66. The expression of, which one of the following types of cyclins in plants is modulated by growth
factors, such as cytokinins, auxins, brassinosteroids, sucrose and gibberelins?
1. A-Type
2. B- Type
3. D-Type
4. H-Type

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Correct Answer: 3. D-Type

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Explanation:

D-Type cyclins in plants are primarily responsible for controlling the transition from the G1 phase to the S phase of the cell cycle. They play a key role in integrating external growth signals into the regulation of the cell cycle, making them unique among cyclins.

Why D-Type Cyclins?

• Responsive to Growth Factors:
  D-Type cyclins are upregulated in response to cytokinins, auxins, gibberellins, brassinosteroids, and sucrose, all of which are key signaling molecules or nutrients that promote growth and division in plant cells.

• Function in Cell Cycle Progression:
  These cyclins form complexes with cyclin-dependent kinases (CDKs), especially CDKAs, to initiate DNA replication by progressing cells through the G1/S checkpoint.

• Environmental and Nutrient Sensitivity:
  Their expression is tightly linked to external environmental cues and internal developmental signals, making them essential for adaptive growth.

Other Cyclin Types in Brief:

• A-Type Cyclins: Involved in S phase and G2/M transitions but not primarily regulated by growth factors.

• B-Type Cyclins: Regulate mitosis (G2 to M transition), expressed in later stages of the cell cycle.

• H-Type Cyclins: Less common and not typically involved in growth factor responses.

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Conclusion:

D-Type cyclins are the key plant cyclins whose expression is modulated by various growth factors like auxins, cytokinins, gibberellins, and nutrients like sucrose. Their role is vital for linking external signals with internal cell cycle control