Problem 7 - Campbell Biology

Question

A plant mutant that shows normal gravitropic bending but does not storch in its plastids would require a reevaluationof the role of __________________ in gravitropism. \begin{equation}\begin{array}{l}{\text { (A) auxin }} \\ {\text { (B) calcium }} \\ {\text { (C) statoliths }} \\ {\text { (D) differential growth }}\end{array}\end{equation}

Step-by-Step Solution

Answer

The role of statoliths in gravitropism.

Step 1: Understanding gravitropism

Gravitropism is the growth response of a plant to gravity. Normally, roots show positive gravitropism by growing downward, while shoots show negative gravitropism by growing upward.

Step 2: Identify the normal process

In a typical plant, plastids (organelles within plant cells) contain starch granules called statoliths. These statoliths settle under the influence of gravity and help the plant determine the direction of growth.

Step 3: Evaluate the mutant plant

The mutant plant in question does not have starch in its plastids but still shows normal gravitropic bending. This suggests that the absence of starch in plastids (and thereby, statoliths) does not hinder gravitropism.

Step 4: Determine the role to reevaluate

Since the mutant plant still undergoes gravitropic bending without statoliths, the role of statoliths in gravitropism must be reevaluated.

Key Concepts

Auxin

Gravitropism is deeply connected with the hormone called auxin. Auxin is crucial for plant growth and development, influencing processes such as cell elongation, root growth, and the plant's response to gravity.
In gravitropism, auxin distribution within the plant causes differential growth. For instance, in roots, the higher concentration of auxin on the lower side (due to gravity) slows down growth on that side, making the root bend downwards. In shoots, the opposite happens: higher auxin concentration on the lower side speeds up growth, causing the shoot to bend upwards.
This hormone plays a pivotal role in ensuring the plant grows in the right direction, helping roots seek out nutrients and water while shoots reach for sunlight.

Calcium

Calcium is another essential player in the plant's response to gravity. It acts as a signaling molecule that helps convey the direction of gravitational pull to the plant cells.
When a plant is tilted, calcium ions accumulate on the lower side of the cells due to gravity. These ions interact with other cellular components to trigger differential growth rates on either side of the plant part.
This mechanism ensures that the plant can adjust its growth directions accurately, even if it's growing on a slant or encountering an obstacle.

Statoliths

Statoliths are specialized starch-filled organelles within plant cells, usually found in plastids such as amyloplasts. They play a key role in sensing gravity.
Under the influence of gravity, statoliths settle to the lower part of the cell, providing a directional signal to the plant about the direction of gravity. This settling process triggers a cascade of cellular events that lead to a reorientation of growth.
However, as mentioned in the original exercise and solution, the mutant plant without starch in its plastids still shows proper gravitropic bending. This suggests that while statoliths are important, they might not be the sole contributors to the plant's ability to sense and respond to gravity.

Differential Growth

Differential growth is a fundamental aspect of how plants respond to environmental stimuli, including gravity. It refers to the different rates of cell elongation on opposite sides of a plant organ, resulting in bending towards or away from the stimulus.
In the context of gravitropism, differential growth ensures that roots grow downwards (positive gravitropism) and shoots grow upwards (negative gravitropism). This is primarily driven by the uneven distribution of growth hormones like auxin.
The process involves:

• A gravitational stimulus that causes redistribution of growth hormones.
• Cells on one side of the plant organ grow faster or slower than those on the opposite side.
• The plant organ bends toward or away from the direction of gravity.

Understanding differential growth is key to comprehending how plants adapt their growth to their environment efficiently.