Vernalization

Abstract

Vernalization, the promotion of flowering in response to a prolonged period of growth at low temperatures, is an important adaptation of plants growing in regions where harsh winters are followed by relatively short growing seasons. In these plants, flowering is triggered by long days but only after the vernalization requirement has been met. Thus, the requirement for vernalization prevents flowering in the long days of autumn and ensures that flowering occurs in the warmer days of spring and summer, allowing sufficient time for seed development before the onset of the next winter. Various crop plants, including the winter cereals and canola, must be vernalized if they are to initiate flowering and set seed. The key genes controlling the response to vernalization differ between monocots and dicots suggesting that this response arose independently after the divergence of the monocot and dicot lineages.

Key concepts:

  • Vernalization is the promotion of flowering in response to prolonged periods of low temperatures, such as those experienced in winter.

  • The physiological properties of vernalization are similar in dicots and monocots but the genes controlling this response differ.

  • The vernalized state is inherited through mitotic cell divisions; this memory of winter is provided by epigenetic changes to the chromatin of key genes in the vernalization response, although the nature of these changes differs between monocots and dicots.

  • Epigenetic regulation results in a heritable but potentially reversible change in gene expression that is modulated by changing the accessibility of a gene for transcription.

  • FLOWERING LOCUS C (FLC) is a key regulator of vernalization responsiveness in the Brassicaceae but the role of FLC homologues in other dicots remains to be elucidated.

  • The regulation of FLC expression is complex and involves both genetic and epigenetic mechanisms and FLC has become an important model for studying epigenetic control of plant gene expression.

  • VERNALIZATION INSENSITIVE 3 (VIN3) is induced by cold and the VIN3 protein is incorporated into the Polycomb Repression Complex (PRC2) that regulates FLC.

  • Polycomb proteins are conserved across kingdoms and play a role in gene repression in Drosophila, Caenorhabditis elegans and mammals.

  • The vernalized state is reset each generation; resetting occurs at different times depending on the gamete transmitting FLC.

  • An FLC orthologue conditions the perennial behaviour of Arabis alpina, a relative of Arabidopsis thaliana.

Keywords: Arabidopsis; winter wheat; FLC; epigenetic regulation; chromatin structure

Figure 1.

The flowering pathways in Arabidopsis. Members of the autonomous pathway are indicated in the yellow box. Some members of this pathway, including FRI and the three proteins with which it interacts FRIGIDA‐LIKE 1 (FRL1), FRIGIDA ESSENTIAL (FES) and SUPPRESSOR OF FRIGIDA 4 (SUF4), upregulate FLC expression. In contrast, the wild‐type function of other members of this pathway is to downregulate FLC. The activation of FLC expression by the FRI complex and by autonomous pathway mutants is dependent on the activity of proteins of the Paf1 complex (VERNALIZATION INDEPENDENT 3 (VIP3) to VIP6, also known as EARLY FLOWERING 8 (ELF8) and ELF7), the SWR1 complex an ATP‐dependent chromatin remodelling complex (SUPRESSOR OF FRIGIDA 3 (SUF3), SWR1 COMPLEX PROTEIN 6 (SWC6), ACTIN RELATED PROTEIN 4 (ARP4), PHOTOPERIOD INDEPENDENT EARLY FLOWERING 1 (PIE1)) which deposits the histone variant H2A.Z into chromatin and lysine methyltransferasesARABIDOPSIS TRITHORAX 1 (ATX1), ATX2 and EARLY FLOWERING SHORT DAYS (EFS), also known as SET DOMAIN GROUP 8 (SDG8). Mono‐ubiquitination of histone H2B is important for FLC activity – this is catalysed by UBIQUTIN CONJUGATING ENZYME 1 (UBC1), UBC2, HUB1 (HISTONE MONOUBIQUTINATION), and HUB2. Other proteins important for FLC activity are EARLY in SHORT DAYS 4 (ESD4), ELF5 and HUA2. The activity of arginine methyltransferasesPROTEIN ARGININE METHYLTRANSFERASE (PRMT) represses FLC expression in nonvernalized plants and may also play a role in the vernalization pathway (reviewed in He, ). The major components of the vernalization pathway are shown in the blue box; VERNALIZATION 5 (VRN5), also known as VERNALIZATION INSENSITIVE‐LIKE 1 (VIL1) is a plant homeo domain (PHD) protein related to VIN3. VRN1, a B3 domain DNA‐binding protein, is also required for stable repression of FLC. The daylength (photoperiod) pathway is represented here by CONSTANS (CO), which is upregulated in long days. CO activates the expression of the floral integrators, SUPPRESSOR OF CONSTANS 1 (SOC1) and FT, which integrate the signals from the autonomous pathway and the photoperiod pathway.

