期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：52
页码：15138-15143
DOI：10.1073/pnas.1619047114
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceFlowering plant reproduction involves two fertilization events: a sperm-egg fusion that forms the embryo, and a sperm-central cell fusion that forms the nutritive extraembryonic endosperm. Chromosomes in the embryo, endosperm, and other plant tissues are modified by methylation, a covalent addition to cytosine in DNA that regulates gene expression. Maternal endosperm chromosomes inherited from the central cell display a pattern of extensive demethylation, which is essential for seed development in Arabidopsis thaliana. Demethylation is presumed to occur in the central cell, but direct evidence for this is very limited. Here, we provide a genome-wide DNA methylation analysis of Arabidopsis and rice central cells, which demonstrates that the demethylation observed in the endosperm is indeed initiated in the central cell. Cytosine methylation is a DNA modification with important regulatory functions in eukaryotes. In flowering plants, sexual reproduction is accompanied by extensive DNA demethylation, which is required for proper gene expression in the endosperm, a nutritive extraembryonic seed tissue. Endosperm arises from a fusion of a sperm cell carried in the pollen and a female central cell. Endosperm DNA demethylation is observed specifically on the chromosomes inherited from the central cell in Arabidopsis thaliana, rice, and maize, and requires the DEMETER DNA demethylase in Arabidopsis. DEMETER is expressed in the central cell before fertilization, suggesting that endosperm demethylation patterns are inherited from the central cell. Down-regulation of the MET1 DNA methyltransferase has also been proposed to contribute to central cell demethylation. However, with the exception of three maize genes, central cell DNA methylation has not been directly measured, leaving the origin and mechanism of endosperm demethylation uncertain. Here, we report genome-wide analysis of DNA methylation in the central cells of Arabidopsis and rice--species that diverged 150 million years ago--as well as in rice egg cells. We find that DNA demethylation in both species is initiated in central cells, which requires DEMETER in Arabidopsis. However, we do not observe a global reduction of CG methylation that would be indicative of lowered MET1 activity; on the contrary, CG methylation efficiency is elevated in female gametes compared with nonsexual tissues. Our results demonstrate that locus-specific, active DNA demethylation in the central cell is the origin of maternal chromosome hypomethylation in the endosperm.
