Auxin

Auxin is a plant hormone. Auxin hormone function is to control growth and developmental processes in plants, including cell expansion, root initiation, and bud and flower growth. Auxin, discovered by Charles Darwin and his son Francis, was first isolated from human urine.

Auxin plays a key role in plant adaptation to the environment, and its functions are associated with signal transduction, transport, homeostasis, and biosynthesis pathways. In this article, we will study the auxin, its types, mechanism of action, and functions in detail.

Definition of Auxin: Auxin is a class of plant hormones that regulate various aspects of plant growth and development.

The word auxin means “to rise”. It is a plant hormone important for growth regulation, mainly in the apical meristem and stem elongation. It controls various physiological processes like cell elongation, tropisms, and apical dominance. Auxin also influences root development, fruit development, and vascular tissue differentiation. The hormone’s distribution within plant tissues is tightly regulated, often moving in a polar manner. Synthetic auxins like indole-3-acetic acid (IAA) are used in agriculture for weed control, rooting hormones, and tissue culture.

Chemical Structure

Auxins are primarily indole derivatives. IAA is the most abundant natural auxin in plants. It has an indole ring and a carboxylic acid group. Other synthetic auxins, like 2,4-dichlorophenoxyacetic acid (2,4-D) and naphthaleneacetic acid (NAA), have similar structures but differ in their substitutions.

Biosynthesis

Auxins are synthesized in various plant tissues, including young leaves, apical buds, and developing seeds. The primary biosynthetic pathway of IAA involves the amino acid tryptophan, which undergoes several enzymatic steps to form IAA. Other pathways also contribute to auxin biosynthesis, especially under stress conditions.

Transport

Auxins can move within plants in a polar manner, from the apical meristems (where they’re synthesized) towards the basal parts. This polar transport occurs through cell-to-cell movement facilitated by specialized carrier proteins and can be both acropetal (upward) and basipetal (downward). This transport mechanism helps coordinate growth and developmental processes throughout the plant.

Also Read: Difference Between Auxin and Gibberellin

Example of auxin are:

• Natural: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
• Synthetic: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid) 

The various types of auxins are explained below:

Indole-3-Acetic Acid (IAA)

• Naturally occurring auxin in plants.
• Regulates cell elongation, root initiation, and apical dominance.
• It is synthesized in young leaves, apical buds, and developing seeds.
• Plays a central role in tropic responses like phototropism and gravitropism and is involved in vascular tissue differentiation and stress responses.
• It also functions as a signaling molecule in various developmental processes and can be transported polarly within plants to coordinate growth.

2,4-Dichlorophenoxyacetic Acid (2,4-D)

• It is synthetic auxin widely used in herbicides and plant growth regulators.
• It mimics the effects of natural auxins and induce uncontrolled growth in susceptible plants.
• Effective in controlling broadleaf weeds without harming grasses in agriculture and is used to regulate plant growth and development in forestry and horticulture.
• It can disrupt hormonal balance in plants at high concentrations.

Also Read: Coordination in Plants

Naphthaleneacetic Acid (NAA)

• It is a synthetic auxin commonly used in agriculture and horticulture.
• Promotes root initiation and development, especially in stem cuttings.
• Helps prevent premature fruit drop and enhances fruit set in some crops.
• Used to induce rooting in difficult-to-root plant species and in tissue culture.
• Can be applied as a foliar spray or as a rooting hormone in rooting media.

Indole-3-Butyric Acid (IBA)

• It is a synthetic auxin with rooting properties similar to NAA.
• Used to stimulate root growth in stem cuttings and to promote root development.
• It is also used in horticulture for propagating plants from cuttings.
• Can be applied as a powder, liquid, or gel formulation for rooting.

1-Naphthaleneacetic Acid (1-NAA)

• It is a synthetic auxin structurally similar to NAA.
• Utilized in plant tissue culture for callus induction and somatic embryogenesis.
• Effective in promoting root formation in plant tissue cultures.
• It is also used in combination with cytokinins for shoot proliferation and regeneration.

