AP Biology Unit 8: Vernalization, FLC Gene Epigenetics, and Cold-Induced Flowering Mechanism
- Introduction to Vernalization: The Cold Memory of Plants
- The Molecular Mechanism: Epigenetics & The FLC Gene Switch
- The Cold Exposure Matrix: Winter Annuals vs. Biennials
- Check Your Understanding: Unit 2 Practice Questions
- Advanced Thinking: Critical Questions
- Data Analysis: Interpreting Graphs
- Vernalization is the physiological process by which the juvenile or vegetative phase of a plant is reduced, thereby accelerating and enhancing the process of flowering through a specific low-temperature (cold) treatment.
- In nature, this phenomenon acts as an evolutionary adaptation, ensuring that plants do not flower prematurely during the harsh winter months but instead wait until the favorable conditions of spring arrive.
- To completely understand floral induction, it is important to distinguish between the two primary environmental response :
- Photoperiodism acts as the primary process in flowering, where the plant responds to the relative lengths of light and dark periods (day length).
- Vernalization acts as the secondary process of flowering, utilizing low temperature as the ultimate physiological switch.
- To understand completely how vernalization works, one must analyze the distinct behavioral differences between the two primary crop varieties:
- These varities are grown and planted in the Spring season. They naturally progress to the flowering stage and produce grains/fruits before the end of the same growing season without requiring any artificial cold exposure.
- These are exceptionally adapted to be planted in the Autumn season. If winter varieties are mistakenly planted in the spring, they will completely fail to flower or produce grain during that season.
- Through the application of artificial or natural Vernalization, the cold-dependent winter varieties are physiologically transformed into spring or summer varieties, allowing them to complete their life cycle successfully.
- The low-temperature stimulus is perceived locally by the plant and does not travel long distances initially. The primary receptive areas are:
- The Shoot Apical Meristem is the metabolically active apical meristem of a mature stem is the main site where cold stimulation is perceived.
- The actively dividing seed embryo of a germinating seed can also receive the vernalization signal.
- All actively dividing meristematic Tissues including shoot tips and embryo tips, are highly receptive.
- Emerging young vegetative leaves show significantly higher receptivity to vernalization than older, senescent structures.
- Vernalization is a highly active biological process and cannot occur in dormant or inactive tissues. The following conditions are mandatory:
- According to the landmark research by scientist Melchers, a specialized physiological flowering hormone called Vernalin is actively produced within the plant tissue during the low-temperature treatment.
- The stimulus is precisely received by the dividing cells of the shoot. Once synthesized, the Vernalin hormone induces critical physiological and biochemical alterations throughout the plant's vascular network, successfully triggering the transition from vegetative growth to floral meristem identity.
- The physiological effect of vernalization is not completely permanent initially.
- If a vernalized plant or seed is immediately exposed to an uncharacteristically high temperature, the cold-induced stimulus is entirely neutralized and reversed.
- This striking reversal phenomenon is scientifically termed Devernalization.
- Integrating vernalization techniques provides massive ecological and agricultural benefits:
- It significantly shortens the vegetative phase, causing early flowering and faster crop maturation.
- Winter variety crops can be successfully converted to spring varieties, allowing multi-season harvesting.
- Tropical plants can be seamlessly adapted and grown in temperate geographical zones where they naturally fail to grow.
- It dramatically optimizes and increases the net agricultural yield per hectare.
- Vernalized plants develop an advanced physiological power to withstand extreme cold, frost, and freezing temperatures. Furthermore, it induces systemic acquired resistance, drastically increasing the crop's defense against various devastating fungal diseases.
- While the physiological effects of vernalization have been known for decades, modern molecular biology has revealed that epigenetics regulation is the true engine behind this process.
- Plants possess a sophisticated genetic "switch" that remembers the duration of cold exposure without altering their underlying DNA sequence.
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| The Molecular Mechanism: Epigenetics & The FLC Gene Switch |
- At the center of this molecular clock is a master regulatory gene known as Flowering Locus C ( FLC).
- FLC codes for a transcription factor that acts as a potent floral repressor .
- In Warm Conditions, The FLC gene is highly active or turned ON.It continuously produces FLC mRNA and protein, which actively block the expression of downstream flowering-induction genes like FT (Flowering Locus T) and SOC1.
- As long as FLC is active, the plant remains strictly in the vegetative/juvenile phase and cannot produce flowers.
