LHP1, the Arabidopsis homologue of HETEROCHROMATIN PROTEIN1, is required for epigenetic silencing of FLC

Abstract

Vernalization is the acceleration of flowering by prolonged cold that aligns the onset of reproductive development with spring conditions. A key step of vernalization in Arabidopsis is the epigenetic silencing of FLOWERING LOCUS C (FLC), which encodes a repressor of flowering. The vernalization-induced epigenetic silencing of FLC is associated with histone deacetylation and H3K27me2 and H3K9me2 methylation mediated by VRN͞VIN proteins. We have analyzed whether different histone methyltransferases and the chromodomain protein LIKE HETEROCHROMATIN PROTEIN (LHP)1 might play a role in vernalization. No single loss-of-function mutation in the histone methyltransferases studied disrupted the vernalization response; however, lhp1 mutants revealed a role for LHP1 in maintaining epigenetic silencing of FLC. Like LHP1, VRN1 functions in both flowering-time control and vernalization. We explored the localization of VRN1 and found it to be associated generally with Arabidopsis chromosomes but not the heterochromatic chromocenters. This association did not depend on vernalization or VRN2 function and was maintained during mitosis but was lost in meiotic chromosomes, suggesting that VRN1 may contribute to chromatin silencing that is not meiotically stable.chromatin ͉ mitosis ͉ vernalization ͉ flowering ͉ meiosis

Impact of nucleosome dynamics and histone modifications on cell proliferation during Arabidopsis development

“…In Arabidopsis, H3K27me3 is spread along the transcribed regions of genes and more rarely in their promoters (Zhang et al, 2007c). Remarkably, plants lack a homolog of PRC1, but recent evidence suggests that LIKE HETEROCHROMATIN PROTEIN (LHP1), also known as TERMINAL FLOWER 2 (TFL2), could perform this function (Mylne et al, 2006;Turck et al, 2007). In fact, LHP1 (i) associates with H3K27me3 in vitro, (ii) is needed to maintain the stable H3K27me3-mediated gene repression in euchromatin (Libault et al, 2005;Nakahigashi et al, 2005) and (iii) although it is not directly involved in histone methylation, disruption of its chromodomain prevents H3K27me3 binding and releases silencing of PcG target genes, indicating that LHP1 could be the functional equivalent of PRC1 in plants (Exner and Hennig, 2008).…”

EMBRYONIC FLOWER1 and ULTRAPETALA1 Act Antagonistically on Arabidopsis Development and Stress Response

“…In Arabidopsis, the PRC1 RINGfinger homologs, AtRING1A/B and AtBMI1A/B, are essential for maintaining cell identity (Sanchez-Pulido et al, 2008;Xu and Shen, 2008;Bratzel et al, 2010). Several other proteins are involved in AtPRC1 functions, namely LIKE HETEROCHROMATIN PROTEIN1/TERMINAL FLOWER2 (TFL2), VRN1, and EMF1 (Mylne et al, 2006;Turck et al, 2007;Zhang et al, 2007;Calonje et al, 2008). EMF1 binds four RING-finger proteins and is required for H2A Lys-119 monoubiquitination activity on select EMF1 target genes (Bratzel et al, 2010).…”

Expression, purification and preliminary X-ray diffraction studies of VERNALIZATION1208–341fromArabidopsis thaliana

“…Immunolocalization showed VRN1 to be excluded from heterochromatic chromocentres. VRN1 protein dissociated from nuclei undergoing meiosis and this was supported by a dramatic drop in mRNA expression in pollen (Mylne et al, 2006).…”

“…In vitro, recombinant fulllength VRN1 protein was shown to bind naked dsDNA in a nonsequence-specific manner (Levy et al, 2002). A fully functional genomic VRN1-GFP fusion associated throughout the nucleus in vivo and remained associated with mitotic chromosomes (Mylne et al, 2006). Immunolocalization showed VRN1 to be excluded from heterochromatic chromocentres.…”

“…Using molecular-genetic approaches, Arabidopsis vernalization has been shown to require a number of proteins including VIN3, a plant homeodomain (PHD) finger protein whose expression is induced by prolonged cold (Sung & Amasino, 2004), VRN5, a VIN3-related protein that is constitutively expressed (Greb et al, 2007;Sung, Schmitz et al, 2006), VRN2, a homologue of the Polycomb protein Su(z)12, a well characterized chromatin regulator in Drosophila and humans (Gendall et al, 2001), and VERNALIZATION1 (VRN1), a DNA-binding protein (Levy et al, 2002). Maintenance of FLC silencing also requires the Arabidopsis homologue of HETEROCHROMATIN PROTEIN1 (Mylne et al, 2006;Sung, He et al, 2006). Recently, TAP-tagged VRN5 was found to be able to pull down a complex of proteins including the aforementioned VRN2 and VIN3 with Polycomb core proteins FIE (an ESC homologue), MSI1 (a p55 homologue) and a PHD-finger protein VEL1 (De Lucia et al, 2008).…”