Also Read: Movement Due to Growth in Plants – Tropical and Nastic Movements

Mechanism of action of auxin involves following steps:

1. Receptor Binding: Auxin binds to specific receptor proteins located in the cell membrane or cytoplasm.
2. Activation of Signal Transduction Pathways: Binding of auxin to its receptors triggers a series of reaction leading to changes in gene expression and cellular responses.
3. Gene Regulation: Auxin signaling pathways regulate the expression of genes involved in various physiological processes, including cell elongation, division, and differentiation.
4. Cell Expansion and Elongation: Auxin promotes cell expansion and elongation by activating proton pumps in the cell membrane, leading to increased acidity and cell wall loosening.
5. Tropism Responses: Auxin gradients in plant tissues mediate tropic responses such as phototropism (response to light) and gravitropism (response to gravity) by influencing cell growth and elongation.
6. Apical Dominance: Auxin inhibits the growth of lateral buds, maintaining apical dominance and promoting the growth of the main shoot.
7. Root Development: Auxin regulates root development by promoting cell division and elongation in the root meristem and influencing the formation of lateral roots.
8. Vascular Tissue Differentiation: Auxin plays a role in the differentiation of vascular tissues, promoting the formation of xylem and phloem elements.

Also Read: What is Senescence?

Auxin performs the following functions:

1. Cell Elongation: Auxin promotes elongation of plant cells by stimulating cell wall loosening and increasing cell membrane flexibility.
2. Apical Dominance: It inhibits the growth of lateral buds and maintains the dominance of the apical meristem. It also promotes vertical growth.
3. Tropisms: Auxin mediates tropic responses such as phototropism (response to light) and gravitropism (response to gravity) by regulating differential growth in plant organs.
4. Root Development: It promotes root growth and development by stimulating cell division and elongation in the root meristem.
5. Vascular Tissue Differentiation: Auxin influences the differentiation of vascular tissues, promoting the formation of xylem and phloem elements necessary for water and nutrient transport.
6. Fruit Development: It regulates fruit development by promoting cell division and enlargement in developing fruit tissues.
7. Leaf Abscission: Auxin inhibits leaf abscission by maintaining the integrity of the abscission zone, delaying the shedding of leaves.
8. Apical Hook Formation: It facilitates the formation of the apical hook in dicot seedlings, protecting the delicate shoot apex during emergence from the soil.
9. Seed Germination: Auxin promotes seed germination by breaking dormancy and stimulating the elongation of the embryonic axis.
10. Leaf Senescence: It delays leaf senescence by inhibiting the breakdown of chlorophyll and other cellular components, prolonging leaf lifespan.

Auxin is a vital plant hormone which regulates growth and development processes in plants. It plays an important role in cell elongation, root initiation, and apical dominance. Auxin promotes phototropism by stimulating cell elongation on the shaded side of plant stems, leading to bending towards light. It also influences gravitropism, directing root growth downward and stem growth upward. Additionally, auxin is involved in tropic responses like geotropism and thigmotropism. Its distribution within plant tissues is tightly regulated through polar transport which ensures control over plant growth and responses to environmental stimuli.

Also Read:

• Difference between Annual and Perennial Plants
• Physiological Effects Of Plant Growth Regulators
• Plant Growth and Development

What is Auxin?

Auxin is a plant hormone that regulates plant growth and development in different environmental conditions. 

Where is Auxin in a Plant?

Auxin is a plant hormone that is produced in the tips of growing roots and stems, also known as apical meristems. It can also be produced in other areas of the plant, including: Differentiating tracheary elements, Root cap cells, Nutritive tissues in developing seeds and flowers, Senescent leaves, and Wounds.

Why is Auxin a Hormone?

Auxin is a plant hormone because it can stimulate different growth in response to light or gravity.

What are 3 Functions of Auxins?

Auxins regulate plant growth, promote phototropism, and stimulate root initiation.

Who Discovered Auxin?

The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. 

What is an Example of an Auxin?

Indole-3-acetic acid (IAA) is a prominent natural auxin found in plants, regulating various growth and developmental processes.

How to Use Auxin?

Auxin can be applied topically or via root drench to promote root formation in plant cuttings.