Auxin and Tropisms

Auxin (Indole-3-Acetic Acid, IAA) is a plant hormone produced at the growing tips (apical meristems) of shoots.

Its job: promote cell elongation — cells with more auxin grow longer.

Key rule: Auxin moves away from light (to the shaded side).

When light hits a shoot from one side:

1. Auxin is produced at the tip
2. Auxin migrates to the shaded side
3. Cells on the shaded side elongate MORE
4. Result: shoot bends toward the light

This is phototropism — growth movement in response to light.

Darwin's experiment (1880): Charles Darwin showed that the tip of the shoot detects light, not the rest of the stem. When he covered the tip, the plant didn't bend. When he covered only the base, it still bent normally.

Went's experiment (1926): Frits Went isolated auxin by placing a cut tip on an agar block, then putting that block on one side of a decapitated shoot — it bent toward the opposite side, proving a chemical was causing growth.

Root and shoot respond oppositely to gravity:

Roots are far more sensitive to auxin — even small concentrations inhibit their growth, causing roots to grow downward.

Auxin moves only in one direction: from tip downward (basipetally). It cannot travel upward. This is called polar auxin transport and is driven by carrier proteins on cell membranes.

The accepted explanation for tropisms:

1. Stimulus (light or gravity) causes uneven distribution of auxin
2. One side gets more auxin than the other
3. Differential growth causes bending

• Predict bend direction from auxin gradient: high auxin = more growth on THAT side
• Root vs shoot response: roots are 10× more sensitive — same auxin level promotes shoots, inhibits roots
• Darwin vs Went — Darwin showed the tip detects, Went isolated the chemical
• Polar transport is basipetal (tip → base), not acropetal
• Auxin = IAA chemically; produced at shoot apices and young leaves

A shoot is placed horizontally. Gravity causes auxin to accumulate on the lower side.

• Lower side of shoot → elongates → shoot curves upward ✓
• Lower side of root → inhibited → root curves downward ✓