Vernal ization-induced trimethylation of histone H3 lysine 27 at FLC is not maintainedin mitotically quiescent cells

Vernalization promotes flowering in Arabidopsis through epigenetic repression of the floral repressor, FLOWERING LOCUS C (FLC) [1-3]. Vernalization, like other polycomb-mediated repression events [4], occurs in two stages; FLCrepression is established at low temperatures, then maintained during subsequent growth at 22 degrees C [5, 6]. Low temperatures induce VIN3 activity, which is required for changes in histone modifications and the associated FLC repression [3]. Plant polycomb proteins FIE, VRN2, CLF, and SWIN, together with VIN3, form a complex that adds histone H3 lysine 27 methylation at FLC in vernalized plants [2,3,7]. VRN1 and LHP1 are required for maintenance of FLC repression [2, 3, 6, 8]. Tissue must be undergoing cell division during lowtemperature treatments for acceleration of flowering to occur [9, 10]. We show that low-temperature treatments repress FLC in cells that are not mitotically active, but this repression is notfully maintained. Trimethyl-lysine 27 (K27me3), is enriched at the start of the FLC gene during the cold, before spreading across the locus after vernalization. In the absence of DNA replication, K27me3 is added to chromatin at the start of FLC but is removed on return to 22 degrees C. This suggests that DNA replication is essential for maintenance of vernalization-induced repression of FLC.