6.1.3 Indoleacetic Acid (IAA)

Production and Function of IAA

• Produced in: The tips of shoots and roots.
• Role: Controls cell elongation in response to environmental stimuli like light (phototropism) and gravity (gravitropism).

Mechanism of IAA Action

1. Uneven Distribution:

• IAA is redistributed in response to stimuli.
• Uneven IAA distribution causes uneven cell growth, leading to the plant's response (e.g., bending towards light or downwards into soil).

2. Effects of IAA:

• In Shoots: IAA promotes elongation of cells.
• In Roots: IAA inhibits elongation, causing differential growth.

Examples of IAA-Driven Tropisms

1. Phototropism:

• IAA accumulates on the shaded side of the shoot.
• Cells elongate faster on the shaded side, causing the shoot to bend towards the light.

2. Gravitropism:

• In roots, IAA accumulates on the lower side due to gravity.
• Growth is inhibited on the lower side, so the root bends downwards.

Key Facts

• IAA moves via diffusion and active transport to areas where it is needed.
• The direction of IAA redistribution depends on the type of stimulus.