plant growth hormones

What is IAA?

A plant growth factor that controls plant cell elongation. A type of auxin

What are plant growth factors and how are they different to animal hormones?

Plant growth factors are chemicals produced in plants that influence their growth and development.

Plant hormones usually act locally at the site of production whereas animal hormones act on distant organs. Plant hormones are produced in a variety of tissues throughout the plant whereas animal hormones are produced in specialised cells within glands.

How does auxin control growth by cell elongation? (Acid growth hypothesis)

Auxin binds to cell surface membrane and activates proton pumps

Hydrogen ions are actively transported from cytoplasm into cell wall (now very acidic)

The cell wall becomes more plastic

Cells elongate and the plant grows

Explain what happens in phototrophic shoots?

Cells in the tip of the shoot produce IAA which is initially transported evenly down the shoot.

Light causes IAA to diffuse to the shaded side of the shoot.

A greater concentration of IAA on the shaded side causes cells to elongate more

The shaded side of the shoot elongates faster causing the shoot to bend towards the light

Explain what happens in the negative phototrophic roots?

Higher concentration of IAA inhibits cell elongation in roots

In roots the elongation of cells is greater on the light side than on the shaded side and so roots bend away from light

Explain what happens in gravitropic roots?

Cells in the tip of the root produce IAA which is initially transported along all sides of the root. As a result of gravity, a greater concentration of IAA builds up on the lower side of the root. IAA inhibits cell elongation on the lower side, the greater elongation on the upper side causes the roots to bend towards the gravity