- When the plant experiences an extended period of low temperature (1 degree celsius to 10 degree Celsius), a brilliant epigenetic transformation takes place.
- The plant cells recruit specialized protein complexes to structurally lock the FLC gene.
- The cold exposure triggers the activation of the Polycomb Repressive Complex 2 (PRC2).
- Polycomb Repressive Complex 2 induces specific chemical modifications on the histone proteins around which the FLC DNA is wrapped. It specifically causes Histone H3 Lysine 27 Trimethylation (H3K27me3) which is a universal molecular signal for gene silencing.
- This methylation causes the chromatin structure to tightly condense (Heterochromatinization), physically blocking the RNA polymerase enzyme from accessing the FLC gene.
- The FLC gene is effectively turned OFF (silenced).
- The down regulation of FLC acts like lifting a massive structural dam, allowing the biochemical cascade of flowering to surge forward
- Genes like VRN1 (Vernalization 1) and VRN2 are upregulated during the cold treatment.
- They ensure that the structural repression of FLC remains stable and permanent even after the winter ends and temperatures rise in spring.
- With FLC successfully silenced, the FT gene is turned ON. The FT protein (often called Florigen) travels via the phloem from the leaves directly to the shoot apical meristem, transforming it into a floral bud.
- To optimize crop yield and understand floral induction, botanists categorize vernalization requiring plants into two distinct ecological cohorts based on their life cycles: Winter Annuals and Biennials.
- While both require a prolonged chilling treatment to flower, their physiological timing and structural maturity requirements differ significantly.
- Winter wheat serves as the classic model organism for understanding winter annual dynamics.
- If a farmer plants winter wheat in the spring, the plant will continuously grow green leaves but will never produce the wheat ear (spike) because its FLC gene remains active.
- When planted in Autumn, the moisture in the soil hydrates the seed embryo. The embryo immediately registers the freezing winter temperatures.
- This triggers the epigenetic silencing of FLC, allowing the shoot apical meristem to transition into a floral state as soon as the warm days of spring arrive.
- Biennial plants like carrots (Daucus carota) or cabbages (Brassica oleracea) exhibit a fascinating multi-stage strategy.
- In the first spring and summer, the plant grows close to the ground in a cluster of leaves called a rosette.
- It focuses entirely on photosynthesis, pumping sugars down into a thick taproot.
- The plant must reach a certain physical size (Juvenile-to-Adult transition) before it can recognize the cold. A small germinating carrot seed cannot be vernalized.
- After overwintering in the cold ground, the winter stimulus activates the SOC1 gene and triggers a massive surge of the plant hormone Gibberellin (GA). This causes Bolting, the rapid, dramatic elongation of the stem.
- The vegetative rosette suddenly shoots upward to produce a massive terminal flower stalk, completing its life cycle by producing seeds.
Total Marks: 30 | Time: 1.5 Hours
Section A: Multiple Choice Questions (8 Marks)
Q1. Vernalization is structurally perceived by which part of the plant? (A) Mature structural xylem elements
(B) Fully expanded senescent leaves
(C) Actively dividing meristematic cells of the shoot apex or embryo
(D) Non-dividing cells within the root elongation zone
Q2. Photoperiodism and Vernalization act as complementary processes in floral induction. Which of the following correctly defines their hierarchy?
(A) Vernalization is the primary trigger, while photoperiodism is completely optional.
(B) Photoperiodism acts as the primary process, while vernalization serves as the secondary physiological trigger.
(C) Both processes are governed entirely by soil moisture content rather than environmental cues.
(D) Photoperiodism regulates root growth, whereas vernalization regulates structural leaf width.
Q3. What is the precise molecular role of the Flowering Locus C (FLC) gene in winter annuals?
(A) It acts as a direct floral promoter that accelerates structural bolting.
(B) It synthesizes the hormone Gibberellin within the root cortex.
(C) It acts as a potent floral repressor that blocks downstream flowering-induction genes.
(D) It encodes a structural protein that protects the seed coat from severe frost damage.
Q4. During prolonged cold exposure (1 degree Celsius to 10 degree Celsius ) what specific epigenetic modification turns OFF the FLC gene?
(A) Complete deletion of the underlying DNA base sequence
(B) Histone H3 Lysine 27 Trimethylation (H3K27me3) causing chromatin condensation
(C) Rapid acetylation of histone tails leading to high transcription rates
(D) Direct phosphorylation of the RNA polymerase II enzyme
Q5. If a winter wheat variety is treated with low temperatures for 6 weeks and is immediately exposed to an uncharacteristically high temperature, what physiological phenomenon occurs?
