背景回顾：Cytosine methylation is a reversible epigenetic modification of DNA. In plants, removal of cytosine methylation is accomplished by the four members of the DEMETER (DME) family of 5-methylcytosine DNA glycosylases, named DME, DEMETER-LIKE2 (DML2), DML3, and REPRESSOR OF SILENCING1 (ROS1) in Arabidopsis thaliana. 

提出问题：Demethylation by DME is critical for seed development, preventing experiments to determine the function of the entire gene family in somatic tissues by mutant analysis. 

主要研究：Here, we bypassed the reproductive defects of dme mutants to create somatic quadruple homozygous mutants of the entire DME family. 

结果1：dme; ros1; dml2; dml3 (drdd) leaves exhibit hypermethylated regions compared to wild-type leaves and rdd triple mutants, indicating functional redundancy among all four demethylases. 

结果2：Targets of demethylation include regions co-targeted by RNA-directed DNA methylation and, surprisingly, CG gene body methylation, indicating dynamic methylation at these less-understood sites. 

结果3：Additionally, many tissue-specific methylation differences are absent in drdd, suggesting a role for active demethylation in generating divergent epigenetic states across wild-type tissues. 

结果4：Furthermore, drdd plants display an early flowering phenotype, which involves 5’ hypermethylation and transcriptional down-regulation of FLOWERING LOCUS C. 

结论：Active DNA demethylation is therefore required for proper methylation across somatic tissues and defines the epigenetic landscape of intergenic and coding regions.

 摘 要 

胞嘧啶甲基化是DNA的可逆表观修饰。在拟南芥中，胞嘧啶甲基化的移除是由5-甲基胞嘧啶DNA糖基化酶DME家族的四个成员，即DME、DML2、DML3和ROS1来完成的。DME的去甲基化对种子的发育至关重要，这阻碍了通过突变来分析体细胞组织中整个基因家族的功能。本文中，作者绕过了dme突变的生殖缺陷，创制了整个DME家族的体细胞四突纯合体。与野生型和rdd三突植株的叶片相比，dme; ros1; dml2; dml3（drdd）四突植株的叶片存在高甲基化区域，表明所有四种去甲基酶之间存在功能冗余。去甲基化的靶标包括由RNA指导的DNA甲基化和CG基因体甲基化共靶向的区域，揭示了这些位点上的甲基化动态。此外，drdd四突中缺少许多组织特异性的甲基化差异，这表明野生型不同组织中活跃的去甲基化产生了不同的表观状态。此外，drdd四突植物表现出早花表型，这涉及了FLC基因的5'超甲基化和转录下调。因此，活跃的DNA去甲基化是体细胞组织适当甲基化所必需的，并决定了基因间和编码区的表观状态。