tr interconversion is regulated by

Options

A. AbA
B. 2, 4-D
C. IAA
D. GA

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the regulation of the interconversion of phytochrome forms, specifically the transition between the Pr (red light-absorbing) and Pfr (far-red light-absorbing) forms of phytochrome.

2. **Identifying Phytochrome Forms**: Phytochromes are photoreceptors in plants that exist in two interconvertible forms: Pr, which absorbs red light, and Pfr, which absorbs far-red light. The conversion between these two forms is crucial for various plant growth and developmental processes.

3. **Role of Light**: The conversion of Pr to Pfr occurs when plants are exposed to red light. Conversely, the conversion of Pfr back to Pr happens in the presence of far-red light.

4. **Influence of Gibberellins**: The video transcript mentions gibberellic acid (a type of gibberellin) as a factor in this interconversion process. Gibberellins are plant hormones that play a significant role in promoting growth and influencing various developmental processes.

5. **Conclusion**: Based on the explanation provided, the regulation of the interconversion of phytochrome forms is primarily influenced by the presence of gibberellins, specifically gibberellic acid, which facilitates the conversion of Pr to Pfr.

6. **Final Answer**: The correct answer to the question is that the interconversion of Pr and Pfr is regulated by gibberellic acid (option number four).