Figure 2.

The requirement for vernalization blocks flowering before winter, ensuring that flowering occurs during the longer and warmer days of spring. In Arabidopsis (top), flowering is blocked before winter as high FLC expression blocks the induction of FT. FLC is repressed by the low temperatures during winter, allowing FT (and SOC1) induction in the longer days of spring to promote flowering. In cereals (bottom), VRN2 represses FT expression in long days before winter. During winter, VRN1 is induced; expression of VRN1 remains high after winter when it represses VRN2, allowing long day induction of FT. Flowering is promoted by the combined action of VRN1 and FT proteins. Adapted from Trevaskis et al., with permission from Elsevier.

Figure 3.

FLC expression is modulated by chromatin structure: (a) In nonvernalized plants, FLC chromatin bears the marks of an active gene – high levels of acetylation of lysine residues in the N‐terminal tails of histones H3 and H4 and high levels of H3K4me3 around the start of transcription and H3K36me3 in the transcribed region of the gene; the blue ovals represent the nucleosomes with the N‐terminal tails indicated as red lines. (b) VIN3 is induced by low temperatures and is required for the changes in histone modifications that are associated with repression of FLC, in plants grown at low temperatures. VIN3 is incorporated into a polycomb repression complex 2 (PRC2) during vernalization. This complex deposits the repressive mark H3K27me3 on chromatin around the start of transcription during growth at low temperatures. Neither the histone deacetylase(s) (HDAC) nor the histone demethylases that remove the active marks have been identified. (c) In vernalized plants that have been returned to warmer temperatures, the repressive mark H3K27me3 spreads bidirectionally across the FLC locus. The PRC2 (lacking VIN3) is associated with the chromatin covering the FLC locus (indicated by the coloured balls). Binding of LHP1 is essential for maintenance of FLC repression.

Figure 4.

FLC:GUS is reset in the somatic and sporogenous tissues in the male reproductive organ, the anther, and the paternally derived gene is expressed in the single‐celled zygotes from vernalized plants. In contrast, the maternally derived FLC:GUS gene is not reset during gametogenesis and is first expressed during embryogenesis. (a) A section through an anther from a vernalized FLC:GUS plant imaged under dark field (GUS crystals are pink). The pollen mother cells (arrow) are entering meiosis. Scale bars in all figures are 50 μm. (b) An ovule from a wild‐type female plant one day after pollination with pollen from a vernalized FLC:GUS plant, imaged under bright field (GUS product is blue). The arrow points to the single‐celled zygote. (c) An ovule from a wild‐type female plant three days after pollination with pollen from a vernalized FLC:GUS plant imaged under bright field (GUS product is blue). The arrow points to the early globular‐stage embryo. (d) A section through an ovule primordia around the time of meiosis (arrow) from a vernalized FLC:GUS plant imaged under dark field. No GUS staining is evident (e). An ovule from a vernalized FLC:GUS female plant one day after pollination with pollen from a wild‐type plant, imaged under bright field. No staining is evident (f). An ovule from a vernalized FLC:GUS female plant three days after pollination with pollen from a wild‐type plant, imaged under bright field (GUS product is blue). The arrow points to the early globular‐stage embryo.

Figure 5.

Integration of photoperiod and vernalization pathways in cereals. The VRN1 gene is induced by low temperatures in a quantitative manner; VRN1 promotes flowering directly and also acts to repress expression of VRN2, which is normally induced by long days. Before winter, VRN2 blocks the long day induction of FT but after winter, when VRN2 is repressed, long days can activate FT expression via the activity of CONSTANS (CO). Flowering is promoted by the combined activities of VRN1 and FT.

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Jean Finnegan, E, Helliwell, Chris, Sheldon, Candice, James Peacock, W, and Dennis, Elizabeth S(Mar 2010) Vernalization. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002048.pub3]