Methylation of CG dinucleotides (mCG), which regulates eukaryotic genome functions, is epigenetically propagated by Dnmt1/MET1 methyltransferases. How mCG is established and transmitted across generations despite imperfect enzyme fidelity remains mysterious. Here we show that MET1 de novo activity, which is enhanced by existing proximate methylation, seeds and stabilizes mCG in Arabidopsis thaliana genes. MET1 activity is restricted by active demethylation and suppressed by histone variant H2A.Z. Based on these observations, we develop a mathematical model that precisely recapitulates mCG inheritance dynamics and predicts intragenic mCG patterns and their population-scale variation given only CG site spacing as input. The model reveals that intragenic mCG undergoes large, millennia-long epigenetic fluctuations, and can therefore mediate evolution on this timescale. Our results demonstrate how genic methylation patterns are created, reconcile imperfect mCG maintenance with long-term stability, and establish a quantitative model that unifies the establishment and epigenetic inheritance of mCG.

In plants, demethylation of 5-methylcytosine (5 mC) residues is controlled by DNA glycosylases, while in mammals it requires oxidation of 5 mC by TET proteins, a group of Fe(II)/2-oxoglutaratedependent dioxygenases. We analysed the effects of expressing the C-terminal catalytic domain of the human TET3 gene (TET3c) in Arabidopsis thaliana, using an rDNA region as a methylation reporter. In TET3c transformants, epialleles with hypomethylation or hypermethylation patterns can be induced, which is each stably retained in progeny lines even after removal of the TET3c transgene. In TET3c transformants, 5-hydroxymethylcytosine (5 hmC) marks are detected, indicative of the oxidative activity of the transgenic enzyme. 5-formylcytosine (5 fC) is only detectable in TET3c transformants with a DNA glycosylase mutant background suggesting further oxidation of 5 hmC residues to 5 fC by TET3c, and efficient recognition and removal of 5 fC by plant glycosylases. The results suggest that TET3c can be employed to induce heritable locus-specific changes in DNA methylation, and that accumulation of 5 hmC can be used as a marker for TET3c target regions.

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Cytosine methylation within CG dinucleotides (mCG) can be epigenetically inherited over many generations. Such inheritance is thought to be mediated by a semiconservative mechanism that produces binary present/absent methylation patterns. However, we show here that in Arabidopsis thaliana h1ddm1 mutants, intermediate heterochromatic mCG is stably inherited across many generations and is quantitatively associated with transposon expression. We develop a mathematical model that estimates the rates of semiconservative maintenance failure and de novo methylation at each transposon, demonstrating that mCG can be stably inherited at any level via a dynamic balance of these activities. We find that DRM2 – the core methyltransferase of the RNA-directed DNA methylation pathway – catalyzes most of the heterochromatic de novo mCG, with de novo rates orders of magnitude higher than previously thought, whereas chromomethylases make smaller contributions. Our results demonstrate that stable epigenetic inheritance of mCG in plant heterochromatin is enabled by extensive de novo methylation.