Storing memories: the distinct phases of Polycomb-mediated silencing of Arabidopsis FLC

Abstract

Polycomb-mediated epigenetic silencing is central to correct growth and development in higher eukaryotes. The evolutionarily conserved Polycomb repressive complex 2 (PRC2) transcriptionally silences target genes through a mechanism requiring the histone modification H3K27me3. However, we still do not fully understand what defines Polycomb targets, how their expression state is switched from epigenetically ON to OFF and how silencing is subsequently maintained through many cell divisions. An excellent system in which to dissect the sequence of events underlying an epigenetic switch is the Arabidopsis FLC locus. Exposure to cold temperatures progressively induces a PRC2-dependent switch in an increasing proportion of cells, through a mechanism that is driven by the local chromatin environment. Temporally distinct phases of this silencing mechanism have been identified. First, the locus is transcriptionally silenced in a process involving cold-induced antisense transcripts; second, nucleation at the first exon/intron boundary of a Polycomb complex containing cold-induced accessory proteins induces a metastable epigenetically silenced state; third, a Polycomb complex with a distinct composition spreads across the locus in a process requiring DNA replication to deliver long-term epigenetic silencing. Detailed understanding from this system is likely to provide mechanistic insights important for epigenetic silencing in eukaryotes generally.

Phenotypic Plasticity: From Theory and Genetics to Current and Future Challenges

“…In addition, the regulation of the lytic cycle in bacteriophages identified a switch mechanism that required a complex genetic locus, and relied on genetic and epigenetic mechanisms (Ptashne 2004). Finally, the regulation of flowering in Arabidopsis thaliana through the flowering locus C is another example of a complex genetic locus that involves genetic and epigenetic mechanisms to regulate a phenotypically plastic trait (Costa and Dean 2019). In all these systems thresholds exist, above or below which responses to the environment result in different phenotypic outcomes.…”

The transition to flowering in winter rapeseed during vernalization

“…In leaves, FLC directly represses the expression of FLOWERING LOCUS T ( FT ), whereas in SAMs, it impairs the up‐regulation of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 ( SOC1 ) and FLOWERING LOCUS D ( FD ), which prevent flowering (Searle et al, 2006). Vernalization leads to a rapid decrease in FLC transcript abundance, and continuous cold exposure results in its epigenetic silencing creating a winter memory state (Costa & Dean, 2019), therefore, liberating FT and SOC1 and other flowering time genes, conferring a seasonal competence to flowering (He, Chen, & Zeng, 2020).…”

Control of Flowering and Runnering in Strawberry

“…H3K27me3 histone marks are deposited on the DAM sequence during cold treatment in a locus specific manner (Leida et al, 2012;De la Fuente et al, 2015). Also Arabidopsis MADSbox gene, floral repressor FLOWERING LOCUS C (FLC), is gradually downregulated by cold by a molecular mechanism including epigenetic modifications of histones and anti-sense transcripts (Swiezewski et al, 2009;Costa and Dean, 2019;Tian et al, 2019), and these studies should pave a way for future research on the epigenetic regulation of DAM expression. Coldregulated FLC-like genes have also been reported in apple (Porto et al, 2015;Kumar et al, 2016), but no functional validation has been carried out.…”