catabolism

Multiple catabolic pathways contribute to decreasing the level of auxin by mediating degradation.

Auxin Perception And Signal Transduction

Plants perceive the signal generated by auxin through an elaborate mechanism of receptors and following cascades.

• The primary receptors of auxins are the TIR1/AFB proteins, comprising Transport Inhibitor Response 1/Auxin Signaling F-Box, which is part of the SCF complex formed by the Skp1-Cullin-F-box type of ubiquitin ligase.

• Upon auxin binding, TIR1/AFB receptors interact with the Aux/IAA repressor proteins and thus target these for degradation via the 26S proteasome.

• Thereby, the auxin response factors are released, now able to activate or repress the expression of auxin-responsive genes.

Importance of Auxin Biosynthesis

The importance of auxin biosynthesis includes:

• Drives organ formation (leaves, roots, flowers)

• Controls the pattern in embryo

• Regulates the apical dominance

• Determines the ratio of root and shoot

• Enables adaptive growth to environmental signals

Auxin Biosynthesis NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Auxin Biosynthesis Pathways

• Regulation of Auxin biosynthesis

• Mechanism of action of Auxin

Practice Questions for NEET

Q1. 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.

Q2. 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.

Q3. The compound that inhibits the action of auxins and compete with auxins for active sites is

1. p-chlorophenoxy isobutyric acid (PCIB)

2. 2,3,5-triiodobenzoic acid (TIBA)

3. Malic hydrazide

4. Both 1 and 2

Correct answer: 4) Both 1 and 2

Explanation:

Both p-chlorophenoxy isobutyric acid (PCIB) and 2,3,5-triiodobenzoic acid (TIBA) are man-made substances that hinder auxins' work in plants.

PCIB is a molecule that stops auxins from doing their job. It binds to the same places auxins would, so the plants don't respond to the signal for growth. This is useful in research to look at how auxins work because they halt activities like root growth and cell stretching.

TIBA is another one that affects how plants grow. It stops auxins from moving to the right places within the plant cells. This messes up important processes like making roots and shoots and can change the way the plant grows as a whole.

Hence, the correct answer is option 4) Both 1 and 2.

Also Read:

• MCQs on Auxin Biosynthesis

• Factors Affecting Plant Growth

• Cytokinins

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