IAA and plant cell elongation

Indoleacetic acid (IAA) is a plant growth factor produced in growing regions of a plant. It is transported to other tissues, where it can affect how much cells elongate. As cells elongate, they become longer, contributing to the growth of the tissue.

IAA concentration and tissue response

The effect of IAA depends on both its concentration and the tissue responding. Roots are more sensitive to IAA than shoots: a concentration that promotes cell elongation in a shoot can inhibit cell elongation in a root.

IAA and bending

If IAA is distributed unevenly across a growing organ, cells on its two sides may elongate at different rates. The side that elongates more grows faster, causing the organ to bend.

For example, where a higher IAA concentration promotes elongation on one side of a shoot, that side grows faster and the shoot bends towards the other side. In a root, a higher concentration can inhibit elongation on one side. The opposite side then grows faster, bending the root towards the side with more IAA.

Phototropism

Plants can change their direction of growth in response to light. This response is called phototropism. Growth towards light is positive phototropism; growth away from light is negative phototropism.

Phototropism in shoots

When light reaches a shoot from one side, the plant growth substance IAA becomes unevenly distributed, with more IAA on the shaded side. In a shoot, a higher IAA concentration promotes greater cell elongation. Cells on the shaded side therefore elongate more than those on the illuminated side.

As the shaded side grows faster, the shoot bends towards the light. This is positive phototropism.

Phototropism in roots

Roots can show negative phototropism, growing away from light. In a simple IAA-based explanation, more IAA on the shaded side inhibits cell elongation there relative to the illuminated side. The illuminated side therefore elongates more, causing the root to bend away from the light.

The same uneven distribution of IAA can thus produce opposite bending responses because the higher IAA concentration promotes elongation in shoots but inhibits it in roots.

Gravitropism

Gravitropism is a directional growth response to gravity. Roots and shoots respond in opposite directions: roots grow towards gravity, while shoots grow away from it.

When a root or shoot is placed horizontally, the plant hormone IAA becomes more concentrated on its lower side. This creates unequal growth between the lower and upper sides. The difference in elongation bends the root or shoot, but the direction of the bend depends on how its cells respond to IAA.

Gravitropism in shoots

In a shoot, the higher IAA concentration promotes greater cell elongation on the lower side. The lower side grows longer than the upper side, bending the shoot upwards, away from gravity. Growth away from gravity is called negative gravitropism.

Gravitropism in roots

In a root, the higher IAA concentration inhibits cell elongation on the lower side. The upper side elongates more than the lower side, bending the root downwards, towards gravity. Growth towards gravity is called positive gravitropism.

Taxis

A mobile organism may respond to a stimulus by moving in a particular direction relative to it. This directional movement response is called a taxis. The direction matters: the organism moves either towards the stimulus or away from it.

In positive taxis, the organism moves towards the stimulus. In negative taxis, it moves away from the stimulus.

Taxis can help an organism remain in an environment suited to its survival. Moving towards a stimulus associated with favourable conditions can bring it into those conditions; moving away from a stimulus associated with unfavourable conditions can help it avoid them.

Kinesis

A mobile organism can respond to its surroundings by changing how actively it moves. In a kinesis, the organism’s movement activity changes with the intensity of a stimulus, but its movement is not directed towards or away from the stimulus.

For example, the organism may move faster or slower, or change how often it turns, as conditions change. These changes affect how long it spends in a particular area.

If an organism moves faster in unfavourable conditions, it is likely to leave that area sooner. If it moves more slowly when it reaches favourable conditions, it is likely to spend longer there. The organism does not need to move directly towards a favourable area for this response to make it more likely to remain in one.

Three-neurone reflex arc

A reflex is a rapid, automatic response to a stimulus. It does not require a conscious decision before it occurs and can protect an organism from harm. For example, touching a hot object can cause the hand to pull away.

A three-neurone reflex arc is the pathway through which nerve impulses pass to produce the response. It contains a sensory neurone, a relay neurone and a motor neurone.

1. A receptor detects a stimulus, such as heat, and a nerve impulse is produced.
2. A sensory neurone carries the impulse from the receptor to the central nervous system, which consists of the brain and spinal cord.
3. In the central nervous system, the sensory neurone communicates with a relay neurone. The relay neurone then communicates with a motor neurone.
4. The motor neurone carries an impulse to an effector, such as a muscle.
5. The effector produces the response. For example, a muscle contracts to pull the hand away.

The connections between the neurones are called synapses. The impulse passes from the sensory neurone to the relay neurone, and then from the relay neurone to the motor neurone, across these connections.

Investigating animal movement

An animal’s movement can be investigated by changing one environmental variable and recording how the animal moves or where it gathers. A choice chamber provides regions with different conditions at the same time. A maze provides a route through which movement can be observed under controlled conditions.

Setting up the investigation

In a choice chamber, arrange the regions so that they differ in the variable being investigated. For example, one region could be shaded and another illuminated to investigate the response to light. Keep other relevant conditions, such as temperature and humidity, as similar as possible. Otherwise, a difference in distribution might be caused by a factor other than light.

In a maze, conditions can be varied between trials. For example, the same maze could be used under different light conditions. Keep the maze layout, starting position and observation time consistent so that movement can be compared between trials. Other relevant conditions should also be controlled as far as possible.

Recording and interpreting movement

Use the same method and timing for each observation. In a choice chamber, count the animals in each region at set times. In a maze, record a measure of movement, such as the distance travelled in a set time. Repeat observations or trials to look for a consistent pattern and to see how much the results vary.

Interpret the evidence according to what was measured. For example, finding more animals in the shaded region shows a difference in distribution, but it does not by itself show why the animals gathered there. They may have moved towards the shaded region, or they may have moved less once they reached it. Direct observations of movement would be needed to distinguish between these possibilities.

Handle animals appropriately and use conditions that avoid unnecessary harm.