SUMMARY Mammalian oocytes can reprogram somatic cells into a totipotent state enabling animal cloning through somatic cell nuclear transfer (SCNT). However, the majority of SCNT embryos fail to develop to term due to undefined reprogramming defects. Here we identify histone H3 lysine 9 trimethylation (H3K9me3) of donor cell genome as a major epigenetic barrier for efficient reprogramming by SCNT. Comparative transcriptome analysis identified reprogramming resistant regions (RRRs) that are expressed normally at 2-cell mouse embryos generated by IVF but not SCNT. RRRs are enriched for H3K9me3 in donor somatic cells, and its removal by ectopic expression of the H3K9me3 demethylase Kdm4d not only reactivates the majority of RRRs, but also greatly improves SCNT efficiency. Furthermore, use of donor somatic nuclei depleted of H3K9 methyltransferases markedly improves SCNT efficiency. Our study thus identifies H3K9me3 as a critical epigenetic barrier in SCNT-mediated reprogramming and provides a promising approach for improving mammalian cloning efficiency.

Histone methylation plays a fundamental role in regulating diverse developmental processes and is also involved in silencing repetitive sequences in order to maintain genome stability. The methylation marks are written on lysine or arginine by distinct enzymes, namely, histone lysine methyltransferases (HKMTs) or protein arginine methyltransferases (PRMTs). Once established, the methylation marks are specifically recognized by the proteins that act as readers and are interpreted into specific biological outcomes. Histone methylation status is dynamic; methylation marks can be removed by eraser enzymes, the histone demethylases (HDMs). The proteins responsible for writing, reading, and erasing the methylation marks are known mostly in animals. During the past several years, a growing body of literature has demonstrated the impact of histone methylation on genome management, transcriptional regulation, and development in plants. The aim of this review is to summarize the biochemical, genetic, and molecular action of histone methylation in two plants, the dicot Arabidopsis and the monocot rice.

Mammalian sperm and oocytes have different epigenetic landscapes and are organized in different fashions. After fertilization, the initially distinct parental epigenomes become largely equalized with the exception of certain loci, including imprinting control regions. How parental chromatin becomes equalized and how imprinting control regions escape from this reprogramming is largely unknown. Here we profile parental allele-specific DNase I hypersensitive sites in mouse zygotes and morula embryos, and investigate the epigenetic mechanisms underlying these allelic sites. Integrated analyses of DNA methylome and tri-methylation at lysine 27 of histone H3 (H3K27me3) chromatin immunoprecipitation followed by sequencing identify 76 genes with paternal allele-specific DNase I hypersensitive sites that are devoid of DNA methylation but harbour maternal allele-specific H3K27me3. Interestingly, these genes are paternally expressed in preimplantation embryos, and ectopic removal of H3K27me3 induces maternal allele expression. H3K27me3-dependent imprinting is largely lost in the embryonic cell lineage, but at least five genes maintain their imprinted expression in the extra-embryonic cell lineage. The five genes include all paternally expressed autosomal imprinted genes previously demonstrated to be independent of oocyte DNA methylation. Thus, our study identifies maternal H3K27me3 as a DNA methylation-independent imprinting mechanism.