title: Induction of RNA-directed DNA methylation upon decondensation of constitutive heterochromatin.
abstract: Centromeric constitutive heterochromatin is marked by DNA methylation and dimethylated histone H3 Lys 9 (H3K9me2) in Arabidopsis. RNA-directed DNA methylation (RdDM) is a process that uses 24-nucleotide (nt) small interfering RNAs (siRNAs) to induce de novo methylation to its homologous DNA sequences. Despite the presence of centromeric 24-nt siRNAs, mutations in genes required for RdDM do not appreciably influence the methylation of centromeric repeats. The mechanism by which constitutive heterochromatin is protected from RdDM remains puzzling. Here, we report that the vegetative cell nuclei (VN) of the male gametophyte (pollen) invariably undergo extensive decondensation of centromeric heterochromatin and lose centromere identity. VN show greatly reduced H3K9me2, phenocopying nuclei carrying a mutation in the chromatin remodeller DECREASE IN DNA METHYLATION 1 (DDM1). However, unlike the situation in ddm1 nuclei, the decondensed heterochromatin retains dense CG methylation and transcriptional silencing, and, unexpectedly, is subjected to RdDM-dependent hypermethylation in non-CG contexts. These findings reveal two assembly orders of silent heterochromatin and implicate the condensed form in blocking the RdDM machinery.
authors.affiliation: Gregor Mendel Institute of Molecular Plant Biology, Histology Department, Austrian Academy of Sciences, Dr Bohr-Gasse 3, 1030 Vienna, Austria.
authors.forename: Vera K
authors.initials: VK
authors.lastname: Schoft
authors.forename: Nina
authors.initials: N
authors.lastname: Chumak
authors.forename: Magdalena
authors.initials: M
authors.lastname: Mosiolek
authors.forename: Lucyna
authors.initials: L
authors.lastname: Slusarz
authors.forename: Vukoslav
authors.initials: V
authors.lastname: Komnenovic
authors.forename: Lynette
authors.initials: L
authors.lastname: Brownfield
authors.forename: David
authors.initials: D
authors.lastname: Twell
authors.forename: Tetsuji
authors.initials: T
authors.lastname: Kakutani
authors.forename: Hisashi
authors.initials: H
authors.lastname: Tamaru
mesh_terms: D017360:Arabidopsis / Q000166:cytology / Q000378:metabolism; D002503:Centromere; D019175:DNA Methylation; D006570:Heterochromatin; D006657:Histones / Q000378:metabolism; D008780:Methyltransferases / Q000378:metabolism; D012313:RNA / Q000378:metabolism*
chemical_list: D006570:Heterochromatin; D006657:Histones; D012313:RNA; D008780:Methyltransferases; C470732:DRM2 protein, Arabidopsis
references.citation: Plant Cell. 2004;16 Suppl:S142-53
references.pmid: 15037731
references.citation: Nat Rev Genet. 2005 Jan;6(1):24-35
references.pmid: 15630419
references.citation: Nature. 2004 Jul 22;430(6998):471-6
references.pmid: 15269773
references.citation: Plant Cell. 2002 May;14(5):1053-66
references.pmid: 12034896
references.citation: Nat Genet. 2003 May;34(1):65-9
references.pmid: 12669067
references.citation: Plant J. 2003 Feb;33(4):743-9
references.pmid: 12609046
references.citation: PLoS Biol. 2004 May;2(5):E104
references.pmid: 15024409
references.citation: PLoS Genet. 2006 Jun 2;2(6):e83
references.pmid: 16741558
references.citation: Genetics. 2002 Aug;161(4):1661-72
references.pmid: 12196409
references.citation: Plant J. 2007 Jun;50(5):848-57
references.pmid: 17470059
references.citation: Science. 2005 Apr 1;308(5718):118-20
references.pmid: 15692015
references.citation: Science. 2001 Aug 10;293(5532):1074-80
references.pmid: 11498575
references.citation: Cell. 2002 Feb 22;108(4):489-500
references.pmid: 11909520
references.citation: EMBO J. 2005 Aug 3;24(15):2783-91
references.pmid: 16001083
references.citation: J Cell Sci. 2007 Apr 1;120(Pt 7):1200-8
references.pmid: 17376962
references.citation: PLoS Genet. 2005 Dec;1(6):e79
references.pmid: 16389298
references.citation: Nat Genet. 2005 Jul;37(7):761-5
references.pmid: 15924141
references.citation: Genetics. 2003 Mar;163(3):1109-22
references.pmid: 12663548
references.citation: Science. 2001 Jun 15;292(5524):2077-80
references.pmid: 11349138
references.citation: Science. 2008 Jan 4;319(5859):94-7
references.pmid: 18174443
references.citation: Genes Dev. 2001 Jul 15;15(14):1753-8
references.pmid: 11459824
references.citation: Cell. 2009 Feb 6;136(3):461-72
references.pmid: 19203581
references.citation: Cell. 2007 Sep 7;130(5):851-62
references.pmid: 17803908
references.citation: EMBO Rep. 2005 Jul;6(7):649-55
references.pmid: 15947783
references.citation: Curr Biol. 2002 Jul 9;12(13):1138-44
references.pmid: 12121623
references.citation: EMBO J. 2002 Dec 2;21(23):6549-59
references.pmid: 12456661
references.citation: Cell. 2005 Mar 11;120(5):613-22
references.pmid: 15766525