Question 2: Causal Relationships in Vernalization In the vernalization response of Arabidopsis, the FLC (FLOWERING LOCUS C) gene is epigenetically silenced to allow flowering. • Task: Explain the causal relationship between the H3K27me3 histone mark and the chromatin state of the FLC locus during and after winter. • Logic Challenge: Based on the molecular players described (e.g., the PRC2 complex), predict what would happen to the plant's flowering behavior if it possessed a "writer" mutation that prevented the deposition of H3K27me3 marks even when exposed to cold.

What is the function of FLC (FLOWERING LOCUS C) in Arabidopsis?

It is a potent repressor of flowering.

What happens to FLC expression during vernalization (exposure to winter cold)?

FLC is epigenetically silenced to allow flowering.

[;; What is the causal relationship between H3K27me3 and the chromatin state of FLC during winter?

What happens to H3K27me3 and FLC chromatin after winter (in spring)?

The H3K27me3 mark expands across the entire FLC gene body, causing chromatin to transition into a tightly packed, inaccessible state where transcription factors cannot bind.

How long is FLC silencing maintained after vernalization?

Permanently, across cell divisions throughout the rest of the plant's life cycle.

What complex acts as the "writer" that deposits H3K27me3 marks?

PRC2 (Polycomb Repressive Complex 2).

What would happen if a plant had a mutation preventing PRC2 from depositing H3K27me3 marks during cold exposure?

The plant would be unable to switch FLC chromatin from accessible to inaccessible, so FLC expression would remain high despite cold treatment.

What flowering behavior would a plant with a defective PRC2 "writer" mutation exhibit?

Extremely delayed flowering or it might never flower at all, remaining in a vegetative state and continuously producing leaves.

Why would a PRC2 mutant fail to flower even after cold exposure?

Because without H3K27me3 deposition, there is no epigenetic memory of winter. FLC stays active as a repressor, so the plant never acquires the developmental competence to flower