Question

The specialized plastids - serving as gravity sensors, that direct root growth downward into the soil, are

• A. Chromoplasts
• B. Chloroplasts
• C. Gerontoplasts
• D. Amyloplasts

Hint:

The starch-statolith hypothesis explains how plants sense gravity. Starch-filled amyloplasts sink to the bottom of statocyte cells, directing root growth downward.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Gravitropism is the coordinated process of plant growth in response to gravity.
Roots exhibit positive gravitropism (growing downward), which is mediated by specialized cells that can sense gravitational pull.

Step 2: Detailed Explanation:
- Gravity sensing occurs in specialized cells called statocytes located in the center of the root cap (the columella).
- Statocytes contain heavy, starch-filled organelles called amyloplasts (specifically referred to as statoliths in the context of gravitropism).
- Because of their high density relative to the surrounding cytoplasm, amyloplasts sediment to the lowest part of the cell in response to gravity.
- This sedimentation exerts physical pressure on the endoplasmic reticulum and cytoskeleton, triggering a signaling cascade that redistributes the plant hormone auxin.
- The resulting asymmetric distribution of auxin inhibits cell elongation on the lower side of the root, causing it to curve downward.
- Other plastids like chloroplasts (photosynthetic), chromoplasts (pigmented), and gerontoplasts (decaying plastids) do not contain dense starch granules and do not function in gravity sensing.

Step 3: Final Answer:
The gravity-sensing plastids that direct downward root growth are amyloplasts.