Auxins: Plant Hormones for Growth and Cell Elongation

Auxins are plant hormones that mainly promote cell elongation in shoots. The most common natural auxin is indole-3-acetic acid, called IAA. It is made in shoot tips and young leaves and moves downward to growing regions.

Auxins matter because they control bending towards light, root initiation, fruit development, and prevention of leaf fall. Farmers and horticulturists use synthetic auxins as rooting powders and even as selective weed killers.

Key Concepts

Auxin: A growth promoting hormone that increases cell wall extensibility and elongation.

IAA: Indole-3-acetic acid, the principal natural auxin in plants.

Phototropism: Bending of shoot towards light, caused by unequal auxin distribution.

Apical dominance: The shoot tip suppresses growth of lateral buds through auxin.

Synthetic auxins: Man made compounds like NAA, IBA, and 2,4-D that mimic natural auxin action.

Units for hormone concentration in experiments are usually milligrams per litre or micromoles per litre.

How It Works

1. Auxin is synthesised in the shoot apex and young expanding leaves.
2. It is transported in a polar manner, from tip towards base, cell to cell.
3. When light comes from one side, auxin moves to the shaded side of the stem.
4. More auxin on the shaded side causes faster cell elongation there.
5. The shoot therefore bends towards the light source.
6. In roots, the same hormone at very low concentration promotes growth, but high concentration inhibits elongation.

Worked Example

A coleoptile is 20 millimetres long. The illuminated side grows to 24 millimetres in 6 hours, while the shaded side with more auxin grows to 32 millimetres. Difference in growth is 32 minus 24 equals 8 millimetres. Average elongation rate on the shaded side is increase 12 millimetres divided by 6 hours equals 2 millimetres per hour. On the lit side it is 4 millimetres divided by 6 hours equals 0.67 millimetres per hour. Unequal rates explain bending without any new cells being added on only one side.

Common Misconceptions

• Auxin always promotes growth everywhere. In roots high auxin inhibits elongation, which is opposite to shoots.
• Auxin is produced in mature leaves. Main sites are shoot tips, young leaves, and developing seeds.
• Synthetic auxins are safe at any dose. At high dose, 2,4-D kills broad leaved weeds by causing uncontrolled growth.

Real-World Applications

IBA and NAA are used as rooting hormones for stem cuttings in nurseries. Auxins help produce parthenocarpic fruits without seeds, such as in tomato under certain conditions. They delay fruit and leaf drop by reducing abscission. Synthetic auxins serve as selective herbicides in cereal fields.

Auxins link light signals to growth by redistributing themselves and changing cell elongation rates. This simple mechanism controls plant shape, rooting, and fruit retention in both nature and agriculture.