The MET1b gene encoding a maintenance DNA methyltransferase is indispensable for normal development in rice
The MET1b gene encoding a maintenance DNA methyltransferase is indispensable for normal development in rice
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DOI：
10.1007/s11103-014-0178-9
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发表时间：
2014-02
影响因子：
5.1
通讯作者：
Takaki Yamauchi;Y. Johzuka-Hisatomi;R. Terada;I. Nakamura;S. Iida
中科院分区：
文献类型：
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作者：
Takaki Yamauchi;Y. Johzuka-Hisatomi;R. Terada;I. Nakamura;S. Iida
拟南芥只有一个MET 1基因编码DNA甲基转移酶，主要负责维持DNA复制后的CG甲基化，而水稻携带两个MET 1基因，MET 1a和MET 1b，在活跃复制和分裂的细胞中表达，MET 1b比MET 1a表达量更高。Amet 1anull突变体没有表现出明显的表型，这意味着MET 1b必须在维持DNA甲基化中发挥主要作用。在这里，我们采用了两个met 1bnull突变体，产生的同源重组介导的敲入靶向和插入内源性反转录转座子Tos 17。这些MET 1a/MET 1a met 1b/met 1b纯合子表现出异常的种子表型，这与胎生萌发或早期胚胎致死有关。他们还显示出在胚胎中重复CentO序列和FIE 1基因位点的DNA甲基化水平降低。此外，独立分离的敲入靶向植物，其中启动子的GUS报告基因与内源MET 1b启动子融合，表现出可重复的，剂量依赖性，和时空表达模式的GUS。通过对异源met 1a-met 1b-null突变体自交后代的基因分型分析表明，弱活性的met 1a在水稻met 1b配子体中似乎是一种遗传备份机制，尽管在水稻met 1b纯合子中表观遗传备份机制的随机和不协调激活的效率低于拟南芥met 1纯合子。此外，在减数分裂后的DNA复制过程中，CG甲基化的被动耗竭导致水稻配子体的早期胚胎致死。这种情况有点类似于拟南芥配子体的情况。
WhileArabidopsisbears only oneMET1gene encoding the DNA methyltransferase that is mainly responsible for maintaining CG methylation after DNA replication, rice carries twoMET1genes,MET1aandMET1b, expressed in actively replicating and dividing cells, andMET1bis more abundantly expressed than isMET1a. Amet1anull mutant displayed no overt phenotypes, implying that MET1b must play a major role in the maintenance DNA methylation. Here, we employed twomet1bnull mutants, generated by homologous recombination-mediated knock-in targeting and insertion of endogenous retrotransposonTos17. TheseMET1a/MET1a met1b/met1bhomozygotes exhibited abnormal seed phenotypes, which is associated with either viviparous germination or early embryonic lethality. They also displayed decreased levels of DNA methylation at repetitive CentO sequences and at theFIE1gene locus in the embryos. In addition, independently isolated knock-in-targeted plants, in which the promoterlessGUSreporter gene was fused with the endogenousMET1bpromoter, showed the reproducible, dosage-dependent, and spatiotemporal expression patterns ofGUS. The genotyping analysis of selfed progeny of heterozygousmet1a met1bnull mutants indicated that weakly active MET1a seems to serve as a genetic backup mechanism in ricemet1bgametophytes, although the stochastic and uncoordinated activation of epigenetic backup mechanisms occurred less efficiently in themet1bhomozygotes of rice than in themet1homozygotes ofArabidopsis. Moreover, passive depletion of CG methylation during the postmeiotic DNA replication in the haploid nuclei of themet1a met1bgametophytes in rice results in early embryonic lethality. This situation somewhat resembles that of themet1gametophytes inArabidopsis.