(A) Hyper-vernalization
(B) Permanent floral arrest
(C) Devernalization
(D) Immediate seed dormancy
Q6. Which of the following correctly contrasts the vernalization requirements of Winter Annuals vs. Biennials?
(A) Winter annuals require two full years of vegetative growth before perceiving cold.
(B) Winter annuals can perceive cold as an active embryo, while biennials must reach a mature vegetative rosette stage.
(C) Biennials can be fully vernalized as dry, completely desiccated seeds.
(D) Neither cohort requires active cell division for the cold stimulus to register.
Q7. The SOC1 gene is often referred to as the molecular "Green Light" for flowering. Which statement accurately describes its activation pathway?
(A) FLC directly activates SOC1 during the warm summer months.
(B) The production of Vernalin hormone completely destroys the SOC1 locus.
(C) Once FLC is epigenetically silenced, the FT protein (Florigen) moves to the shoot apex and activates SOC1.
(D) SOC1 undergoes permanent degradation when exposed to low temperatures.
Q8. A scientist targets a plant with a chemical inhibitor that completely blocks the Polycomb Repressive Complex 2 (PRC2). What will be the most likely phenotypic result when winter annuals are exposed to winter? (A) The plants will flower much earlier than normal.
(B) The plants will fail to flower because FLC cannot be epigenetically silenced without PRC2.
(C) The plants will immediately undergo stem bolting without any cold cue.
(D) The plant will bypass the vegetative rosette phase entirely.
Section 2: Short Answer Questions (12 Marks)
Q9. State the mandatory environmental factors required for vernalization to successfully occur within plant tissue, and explain why it cannot occur in dry seeds.
Q10. Briefly explain the experimental hypothesis proposed by scientist Melchers regarding the hormone Vernalin.
Q11. Define the term "Bolting" and explain which plant cohort (Winter Annuals or Biennials) undergoes this process in its second growing season.
Q12. Describe the inverse relationship observed on a graph plotting "Weeks of Chilling Exposure" vs. "Days to Flowering."
Section 3 : Long Answer Questions (12 Marks)
Q13. The Epigenetic Switch Model A regulatory network controls the transition from vegetative to reproductive growth in Arabidopsis thaliana.
(a) Describe how the FLC gene regulates the downstream targets FT and SOC1 before the winter chilling treatment.
(b) Explain the specific biochemical steps through which prolonged cold exposure transitions the FLC locus from an open chromatin state to a closed heterochromatin state. Include the role of the PRC2 complex.
(c) Predict the evolutionary advantage of having a quantitative response (memory of winter) rather than an instant reaction to a single cold night.
Total Marks: 30 | Time: 1.5 Hours
Section A: Multiple Choice Questions (8 Marks)
Q 1. Which of the following best describes the molecular state of the FLC gene in a winter annual before it experiences winter?
(A) Condensed heterochromatin with high levels of H3K27me3 modifications.
(B) Open, transcriptionally active chromatin with high levels of H3K4me3 modifications.
(C) Completely methylated DNA strands that block RNA Polymerase II binding.
(D) Linearized plasmids completely detached from the main plant chromosome.
Q2 In biennial plants, the physiological process of "Bolting" is heavily driven by which plant hormone after cold exposure?
(A) Abscisic Acid (ABA)
(B) Ethylene gas
(C) Gibberellins (GA)
(D) Cytokinins
Q3 Why does vernalization fail to occur in completely dry, desiccated seeds?
(A) Dry seeds cannot absorb the low-temperature wavelengths of light.
(B) Metabolic pathways and active cell divisions required to process the cold stimulus are suspended without hydration.
(C) The FLC gene is physically absent from the plant genome until water enters.
(D) High humidity is required to chemically burn the outer seed coat.
Q4 . If a plant experiences a single cold night 2 degree celsius followed by weeks of warm weather, flowering is usually not induced. This is because vernalization is:
(A) An instant genetic mutation that occurs only during daylight.
(B) A qualitative, all-or-nothing response triggered by any temperature drop.
(C) A quantitative process requiring a sustained cumulative period of cold to lock the epigenetic switch.
(D) Completely dependent on root elongation rather than shoot apical meristems.
