Polycomb repression: It's all in the balance.

In our recent paper1 we suggested a molecular explanation for the long standing observation that plants need to be mitotically active to respond to a prolonged period of low temperatures by flowering early (vernalization).2 In Arabidopsis, vernalization is associated with the epigenetic repression of the floral repressor, FLC.3-5FLC repression is established during the low temperature treatment and is marked by the loss of chromatin marks associated with active genes and the gain of histone H3 trimethyl-lysine 27 (K27me3) at the start of transcription/translation.1 After the end of the cold treatment, this repressive modification spreads across FLC chromatin to mark the entire locus.1 In cells not undergoing mitosis, we found that FLC is repressed by low temperatures, but that this repression is only partially maintained. We concluded that DNA replication is not required for the initial response to low temperatures, but rather for the maintenance of this response. Here we discuss the implications of our observations in terms of the plasticity of chromatin modifications in plants.