Auxin is a plant growth hormone responsible for directional growth, cell elongation, and bending movements in plants. It plays a central role in processes such as phototropism (growth toward light), apical dominance, root formation, vascular differentiation, and fruit development. Without auxin, plants would not grow in an organized, directional manner.
Auxin is primarily produced in the shoot apex (growing tips), young leaves, and developing seeds. From these regions, it moves downward through plant tissues in a controlled pattern. One of the most fascinating features of auxin is its unequal distribution in response to environmental stimuli like light and gravity. This uneven concentration causes one side of the plant to grow faster than the other, resulting in bending.
In shoots, auxin promotes cell elongation. When light falls on one side of a plant, auxin shifts toward the shaded side. The shaded side grows more, causing the plant to bend toward the light—this is called positive phototropism. In roots, however, higher auxin concentration inhibits elongation, leading to different growth responses.
Understanding auxin is essential for exams, plant physiology concepts, agriculture, and horticulture. This guide explains auxin structure, mechanism, functions, distribution, phototropism, experiments, applications, comparisons with other hormones, and frequently asked questions.
Definition of Auxin
Auxin is a plant growth regulator that controls cell elongation, apical dominance, root initiation, and directional growth responses.
The most common natural auxin is:
Indole-3-acetic acid (IAA)Synthetic auxins include:
- NAA (Naphthalene acetic acid)
- 2,4-D (used as herbicide)
Discovery of Auxin (Historical Background)
The discovery of auxin began with experiments on phototropism:
- Charles Darwin (1880) observed that coleoptiles bend toward light.
- He proposed that a signal from the tip controls bending.
- Later scientists isolated and identified auxin as the chemical responsible.
This was one of the earliest discoveries in plant hormone research.
Phototropism: How Auxin Causes Bending
What Is Phototropism?
Phototropism is the growth of a plant part in response to light.
Positive phototropism: Growth toward light (shoots)Step-by-Step Mechanism in Shoots
1. Light strikes one side of the stem.This unequal growth creates curvature.
Auxin Distribution
| Side of Stem | Auxin Level | Growth Rate |
|---|---|---|
| Light Side | Less auxin | Less elongation |
| Dark Side | More auxin | More elongation |
Auxin and Cell Elongation Mechanism
Auxin promotes elongation by:
- Activating proton pumps in the cell membrane
- Lowering pH in cell wall (acid growth hypothesis)
- Loosening cell wall structure
- Allowing water entry
- Increasing cell length
This process is called the Acid Growth Theory.
Auxin in Roots vs Shoots
Auxin behaves differently in roots and shoots.
| Organ | Effect of High Auxin |
|---|---|
| Shoot | Stimulates elongation |
| Root | Inhibits elongation |
This explains why:
- Shoots bend toward light
- Roots often show negative phototropism
Roots are more sensitive to auxin than shoots.
Major Functions of Auxin
1. Apical Dominance
Auxin suppresses lateral bud growth, maintaining dominance of the main shoot.
2. Root Initiation
Low concentration promotes adventitious root formation.
3. Vascular Differentiation
Auxin helps in xylem and phloem formation.
4. Fruit Development
Auxin triggers fruit growth even without fertilization (parthenocarpy).
5. Tropic Movements
- Phototropism
- Geotropism
- Thigmotropism
Auxin Transport
Auxin movement is:
- Polar (from apex downward)
- Energy-dependent
- Through specialized carrier proteins
This directional transport ensures controlled plant architecture.
Auxin vs Other Plant Hormones
| Hormone | Main Function |
|---|---|
| Auxin | Growth & elongation |
| Gibberellin | Stem elongation & seed germination |
| Cytokinin | Cell division |
| Ethylene | Fruit ripening |
| Abscisic Acid | Stress response |
Auxin works in coordination with these hormones.
Practical Applications of Auxin
Agriculture
- Rooting hormone powders
- Weed control (2,4-D)
- Fruit enlargement
Horticulture
- Plant propagation
- Bonsai shaping
- Tissue culture
Commercial Use
- Parthenocarpic fruit production
- Crop yield management
Experiments Demonstrating Auxin
Darwin’s Experiment
Covered coleoptile tips did not bend toward light.
Went’s Experiment
Agar block soaked in auxin caused bending when placed asymmetrically.
These experiments confirmed auxin as a growth hormone.
Common Misconceptions About Auxin
- Auxin does not always stimulate growth (in roots it inhibits).
- More auxin is not always better.
- Synthetic auxins can act as herbicides in high doses.
FAQs
What is auxin in simple words?
Auxin is a plant hormone that controls growth and bending movements in plants.
Where is auxin produced?
It is mainly produced in shoot tips and young leaves.
Why do plants bend toward light?
Auxin accumulates on the shaded side, causing more growth there.
What is positive phototropism?
Growth of shoots toward light.
Why do roots show negative phototropism?
Higher auxin concentration inhibits root elongation.
What is the acid growth theory?
Auxin loosens cell walls by lowering pH, allowing cells to elongate.
Is auxin used in agriculture?
Yes, for rooting, fruit growth, and weed control.
What happens if auxin transport is blocked?
Plants lose directional growth and proper development.