Q5 . The protein complex PRC2 plays a vital role during vernalization. What is its direct enzymatic function?
(A) It acts as a ligase to splice the FT gene into the chloroplast genome.
(B) It functions as a histone methyltransferase that deposits repressive marks on the FLC locus.
(C) It degrades the Vernalin hormone in the phloem tissue.
(D) It synthesizes structural cellulose to strengthen the shoot apical meristem against frost.
Q6 . What is the true evolutionary purpose of the FLC gene acting as a floral repressor?
(A) To force the plant to complete its entire life cycle within the dark winter months.
(B) To maximize root growth by preventing any energy from reaching the leaves.
(C) To prevent premature flowering in Autumn before the winter ends, protecting delicate reproductive structures from frost.
(D) To permanently stop seed germination in tropical regions.
Q7 . Once the winter cold passes, what ensures that the FLC gene remains "locked" in its silenced state during the warm spring?
(A) Rapid degradation of all ribosome structures within the plant cells.
(B) The upregulation and continuous activity of maintenance genes like VRN1 and VRN2.
(C) A permanent decrease in global soil moisture levels.
(D) Continuous application of high-temperature stress.
Q8 . The transition from a vegetative meristem (leaf production) to a floral meristem (flower bud production) is ultimately executed when SOC1 activates which downstream master genes?
(A) Auxin efflux carriers and Rubisco enzymes
(B) LEAFY (LFY) and APETALA1 (AP1)
(C) FLC and Polycomb Repressive Complex 1
(D) Histone acetyl transferases and RNA Polymerase I
Section 2 : Short Answer Questions (12 Marks)
Q1. Explain the molecular outcome if a winter annual plant is subjected to severe cold treatment but lacks a functional PRC2 (Polycomb Repressive Complex 2) assembly.
Q2. Distinguish between the structural maturity required for vernalization in Winter Wheat versus a biennial crop like Carrot (Daucus carota).
Q3 . A student claims that "Once a plant is vernalized, its DNA sequence is permanently mutated to enable fast flowering." Evaluate the accuracy of this statement based on principles of epigenetics.
Q4 . What is the functional role of the FT protein (Florigen) once the FLC gene is successfully silenced, and how does it interact with SOC1?
Section 3 : Long Answer Questions (12 Marks)
Q1. Agricultural Manipulation Case Study An industrial agricultural facility wants to grow a high-yield temperate biennial crop in a sub-tropical region where winters are short and mild (>18 degree Celsius ).
(a) Predict whether the biennial crop will naturally produce seeds under these sub-tropical conditions. Justify your prediction using your knowledge of the juvenile-to-adult transition and chilling thresholds.
(b) Design an artificial protocol using temperature chambers or hormone applications (such as Gibberellins) that would allow the farmers to successfully induce flowering and secure a high seed yield.
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📝 Advanced thinking Critical question
Question: With global climate change causing erratic winter patterns, a temperate region experiences an unusually warm week 22 degree Celsius right in the middle of its winter chilling period. At a molecular level, explain how this "heat shock" impacts a winter annual plant that has only undergone 3 weeks of its required 6-week vernalization threshold. Focus on the competition between PRC2-mediated methylation and potential re-activation of the FLC locus.
Answer : 1 If a plant experiences a sudden, prolonged spike in temperature before its vernalization period is chemically saturated, it undergoes devernalization.
At 3 weeks of cold exposure, the silencing of the FLC gene is incomplete and unstable (metastable state). The PRC2 complex has deposited some repressive H3K27me3 marks, but the chromatin structure has not yet fully condensed into permanent heterochromatin.
The sudden warmth triggers cellular signals that counteract the cold memory. Without the stabilization proteins (like VRN1), active histone demethylases are recruited to strip away the repressive H3K27me3 tags and replace them with active H3K4me3 marks.
The FLC gene turns back ON (reactivates), completely erasing the plant’s cellular memory of the first 3 weeks of winter. The plant is reset to its baseline vegetative state and will fail to flower unless it experiences a fresh, uninterrupted 6-week cycle of cold.
Question: 2 Experimental grafting studies show that if a vernalized shoot apex of an Arabidopsis plant is grafted onto an unvernalized non-planted rootstock, the entire structural unit successfully flowers. However, if an unvernalized shoot apex is grafted onto a pre-vernalized rootstock kept in the dark, flowering is severely delayed or absent. Analyze these findings to explain the tissue specificity of cold perception and the systemic transport of the flowering signal.
