How does the effect of a high IAA concentration differ between shoot cells and root cells?

In shoot cells, a high concentration of IAA stimulates cell elongation, leading to faster growth on that side. Conversely, in root cells, a high concentration of IAA inhibits cell elongation, causing that side to grow more slowly.

When comparing phototropism and gravitropism, how does the redistribution of IAA differ?

In phototropism, IAA redistributes toward the shaded side of the organ in response to light. In gravitropism, IAA redistributes toward the lower side of the organ due to the influence of gravity and the sinking of amyloplasts.

What is the difference between the transport of IAA over short distances versus long distances?

Short-distance transport occurs via diffusion and active transport between adjacent cells. Long-distance transport is achieved through the phloem, which moves the growth factor throughout the plant body.

What is a common misconception regarding the effect of IAA on cell numbers?

A common error is stating that IAA increases the rate of cell division (mitosis). In reality, IAA primarily influences the rate of cell elongation, increasing the size of individual cells rather than their quantity.

Why might a student incorrectly predict that a root will bend toward light if IAA accumulates on the shaded side?

The student may be applying the 'shoot rule' where IAA stimulates growth. They must remember that in roots, IAA inhibits elongation, so the shaded side (with more IAA) grows slower, causing the root to bend away from light.

What error occurs if an experimenter fails to include a 'tip-removed' group when studying IAA?

Without a tip-removed control, the experimenter cannot conclude that the growth response is specifically caused by a factor produced in the tip. It is necessary to prove the tip is the source of the stimulus-response mechanism.

Define Indoleacetic Acid (IAA) and identify its chemical group.

IAA is a plant growth factor that regulates cell elongation. It belongs to a group of growth factors known as auxins.

What are amyloplasts and what is their role in plant growth?

Amyloplasts are specialized organelles containing starch grains that settle at the bottom of cells due to gravity. Their movement triggers the redistribution of IAA to the lower side of the tissue, facilitating gravitropism.

Where is IAA primarily synthesized in a flowering plant?

IAA is synthesized in the growing tips of roots and shoots, known as the apical meristems, before being transported to other regions.

How does the concentration of IAA relate to the rate of growth in a plant tissue?

The concentration of IAA determines the rate of cell elongation. An uneven concentration across a tissue results in uneven growth rates, which causes the organ to bend.

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Indoleacetic Acid (IAA)

Summary

Indoleacetic Acid (IAA) is a fundamental plant growth factor belonging to the auxin family. It coordinates complex growth responses by regulating the rate of cell elongation, exhibiting distinct, opposite effects in shoot and root tissues to facilitate directional movement toward or away from environmental stimuli.

1. Definition & Core Concepts

Indoleacetic Acid (IAA) is a naturally occurring chemical messenger in plants classified as an auxin, which is a type of plant growth factor similar in function to animal hormones.
It is primarily synthesized in the growing tips of plant organs, specifically the apical meristems of shoots and roots, where it initiates developmental signals.
Unlike animal hormones that often travel through complex circulatory systems, IAA acts locally in the regions adjacent to the tip or is transported to other tissues to regulate growth.

2. Transport Mechanisms

IAA moves over short distances through diffusion and active transport between individual cells, allowing for the creation of precise concentration gradients.
For long-distance translocation throughout the plant body, IAA is transported via the phloem, which allows it to reach mature tissues far from the site of synthesis.
The ability of IAA to redistribute unevenly across a tissue is the mechanical basis for all tropic growth responses, as it creates differential growth rates on opposite sides of an organ.

3. Mechanism of Action: Cell Elongation

The primary function of IAA is to control the elongation of plant cells rather than increasing the rate of cell division (mitosis).
In shoots, a higher concentration of IAA stimulates faster cell elongation, causing the side with more IAA to grow longer and push the shoot in the opposite direction.
In roots, the relationship is inverted; a higher concentration of IAA inhibits cell elongation, meaning cells with more IAA grow more slowly than those with lower concentrations.

Diagram showing how high IAA concentrations lead to bending in shoots (stimulation) versus roots (inhibition).

4. Tropic Responses

5. Key Distinctions

6. Exam Strategy & Tips