Hormones- AUXIN

What is the main function of auxins?

Promote cell elongation.

What did Charles Darwin study in seedlings?

How seedlings bend toward light (phototropism).

What did Darwin conclude about the seedling tip?

It detects light and sends an “influence” (auxin) to the region below it.

Where does bending occur in a seedling?

In the region below the tip, where cells elongate.

What is phototropism?

Directional plant growth in response to light.

Which side of a shoot accumulates more auxin during phototropism?

The shaded side.

Naturally occurring auxins

• IAA – indole-3-acetic acid; the main and most widely studied natural auxin.

• 4-chloroindole-3-acetic acid

• Phenylacetic acid (PAA)

• Indole-3-butyric acid (IBA)

• Indole-3-propionic acid (IPA)

Where are auxins synthesized?

• Shoot apical meristems: Growing tips of shoots.

• Young leaves: Developing leaves.

• Root apical meristems: Growing root tips, but generally in smaller amounts.

Synthesized auxins and what they are used for:

2, 4-D, NAA

→ widely used as herbicide

→ employed to induce the formation of adventitious roots in cuttings

Influx of auxin process (how auxin IAA enters plants)

1. The apoplast (outside the cell) is relatively acidic, around pH 5.

2. Some IAA becomes protonated, forming uncharged IAAH.

3. IAAH crosses the plasma membrane by passive diffusion.

4. Inside the cytoplasm (around pH 7), IAAH loses a proton (H⁺) and becomes charged IAA⁻.

5. Because IAA⁻ is charged, it cannot easily cross the membrane by simple diffusion.

IAAH = uncharged → can diffuse IN.

IAA⁻ = charged → needs a transporter to get IN.

Efflux of auxin process (how auxin IAA leaves plants through proteins, and the location of these proteins determines the direction in which auxin moves)

Auxin regulates lateral root formation, where is it initiated?

Lateral roots are initiated in the pericycle.

What does auxin produced by developing embryos promote?

Maturation of the ovary wall and the development of freshy fruits eg strawberries.