Promotion of Cell Growth by Auxin
- Auxin is a key phytohormone that regulates plant growth and development.
- One of its most important roles is in promoting cell elongation, which is essential for processes like
- phototropism (growth toward light),
- gravitropism (growth in response to gravity),
- apical dominance (suppression of lateral growth to favor upward growth).
- Auxin promotes cell elongation by acidifying the cell wall, loosening its structure, and allowing cells to expand under turgor pressure.
- Many students mistakenly think auxin directly pushes the plant toward light.
- In reality, it increases growth on one side, causing the bending effect.
How Auxin Promotes Cell Growth
- Auxin triggers hydrogen ion (H⁺) secretion into the apoplast (the space outside the cell membrane but within the cell wall).
- This acidifies the cell wall, weakening the cross-links between cellulose molecules.
- The loosened cell wall allows the cell to expand under turgor pressure.
This process is known as the acid growth hypothesis, a key mechanism in plant biology.
Step-by-Step: How Auxin Works
- Auxin Activation: Auxin binds to receptors on the plasma membrane, activating proton pumps.
- Proton Pumping: These pumps transport H⁺ ions from the cytoplasm into the apoplast, lowering the pH of the cell wall.
- Cell Wall Loosening: The acidic environment activates expansins, enzymes that break the bonds between cellulose fibers, making the wall more flexible.
- Cell Elongation: The cell absorbs water, and the loosened wall stretches, allowing the cell to grow.
Remember: auxin doesn’t directly cause growth. It creates the conditions for growth by modifying the cell wall.
Why Acidification Matters
- The acidic environment created by auxin is crucial because it activates enzymes like expansins.
- Without this step, the cellulose microfibrils in the cell wall would remain rigid, preventing expansion.
- Imagine pouring water into a balloon.
- If the balloon is stiff, it won’t expand.
- But if it’s flexible, it will stretch and grow.
- Auxin makes the cell wall flexible, like a stretchy balloon.