Answer : 2 Vernalization must be perceived by actively dividing cells, primarily located in the Shoot Apical Meristem (SAM) or active leaf primordia where chromatin remodeling can safely occur during cell division. The root system, while experiencing the cold, does not act as the primary signaling hub for floral transition.
The ultimate product of a successfully vernalized and light-stimulated leaf is the FT protein (Florigen). FT is synthesized in the leaves and must travel through the phloem up to the shoot apex.
When the rootstock alone is vernalized and kept in the dark, it cannot synthesize or transport sufficient FT protein upward against the natural source-to-sink phloem gradient to activate the SOC1 gene at the unvernalized shoot apex. Thus, the shoot apex remains under the strict transcriptional repression of its own active FLC gene, proving that vernalization is locally cell-autonomous at the shoot tip but requires systemic support from photosynthetic leaves to execute flowering.
Question: 3 Arabidopsis thaliana is monocarpic (flowers once and dies), and it permanently silences FLC during spring to finish its lifecycle. In contrast, its close relative Arabis alpina is a polycarpic perennial (flowers every spring but survives for many years). In Arabis alpina, the FLC ortholog (known as PEP1) is silenced during winter but turns completely back ON when summer arrives. From an evolutionary and energetic perspective, evaluate why the permanent locking mechanism of VRN1 must be bypassed or modified in perennial plants.
Answer : 3 For an annual plant, finishing the lifecycle quickly in summer is an evolutionary win. It completely exhausts its energy reserves to make seeds, so permanently locking FLC via VRN1 ensures all meristems switch to flowers.
A perennial plant cannot turn all of its growing tips into flowers, otherwise it would die after production. It needs to save some buds to stay vegetative for the next year.
During winter, the perennial silences PEP1 to allow spring flowering. However, because it lacks the permanent epigenetic locking mechanism found in annuals, the rising temperatures of summer cause PEP1 to turn back ON.
Reactivating the repressor (PEP1) forces the newly formed summer shoots back into a vegetative stage. This preserves the plant's structural energy, allows it to build up carbohydrate reserves through the summer, and ensures it survives the autumn to flower again the following year.
📝Experimental Design and Data Analysis
Background Information: A group of plant physiologists wanted to test the quantitative effect of vernalization on a newly discovered mutant variety of winter wheat (Triticum aestivum). They exposed separate groups of germinating seeds to a constant chilling threshold of 3 degree Celsius for varying durations (weeks).
Following the cold treatment, all groups were transferred to a controlled greenhouse maintained at a warm 22 degree Celsius under uniform long-day light conditions.
The scientists then measured two variables: the average number of days taken to observe the first floral bud and the relative expression level of FLC mRNA in the shoot apical meristem.
Collected Experimental Data Table
Based on the quantitative data provided in the table, which of the following statements provides the most valid biological conclusion regarding the molecular mechanism of vernalization in this winter wheat variety?
(A) Vernalization is a qualitative, all-or-nothing physiological switch where any exposure to cold completely deletes the FLC locus instantly.
(B) There is a direct positive correlation between FLC mRNA expression levels and the acceleration of flowering time.
(C) Cold exposure acts as a quantitative environmental trigger that downregulates the floral repressor FLC, with the physiological response reaching its saturation threshold around 6 weeks of exposure.
(D) Extending the cold treatment from 6 weeks to 8 weeks continues to linearly decrease the days to flowering by suppressing SOC1 activation.
Correct Answer: (C)
Detailed Explanation for Students (Data Breakdown):
The data clearly shows an inverse relationship between the duration of cold exposure and both FLC expression and days to flowering. As cold duration increases from 0 to 6 weeks, FLC mRNA expression drops significantly from 100% down to 5%, causing the days to flowering to decrease drastically from over 150 days to just 28 days.
Between 6 weeks and 8 weeks, the relative expression of FLC stays locked at 5%, and the days to flowering remain virtually unchanged (28 vs 27 days). This indicates that the epigenetic silencing mechanism (histone methylation by the PRC2 complex) has reached its maximum biological saturation point at 6 weeks.
Why other options are wrong: (A) is wrong because the process is gradual and quantitative, not instant. (B) is wrong because the correlation is inverse/negative, not positive. (D) is wrong because the graph flattens out after 6 weeks and does not continue a linear decrease.
