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. Author manuscript; available in PMC: 2013 Jul 3.
Published in final edited form as: Cold Spring Harb Symp Quant Biol. 2012 Nov 28;77:161–173. doi: 10.1101/sqb.2012.77.014936

Figure 1.

Figure 1

Global DNA demethylation during Arabidopsis early development. (A) Multicellular female gametophyte consisting of three antipodal cells (AC), two synergid cells (Syn), one haploid egg cell (EC), and one diploid central cell nucleus (CCN). DME is expressed in the central cell before fertilization and thereby may account for the global demethylation in the maternal endosperm genome (Gehring et al. 2009; Hsieh et al. 2009). (B) Tricellular male gametophyte consisting of two sperm cells (SC) and one vegetative cell nucleus (VCN). Desilencing of transposable elements (TEs) and production of 21-nucleotide siRNA occur only in the VCN (Slotkin et al. 2009). These 21-nt siRNA may travel to SC and reinforce TE silencing. (C) The endosperm and embryo are produced after double fertilization. The endosperm exhibits genome-wide hypomethylation compared with the embryo, with TEs being extensively demethylated. Compared with aerial tissues, both endosperm and embryo show local CHH hypermethylation in TE loci targeted by siRNA, which are likely produced by transcriptional activation of endosperm TEs (Gehring et al. 2009; Hsieh et al. 2009). The 21-nucleotide siRNA originating from the male gametophyte may contribute to posttranscriptional silencing of TE primary transcripts in the endosperm.