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Plant bio
Cold and flowering environmental responses
| Question | Answer |
|---|---|
| What is phenology? | Study of periodic natural biological phenomena controlled by climate |
| Seasons | - autumn: germination - winter: vegetative phase - spring: flowering - summer: seed dispersal - seasonal alignment important for developmental transitions - Mis-alignment of floral transition can have fatal consequences |
| How do plants know what season they're in? | Individual seasonal cues are not sufficient for plants to distinguish between Spring and Autumn A requirement for exposure to a prolonged period of cold ensures spring-time flowering |
| Why is this important? | A cold requirement that promotes spring flowering is an important agronomic trait |
| Crop varieties have been bred with different levels of cold requirement | Longer cold requirement - vegetative growth Shorter cold requirement - flowering Insufficient cold for varieties with long chill requirements delays flowering and impairs yield |
| Lysenko | Lysenko observed a quantitative effect of prolonged cold exposure Longer cold periods progressively reduced the time taken for the wheat plants to flower |
| Arabidopsis research has revealed the complex molecular processes underlie vernalization | The transcription factor FLC is a major repressor of flowering FLC is a MADS-Box transcription factor that binds to the promoters of flowering genes and inhibits their transcription |
| In winter annual Arabidopsis accessions, FRIGIDA (FRI) increases FLC transcription | FRI upregulates FLC High FLC expression = Plants flower much later without cold |
| High FLC and late flowering requires FRIGIDA | fri mutants Low FLC expression = flowering is repressed until plants are competent |
| Prolonged cold exposure gradually silences FLC expression | Prolonged COLD silences FLC Low FLC expression = Rapid flowering under permissive conditions |
| Why can’t simple gene regulation provide a long-term memory of cold? | Perception of long-term, reducing, fluctuating temperatures is required Requirement of a “counting” mechanism Requirement of “memory” component The process must be reset for the next generation |
| So cold registration at FLC loci is achieved via epigenetic changes.. | - epigenetics Any potentially stable and heritable change in gene expression that occurs without a change in DNA sequence Modifications to DNA or packaging proteins that impact gene activity |
| Epigenetic modifications affect chromatin structure | Chromatin = DNA + Associated RNA + Packaging Proteins Heterochromatin: Gene Silencing = Transcription Repression Euchromatin: active transcription |
| Epigenetic modifications affect chromatin structure | DNA methylation: This is mainly associated with gene silencing Histone modification: these open or close chromatin |
| Histone proteins can be modified to affect chromatin structure | Histone tails can be modified by: Methylation (Me), Acetylation (Ac), Ubiquitination (Ub), Phosphorylation (P), Sumoylation (Su) |
| There are several examples of Histone H3 modifications | FRIGIDA - Active Marks H3K36me3/H3K4me3 = FLC transcription COLD - Repressive Mark H3K27me3 = FLC transcription |
| Does FLC silencing occur gradually in all cells? | FLC-GUS transgenic lines were used to visualize switching between active and inactive states |
| Does FLC silencing occur gradually in all cells? | FLC is switched on in all cells = FLC is fully silencing in all cells H3K27me3 H3K36me3 H3K4me3 - after 4 weeks of cold |
| Cell-autonomous epigenetic switching was demonstrated | Vernalization works through digital chromatin switching Some cells return to an ON state and others remain OFF The overall proportion of cells expressing FLC provides plants with a memory of cold exposure |
| Digital epigenetic switching is a key feature of vernalization | During cold exposure there is a low probability of FLC in each cell from switching from an “on” to an “off” state |
| Digital epigenetic switching is a key feature of vernalization | During cold exposure there is a low probability of FLC in each cell from switching from an “on” to an “off” state This off state is propagated to daughter cells after each cell division |
| Digital epigenetic switching is a key feature of vernalization | The proportion of cells that remain stably silenced determine the barrier to flower initiation |
