Distinct phases of Polycomb silencing to hold epigenetic memory of cold in Arabidopsis ($)

Some plants can only flower after a cold period. This cold period has been shown to cause epigenetic gene silencing of an inhibitor of flowering, which in Arabidopsis is encoded by FLC. Yang and Berry et al. explore the mechanism by which FLC is epigenetically silenced in response to cold. Previous studies showed that the Polycomb Repressive Complex 2 (PRC2), including the subunit CURLY LEAF (CLF), is required for FLC silencing. The authors explored the extent of histone methylation and FLC silencing in various mutant backgrounds. Mutation of clf allowed initial but impersistent silencing.  These results suggest a two-step silencing process, starting with nucleation of the histone marks followed by spreading of these silencing marks across the locus, and they propose a model in which spreading is required for long-term silencing that persists through cell divisions. Science 10.1126/science.aan1121