Yaari Rafael, Noy-Malka Chen, Wiedemann Gertrud, Auerbach Gershovitz Nitzan, Reski Ralf, Katz Aviva, Ohad Nir
Department of Molecular Biology and Ecology of Plants, Tel-Aviv University, 69978, Tel Aviv, Israel.
Plant Mol Biol. 2015 Jul;88(4-5):387-400. doi: 10.1007/s11103-015-0328-8. Epub 2015 May 6.
DNA methylation has a crucial role in plant development regulating gene expression and silencing of transposable elements. Maintenance DNA methylation in plants occurs at symmetrical (m)CG and (m)CHG contexts ((m) = methylated) and is maintained by DNA METHYLTRANSFERASE 1 (MET1) and CHROMOMETHYLASE (CMT) DNA methyltransferase protein families, respectively. While angiosperm genomes encode for several members of MET1 and CMT families, the moss Physcomitrella patens, serving as a model for early divergent land plants, carries a single member of each family. To determine the function of P. patens PpMET we generated ΔPpmet deletion mutant which lost (m)CG and unexpectedly (m)CCG methylation at loci tested. In order to evaluate the extent of (m)CCG methylation by MET1, we reexamined the Arabidopsis thaliana Atmet1 mutant methylome and found a similar pattern of methylation loss, suggesting that maintenance of DNA methylation by MET1 is conserved through land plant evolution. While ΔPpmet displayed no phenotypic alterations during its gametophytic phase, it failed to develop sporophytes, indicating that PpMET plays a role in gametogenesis or early sporophyte development. Expression array analysis revealed that the deletion of PpMET resulted in upregulation of two genes and multiple repetitive sequences. In parallel, expression analysis of the previously reported ΔPpcmt mutant showed that lack of PpCMT triggers overexpression of genes. This overexpression combined with loss of (m)CHG and its pleiotropic phenotype, implies that PpCMT has an essential evolutionary conserved role in the epigenetic control of gene expression. Collectively, our results suggest functional conservation of MET1 and CMT families during land plant evolution. A model describing the relationship between MET1 and CMT in CCG methylation is presented.
DNA甲基化在植物发育过程中对基因表达调控和转座元件沉默起着关键作用。植物中维持性DNA甲基化发生在对称的（m）CG和（m）CHG序列环境中（（m）=甲基化），分别由DNA甲基转移酶1（MET1）和染色质甲基化酶（CMT）DNA甲基转移酶蛋白家族维持。虽然被子植物基因组编码了MET1和CMT家族的多个成员，但作为早期分化陆地植物模型的小立碗藓，每个家族仅携带一个成员。为了确定小立碗藓PpMET的功能，我们构建了ΔPpmet缺失突变体，该突变体在测试位点失去了（m）CG甲基化，并且意外地失去了（m）CCG甲基化。为了评估MET1对（m）CCG甲基化的影响程度，我们重新检测了拟南芥Atmet1突变体的甲基化组，发现了类似的甲基化缺失模式，这表明MET1介导的DNA甲基化维持在陆地植物进化过程中是保守的。虽然ΔPpmet在配子体阶段没有表现出表型改变，但它无法发育出孢子体，这表明PpMET在配子发生或早期孢子体发育中发挥作用。表达阵列分析显示，PpMET的缺失导致两个基因和多个重复序列的上调。同时，对先前报道的ΔPpcmt突变体的表达分析表明，缺乏PpCMT会触发基因的过表达。这种过表达与（m）CHG的缺失及其多效性表型相结合，意味着PpCMT在基因表达的表观遗传控制中具有重要的进化保守作用。总的来说，我们的结果表明MET1和CMT家族在陆地植物进化过程中具有功能保守性。本文提出了一个描述MET1和CMT在CCG甲基化中关系的模型。