| But do FLC alleles switch independently in the same cell? | Transgenic plants were generated to contain two FLC alleles with different fluorescent proteins |
| Yes, FLC alleles switch independently! | After cold exposure, fluorescent marker imaging indicated that individual loci had switched off independently |
| During vernalization FLC switches off independently, allele by allele…. But what are the key molecular players in this process? | Cold induced FRIGIDA nuclear condensates contribute to FLC Repression During cold, FRI is sequestered away from FLC loci into nuclear condensates This prevents FRI from binding to FLC promoters and promoting an active chromatin state |
| Another key player in vernalization: FLC antisense long noncoding RNA (lncRNA) COOLAIR | Antisense: Transcribed from a promoter at the 3’ end of the sense gene on the opposing DNA strand |
| Another key player in vernalization: FLC antisense long noncoding RNA (lncRNA) COOLAIR | Long non-coding: Does not code for protein, but unlike short-interfering or micro RNAs, lncRNAs are longer (>200 nt) Cold Induced: COOLAIR transcription peaks after 14 days cold exposure |
| RNA labelling showed that COOLAIR intron RNA associates with FLC loci | Cold-induced protein VIN3 is required for FLC chromatin switching COOLAIR and VIN3 initiate FLC chromatin switching PHD PRC2 complexes add H3K27me3 across the locus and LHP1 ensures stable repression of FLC |
| In autumn, active chromatin marks promote a high level of FLC expression | Autumn: FRIGIDA promotes high FLC expression that prevents warm periods from activating flowering genes |
| In low temperatures, FRI is sequestered away from FLC, there is a reduction of sense and an increase in antisense transcription | Winter: An increase in antisense lncRNA COOLAIR transcription FRIGIDA protein condensates form in the nucleus |
| The absence of warmth causes VIN3 to associate with PRC2 to form an active complex at FLC loci | Slow growth in the cold causes VIN3 levels to accumulate VIN3 recruits the PHD PRC2 complex to the FLC gene H3K27me3 modifications occur first in the ”nucleation” region of FLC |
| In prolonged cold, an increasing number of individual loci switch from having active to silencing chromatin marks | Gradually, H3K36me3 active marks are lost across individual FLC loci A switch to H3K27me3 repressive marks is coordinated by the lncRNA COOLAIR An overall reduction in FLC transcription is observed during cold exposure |
| Fully silenced FLC loci enable long, warm days to trigger flowering in spring | LHP1 prevents FLC reactivation by maintaining repressive marks throughout mitotic cell division Permissive conditions in spring can then activate flowering genes e.g. FT |
| Reactivation of FLC expression through demethylation during embryogenesis resets the system | ELF6 reactivates FLC occurs during seed development to ensure the next generation inherits a vernalization requirement |
| SUMMARY: The importance of cold perception to plant phenology | This provides plants with the ability to distinguish autumn from spring Flowering is repressed until spring to ensure optimal reproductive success Cold requirement is an important agronomic trait |
| SUMMARY: Major molecular pathways involved in flowering time control | FLOWERING LOCUS C (FLC) is a major flowering repressor FRIGIDA (FRI) increases FLC expression Prolonged cold represses high FLC expression, and this allows permissive environmental conditions to trigger flowering |
| SUMMARY: Molecular mechanisms that monitor and remember cold exposure | Vernalization provides plants with an epigenetic memory of winter During cold exposure, an increasing number of FLC alleles switch from active H3K36me3 to silencing H3K27me3 marks |
| SUMMARY: Molecular mechanisms that monitor and remember cold exposure | Chromatin switching is promoted by FRIGIDA condensation and upregulation of both VIN3 and the FLC antisense, long non-coding RNA, COOLAIR LHP1 stabilizes H3K27me3 marks over many cell divisions |
| SUMMARY: Molecular mechanisms that monitor and remember cold exposure | The histone demethylase ELF6 removes H3K27me3 during embryogenesis to ensure the next generation inherits a vernalization requirement |
| An example of cold memory adaptation in northern Sweden..and beyond | Adaptation to harsh winters has led to a seasonal shift of vernalization to autumn Brassica napus (oilseed rape) has also been found to vernalize in autumn |
Created by:
rose.coo
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