📈 Graph Interpretation
Directions: Analyze the quantitative response curve provided in the graph carefully. Answer the following analytical and descriptive prompts using precise molecular biology terminology (FLC, PRC2, chromatin remodeling, etc.).
Question 1: Based on the data curve, the Daily Vernalization Rate (d) remains locked at its maximum value of 1.0 within a specific temperature window of 0 to 7 degree Celsius.
(a) Describe the molecular event occurring at the chromatin level within this optimal plateau phase.
(b) Predict the physiological consequence on the plant’s flowering timeline if the ambient temperature remains constant at 4 degree Celsius for 6 weeks versus fluctuating daily between 8 degree Celsius and 12 degree Celsius. Justify your prediction using specific rate data points from the graph.
Question 2: The graph clearly shows a sharp drop in the daily vernalization rate as temperatures decrease below 0 degree Celsius, reaching an absolute value of 0.0 at -5 degree Celsius and below.
(a) Explain why a freezing temperature (such as -7 degree Celsius), which is technically a "winter cue," fails to induce any molecular vernalization effect in the plant tissue.
(b) Identify the biological requirement that must be met within the meristematic cells for the PRC2 (Polycomb Repressive Complex 2) to successfully deposit repressive histone marks, and explain why this requirement is completely arrested below -5 degree Celsius.
Answer to Question 1
(a) Within the optimal plateau phase 0 degree Celsius to 7 degree Celsius, the PRC2 (Polycomb Repressive Complex 2) functions at its maximum enzymatic efficiency. It actively binds to the nucleation site of the FLC (Flowering Locus C) gene. At this site, PRC2 catalyzes the transfer of methyl groups to histone tails, specifically causing Histone H3 Lysine 27 Trimethylation (H3K27me3). This specific epigenetic tag acts as a molecular "repressive mark," triggering the chromatin structure to tightly wind up and condense into an inactive state (heterochromatin). As a result, RNA Polymerase II is physically blocked, completely shutting down FLC mRNA transcription.
(b) Prediction: The plant kept at a constant 7 degree Celsius will flower significantly faster (earlier timeline) when spring arrives. The plant exposed to a fluctuating 8 degree Celsius to 12 degree Celsius cycle will face a massive flowering delay or may remain completely vegetative.
Justification based on Graph Data: According to the graph, a constant temperature of 4 degree Celsius} falls directly inside the optimum plateau, keeping the Daily Vernalization Rate (d) locked at maximum efficiency (d = 1.0). This ensures total saturation and permanent silencing of the FLC gene over 6 weeks. Conversely, at 8 degree Celsius to 12 degree Celsius, the graph shows a steep decline where the daily rate drops significantly (ranging roughly from d approx 0.7 down to d approx 0.3). Because the daily rate is so low and unstable, the cumulative cold memory is never fully locked, leaving enough active FLC protein in the shoot apex to continue blocking the downstream floral promoters (FT and SOC1).
Answer to Question 2
(a) Why Freezing Temperatures (e.g., -7 degree Celsius) Fail: Vernalization is an active, adaptation-driven physiological process, not a passive physical reaction to cold. At extreme freezing temperatures like -7 degree Celsius, although it is a clear environmental winter cue, the cellular water within the plant tissue undergoes crystallization or enters metabolic stasis. For vernalization to successfully progress, the plant requires running cellular respiration and active protein synthesis to create proteins like VIN3 (Vernalization Insensitive 3). Since metabolic enzyme kinetics are completely arrested at -7 degree Celsius, the plant cannot sense or process the cold signal, bringing the daily rate down to 0.0.
(b) The Biological Requirement and Its Arrest Below -5 degree Celsius}: The Requirement: The fundamental biological requirement for the PRC2 complex to permanently lock and spread the repressive H3K27me3 marks across the FLC gene is active cell division (mitosis). Epigenetic chromatin remodeling and the spreading of stable histone modifications are cell-autonomous events that require the DNA replication fork to open during the S-phase of the cell cycle.
Why it is Arrested: Below -5 degree Celsius, cell division in the shoot apical meristem is completely stopped. Mitotic spindle formation, ATP production via mitochondria, and cytoplasmic transport are entirely frozen. Without active cellular replication and metabolic flow, the PRC2 complex cannot physically spread or maintain the silencing marks throughout the meristematic tissue, which perfectly aligns with the graph showing the vernalization rate dropping straight to absolute zero (0.0).
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