Question

Phytochrome is localised within: [Question ID = 709][Question Description = 209_34_PLB_AUG22_Q109]

• A. Chloroplast [Option ID = 2833]
• B. Mitochondria [Option ID = 2834]
• C. Plasma membrane [Option ID = 2835]
• D. Cytosol [Option ID = 2836]

Hint:

Red light converts \(P_r \rightarrow P_{fr}\) (active, moves to nucleus). Far-red light converts \(P_{fr} \rightarrow P_r\) (inactive, remains in cytosol).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Phytochrome is a photoreceptor pigment used by plants to detect light, specifically in the red and far-red regions of the spectrum. It exists in two interconvertible forms: \(P_r\) (inactive) and \(P_{fr}\) (active). It regulates processes like seed germination, shade avoidance, and flowering.

Step 2: Detailed Explanation:
The intracellular localization of phytochrome is dynamic and crucial for its signaling mechanism:
- Cytosol: Phytochrome is synthesized on free ribosomes in the cytosol in its inactive \(P_r\) form. In the dark, nearly all the phytochrome remains in the cytosol.
- Translocation: Upon absorption of red light, \(P_r\) is converted to the active \(P_{fr}\) form. This conversion triggers the translocation of the phytochrome protein from the cytosol into the nucleus. Inside the nucleus, it interacts with transcription factors (PIFs) to regulate gene expression.
- Although some research suggests transient associations with the plasma membrane for rapid signaling, the primary synthesis and fundamental reservoir of the protein in its basal state is the cytosol. In the context of standard ICAR biotechnology questions, the answer focuses on where the bulk of the protein resides initially or overall.

Step 3: Final Answer:
Phytochrome is synthesized and primarily localized within the cytosol before moving to the nucleus upon activation.