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.
- Don’t assume auxin moves toward the light.
- It actually accumulates on the opposite side, causing the plant to bend toward the light.
How Does Auxin Cause Phototropic Bending?
- Lateral redistribution of auxin
- When light shines on one side of a shoot, auxin is transported laterally to the shaded side.
- This creates a higher auxin concentration on the shaded side compared to the illuminated side.
- Faster growth on the shaded side
- The higher auxin concentration on the shaded side enhances acid growth, leading to more cell elongation.
- The illuminated side has less auxin, so its cells grow more slowly.
- Curving toward light
- The unequal growth rates between the two sides cause the shoot to bend toward the light.
- This maximizes light absorption for photosynthesis, improving energy production for the plant.
Think of auxin as a traffic director. By redistributing itself, it tells cells on one side of the plant to grow faster, steering the shoot toward the light.
Why Auxin Matters
- Auxin is essential for:
- Phototropism: Ensuring shoots grow toward light for optimal photosynthesis.
- Gravitropism: Helping roots grow downward by redistributing auxin in response to gravity.
- Apical Dominance: Inhibiting the growth of side branches to prioritize upward growth.
- How does the ability of plants to respond to light and gravity reflect the interconnectedness of structure and function in biology?
- Can you think of parallels in animal systems?
- Explain how auxin causes a plant shoot to bend toward light.
- How does auxin trigger cell elongation in plant cells?
- What is the role of hydrogen ions (H⁺) in auxin-induced growth?
- Why does a plant shoot bend toward light when exposed to a unilateral light source?
