A plant mutant that shows normal gravitropic bending but does not store starch in its plastids would require a reevaluation of the role of
a. Statoliths
b. Auxin
c. Calcium
d. Differential growth

Verified step by step guidance

1. Understand the concept of gravitropism: Gravitropism is the orientation or growth of a plant in response to gravity. Typically, roots grow downward (positive gravitropism) and shoots grow upward (negative gravitropism).
2. Identify the role of statoliths: Statoliths are starch-filled plastids that are thought to play a key role in sensing gravity in plant cells. They settle at the bottom of the cell, helping the plant sense the direction of gravity.
3. Consider the mutant plant's characteristics: The mutant plant shows normal gravitropic bending but does not store starch in its plastids. This suggests that statoliths, which are typically starch-filled, may not be essential for gravitropism in this case.
4. Evaluate the role of other factors: Since the mutant plant can still perform gravitropic bending without starch storage, other factors like auxin, calcium, or differential growth might be involved in the gravitropic response.
5. Reassess the role of statoliths: Given the mutant's ability to bend normally without starch, the role of statoliths in gravitropism may need to be reevaluated, indicating that other mechanisms might compensate for the lack of starch storage.

Key Concepts

Gravitropism

Gravitropism is the orientation or growth of a plant in response to gravity. Roots typically exhibit positive gravitropism, growing downward, while shoots show negative gravitropism, growing upward. This process involves the perception of gravity, signal transduction, and differential growth, often mediated by plant hormones like auxin.

Statoliths

Statoliths are specialized starch-filled organelles found in plant cells, particularly in the root cap, that play a crucial role in gravity perception. They settle under the influence of gravity, helping the plant sense its orientation. The absence of starch in plastids, as in the mutant plant, suggests a need to reassess the role of statoliths in gravitropism.

Auxin

Auxin is a plant hormone that regulates growth and plays a pivotal role in gravitropism by promoting differential growth. It redistributes in response to gravity, causing cells on one side of the plant to elongate more than those on the opposite side, leading to bending. The mutant's normal gravitropic response without starch storage suggests auxin's role might be independent of statoliths.