During development, cell type-specific epigenetic states must be duplicated accurately during mitosis to maintain cellular memory. The maintenance of epigenetic memory seems to involve the DNA replication machinery but is poorly understood. Here (), Yeonhee Choi and co-workers show that INCURVATA2 (ICU2), the catalytic subunit of DNA polymerase α in Arabidopsis, ensures the stable maintenance of histone modifications. Vernalisation, the acquisition of floral competence though exposure to prolonged cold, requires repression of Flowering Locus C (FLC) through accumulation of the histone mark H3K27me3. The researchers show that the missense mutant allele icu2-1 does not affect the recruitment of CLF, a component of polycomb repressive complex 2 (PRC2), and the resultant deposition of the H3K27me3 mark on the FLC locus. However, icu2-1 mutants do not stably maintain this epigenetic mark, which results in mosaic FLC de-repression after vernalisation. ICU2 also maintains repressive histone modifications at other PRC2 targets and at retroelements, and it facilitates histone assembly in dividing cells, which suggests a possible mechanism for ICU2-mediated epigenetic maintenance.
