Auxins

What is a plant growth factor (plant hormone)?

• A chemical messenger produced by the plant (in low concentrations) that controls and coordinates growth, development, and responses to stimuli.

What is Auxin and where is it produced?

• What: A key plant growth factor (main example is IAA) that primarily controls cell elongation.

• Where: Produced in the apical meristem (growing tips) of shoots and roots.

What is the "acid growth hypothesis"?

• The accepted mechanism explaining how auxin promotes cell elongation. It states that auxin makes the cell wall acidic, which "loosens" it, allowing the cell to expand.

How does auxin cause cell elongation?

1. Auxins are synthesised by the meristem cells in shoot tips.

2. Auxins diffuse down the shoot away from the tip.

3. Auxins bind to receptor sites on cell-surface membranes.

4. A low pH develops in cell walls.

5. The cells absorb water by osmosis, forming vacuoles and increasing the internal pressure, causing the cell walls to expand.

6. Cells elongate and the plant grows.

What is apical dominance?

• The phenomenon where the growth of the central, apical bud (main stem) is strong, while the growth of the lateral (side) buds is inhibited.

How does auxin control apical dominance?

• The apical bud produces a high concentration of auxin, which diffuses down the stem and inhibits the growth of the lateral buds. (If the tip is cut off, the inhibition stops).

How does auxin control phototropism in shoots?

• Auxin moves to the shaded side of the shoot. This high concentration stimulates cell elongation on the shaded side, causing the shoot to bend towards the light.

How does auxin control geotropism in roots?

• Auxin accumulates on the lower side due to gravity. In roots, a high concentration INHIBITS cell elongation, so the top side grows faster, causing the root to bend down.

How are auxins used as hormonal weedkillers?

• They are selective. They are absorbed by broad-leaved plants (dicot weeds), causing them to grow uncontrollably fast, which disrupts their metabolism and kills them. Monocot crops (like wheat) are not affected.

How are auxins used in rooting powders?

• A cutting (a piece of stem) is dipped in the powder. The auxin stimulates the growth of new roots (adventitious roots) from the cut stem, helping the cutting to propagate.