Plant hormones: How IAA, the most common form of auxin, works

Summary

This video explores the role of plant hormones, specifically indole-3-acetic acid (IAA), in regulating plant growth. It explains how IAA is produced at the tip of the stem and redistributed to the shaded side when exposed to unilateral light, causing cells to elongate and the shoot tip to bend towards the light. Experimental evidence, such as Darwin's experiments with oat coleoptiles and the work of scientists like Peter Boon Jensen, further supports the chemical nature of this process. These studies help uncover the mechanisms behind hormone distribution and the bending response to light in plants.

Takeaways

• 😀 Plant hormones, also known as plant growth factors, are chemical substances that affect growth by accelerating or inhibiting processes.
• 😀 Indole-3-acetic acid (IAA) is the most common form of auxin, a plant hormone involved in growth regulation.
• 😀 IAA is produced at the apex (tip) of the plant stem and is redistributed to the shaded side when exposed to unilateral light.
• 😀 A gradient of IAA across the stem causes cell elongation on the shaded side, leading the shoot tip to bend towards the light.
• 😀 The process of IAA movement within the plant is driven by diffusion between cells.
• 😀 IAA activates proton pumps that lower the pH, triggering enzymes called expansins to break bonds in the cell wall and allow water to enter.
• 😀 The increase in cell turgor pressure from water intake causes the cells on the shaded side to elongate.
• 😀 Charles Darwin's experiments with oat coleoptiles demonstrated that the tip of the plant must be stimulated by light for bending to occur.
• 😀 Peter Boysen-Jensen's experiment using a mineral mica flake showed that the bending response occurs only when the shaded side receives the message to bend.
• 😀 The discovery that auxin acts chemically (not electrically) was confirmed when mica blocked chemical movement but not electrical signals.
• 😀 Further research by Fritz Went confirmed that hormone distribution, even in the dark, causes the plant to bend toward the light when placed in agar blocks with IAA.
• 😀 Experiments by Winthrop Briggs demonstrated that a glass plate could block the movement of auxins and prevent bending, showing that auxins need to travel to induce the response.

Q & A

What are plant hormones, and what role do they play in plant growth?

Plant hormones, also known as plant growth factors, are chemical substances that accelerate, inhibit, or otherwise affect plant growth. They are crucial in regulating various physiological processes in plants, such as growth, development, and responses to environmental stimuli.

What is indole-3-acetic acid (IAA), and how does it function in plants?

Indole-3-acetic acid (IAA) is the most common form of auxin, a type of plant hormone. It is produced at the apex (tip) of the stem and is responsible for promoting elongation in plant cells, particularly on the shaded side of the stem when exposed to unilateral light.

How does light influence the distribution of IAA in plants?

When unilateral light shines on one side of the plant, IAA is redistributed towards the shaded side, creating a gradient of the hormone. This gradient is essential for the differential growth of cells, causing the stem to bend towards the light.

What mechanism allows IAA to move between cells in the plant?

IAA moves between plant cells by diffusion. This process is influenced by various factors, including the activation of proton pumps that lower the pH, which in turn activates enzymes called expansins that help loosen the cell wall.

What effect does IAA have on plant cell walls?

IAA activates proton pumps that lower the pH, which in turn activates expansins. These enzymes break bonds in the cell wall, loosening it and allowing water to enter the cell, which causes the cell to expand and increase turgor pressure.

Why do plants bend towards light in response to IAA?

Plants bend towards light due to the elongation of cells on the shaded side of the stem. This is caused by a higher concentration of IAA on the shaded side, which promotes cell expansion and increases turgor pressure, resulting in the bending of the shoot tip towards the light.

What was Darwin’s experiment to investigate plant bending in response to light?

Darwin's experiment involved using oat coleoptiles (shoots) to study plant bending in response to light. He found that when the tip of the coleoptile was removed, the plant did not bend towards the light, suggesting that the tip was the origin of the signal. He further showed that the tip must be stimulated by light to initiate the bending response.

What was Peter Boon Jensen's contribution to the understanding of plant hormone movement?

Peter Boon Jensen used a thin flake of mica, a mineral that blocks chemical movement, to show that when it was inserted on the shaded side of the coleoptile, there was no bending. This experiment demonstrated that the hormone’s message to bend travels down the shaded side, and that the message is chemical in nature.

How did Fritz Wendt contribute to the understanding of auxin distribution in plants?

Fritz Wendt's experiment involved removing the coleoptile tip and incubating it on a block of agar. The agar absorbed the hormone, which then traveled down the plant, causing bending. This experiment showed that the amount of bending was proportional to the amount of hormone in the agar, providing further evidence for the role of auxin in plant growth.

What role did Windslow Briggs' experiment with a glass plate play in understanding auxin movement?

Windslow Briggs inserted a thin glass plate vertically through the tip of the coleoptile. The glass did not prevent light transmission, but it prevented the movement of auxin. The experiment showed that auxin cannot travel through glass and that its concentration remains equal on both sides of the plant when blocked by the glass. This demonstrated the importance of a gradient of auxin for the bending response.