Trends in Plant Science
ReviewChromatin regulation of flowering
Section snippets
Control of the transition to flowering
The timing of the transition from a vegetative to reproductive phase is critical for reproductive success in the angiosperm life cycle. Many species have evolved multiple pathways responding to environmental cues and endogenous factors to regulate flowering time properly. In Arabidopsis thaliana, a facultative long-day plant, the vernalization, thermosensory, and photoperiod pathways, sensing cold winter, ambient temperature, and long days, respectively, together with other pathways responding
FRI mediates chromatin modifications at the FLC locus to establish the winter-annual growth habit
FRI, encoding a plant-specific scaffold protein, is a major determinant of natural variation in Arabidopsis flowering time [6], and many players involved in FRI-dependent FLC activation have been identified in genetic screens for mutants that suppress FLC activation in an FRI-containing line. These include both conserved chromatin modifiers and plant-specific components 3, 5. Loss-of-function mutations in these components suppress FLC expression and so render a FRI-containing line early
Chromatin silencing of FLC in rapid-cycling accessions involves lncRNAs
In rapid-cycling accessions lacking a functional FRI, FLC is typically repressed by the autonomous or constitutive FLC repressors, among which are FPA, FCA, FY, FVE, FLOWERING LOCUS D (FLD), Polycomb (PcG) components, and HISTONE DEACETYLASE 6 (HDA6) 5, 17, 51, 52. These components directly interact with the FLC locus and are involved in FLC chromatin silencing, leading to flowering promotion. It should be noted that, in addition to FLC, these genes also silence other loci. Here, I focus on
Epigenetic ‘Memory of Winter’ involves lncRNAs and Polycomb components
Winter annuals require vernalization to acquire the competence to flower. Vernalization silences FLC expression, leading to acceleration of the floral transition upon the return of plants to a warm condition (typically approximately 20–25 °C). This silencing involves lncRNAs and PcG components, is maintained after the plants resume growth in warm conditions, and so is mitotically stable, conferring the cold-experienced plants the ‘memories of winter’ 4, 63, 64.
Previous genetic screens for
Chromatin-mediated regulation of FT expression
FT expression is induced in the vasculature by a long-day photoperiod and ambient temperature rise. Recent studies have shown that various chromatin modifiers are involved in the regulation of FT expression, including SWR1c, PRC2, LHP1, the REF6 H3K27 demethylase, and the PKDM7B (also known as AtJMJ4 or JMJ14) H3K4 demethylase (Figure 4).
FT expression is repressed by PcG activity in both long and short days. It has been shown that CLF binds to FT chromatin and is required for H3K27me3
Concluding remarks
Chromatin-mediated mechanisms play crucial roles in flowering-time regulation in Arabidopsis through the expression control of the key flowering genes FLC and FT. Chromatin regulation of FLC involves various conserved chromatin modifiers, plant-specific factors, and lncRNAs. Regulation of other developmental genes often involves chromatin modification. The study of FLC regulation has provided a paradigm for chromatin control of other developmental genes and thus developmental processes. FLC is
Acknowledgments
I thank Tongda Xu for critically reading this manuscript. Research in the He laboratory is supported by grants from the Singapore Ministry of Education (AcRF Tier 2; MOE2009-T2-1-081) and the Singapore National Science Foundation (2010NRF-CRP002-018), and by the Temasek Life Sciences Laboratory.
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