Kumar Sushil, Kumari Renu, Sharma Vishakha, Sharma Vinay
National Institute of Plant Genome Research (NIPGR), Aruna Asaf Ali Marg, New Delhi 110 067, India.
J Genet. 2013 Dec;92(3):629-66. doi: 10.1007/s12041-013-0273-8.
Heritable information in plants consists of genomic information in DNA sequence and epigenetic information superimposed on DNA sequence. The latter is in the form of cytosine methylation at CG, CHG and CHH elements (where H = A, T orC) and a variety of histone modifications in nucleosomes. The epialleles arising from cytosine methylation marks on the nuclear genomic loci have better heritability than the epiallelic variation due to chromatin marks. Phenotypic variation is increased manifold by epiallele comprised methylomes. Plants (angiosperms) have highly conserved genetic mechanisms to establish, maintain or erase cytosine methylation from epialleles. The methylation marks in plants fluctuate according to the cell/tissue/organ in the vegetative and reproductive phases of plant life cycle. They also change according to environment. Epialleles arise by gain or loss of cytosine methylation marks on genes. The changes occur due to the imperfection of the processes that establish and maintain the marks and on account of spontaneous and stress imposed removal of marks. Cytosine methylation pattern acquired in response to abiotic or biotic stress is often inherited over one to several subsequent generations.Cytosine methylation marks affect physiological functions of plants via their effect(s) on gene expression levels. They also repress transposable elements that are abundantly present in plant genomes. The density of their distribution along chromosome lengths affects meiotic recombination rate, while their removal increases mutation rate. Transposon activation due to loss of methylation causes rearrangements such that new gene regulatory networks arise and genes for microRNAs may originate. Cytosine methylation dynamics contribute to evolutionary changes. This review presents and discusses the available evidence on origin, removal and roles of cytosine methylation and on related processes, such as RNA directed DNA methylation, imprinting, paramutation and transgenerational memory in plants.
植物中的可遗传信息由DNA序列中的基因组信息以及叠加在DNA序列上的表观遗传信息组成。后者以CG、CHG和CHH元件（其中H = A、T或C）处的胞嘧啶甲基化以及核小体中多种组蛋白修饰的形式存在。由核基因组位点上的胞嘧啶甲基化标记产生的表观等位基因比由染色质标记引起的表观等位基因变异具有更好的遗传性。由甲基化组构成的表观等位基因使表型变异大幅增加。植物（被子植物）具有高度保守的遗传机制来建立、维持或消除表观等位基因上的胞嘧啶甲基化。植物生命周期中营养和生殖阶段的甲基化标记会根据细胞/组织/器官而波动。它们也会因环境而改变。表观等位基因通过基因上胞嘧啶甲基化标记的获得或丢失而产生。这些变化是由于建立和维持标记的过程不完善以及自发和应激导致的标记去除。响应非生物或生物胁迫而获得的胞嘧啶甲基化模式通常会在随后的一到几代中遗传。胞嘧啶甲基化标记通过影响基因表达水平来影响植物的生理功能。它们还抑制植物基因组中大量存在的转座元件。它们沿染色体长度的分布密度影响减数分裂重组率，而其去除会增加突变率。由于甲基化缺失导致的转座子激活会引起重排，从而产生新的基因调控网络，并且可能产生microRNA的基因。胞嘧啶甲基化动态变化促成了进化改变。本综述展示并讨论了关于植物中胞嘧啶甲基化的起源、去除和作用以及相关过程（如RNA指导的DNA甲基化、印记、副突变和跨代记忆）的现有证据。
