Replication-uncoupled CG methylation
In most eukaryotes, CG methylation (mCG) is an essential epigenetic modification, yet the mechanisms underlying its stable maintenance across cell divisions remain elusive. Recent findings in mammals suggest that mCG maintenance includes a replication-uncoupled phase during G2 phase.  Whether a similar phase occurs in plants is still unknown. By developing methylation-sensitive reporters coupled with cell cycle markers, we found that mCG maintenance is incomplete during S phase and extends to G2 in Arabidopsis, with MET1 binding chromatin from S to G2. Consistently, we observed that, similar to DNMT1, MET1 methylates DNA wrapped around nucleosomes. Duplex sequencing of mutants deficient of confounding enzymes reveals that nucleosomal CG methylation is shaped by chromatin accessibility, indicating methylation of chromatinized DNA. Moreover, our findings indicate that this replication-uncoupled methylation not only enables stable mCG inheritance but also facilitates restoration of accidentally lost mCG and generates stochastic epimutations that accumulate in nucleosomal DNA.
In most eukaryotes, CG methylation (mCG) is an essential epigenetic modification, yet the mechanisms underlying its stable maintenance across cell divisions remain elusive. Recent findings in mammals suggest that mCG maintenance includes a replication-uncoupled phase during G2 phase.  Whether a similar phase occurs in plants is still unknown. By developing methylation-sensitive reporters coupled with cell cycle markers, we found that mCG maintenance is incomplete during S phase and extends to G2 in Arabidopsis, with MET1 binding chromatin from S to G2. Consistently, we observed that, similar to DNMT1, MET1 methylates DNA wrapped around nucleosomes. Duplex sequencing of mutants deficient of confounding enzymes reveals that nucleosomal CG methylation is shaped by chromatin accessibility, indicating methylation of chromatinized DNA. Moreover, our findings indicate that this replication-uncoupled methylation not only enables stable mCG inheritance but also facilitates restoration of accidentally lost mCG and generates stochastic epimutations that accumulate in nucleosomal DNA.
Institut de Sciences des Plantes - Paris-Saclay