Heat can erase epigenetic marks of vernalization in Arabidopsis

Vernalization establishes a memory ofwinter that must be maintained for weeks or months in order to promote flowering the following spring. The sta-bility of the vernalized state varies among plant species and depends on the dura-tion of cold exposure. In Arabidopsis thaliana, winter leads to epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC) and the duration of cold is measured through the dynamics of chromatin modifications during and after cold. The growing con-ditions encountered post-vernalization are thus critical for the maintenance of the vernalized state. We reported that high temperature leads to devernalization and, consistently, to FLC reactivation in Arabidopsis seedlings. Here we show that the repressive epigenetic mark H3K27me3 decreases at the FLC locus when vernalized seedlings are grown at 30C, unless they were first exposed to a stabilizing period at 20C. Ambient tem-perature thus controls the epigenetic memory of winter. Vernalization-requiring plants are unable to flower or flower very late unless they are exposed to prolonged cold tem-peratures. The ecological significance of this trait is to block flowering prior to winter and permit the floral transition under the favorable conditions of follow-ing spring, when ambient temperature and daylength increase.1 In Arabidopsis thaliana, vernalization requirement is largely due to the high expression level of the FLOWERING LOCUS C (FLC) gene encoding a MADS-domain protein.2,3 FLC acts as a transcriptional inhibitor of the flowering integrator genes FLOWER