Plants grow and flower at appropriate times in a year by detecting seasons from environmental signals. For proper detection of seasons, plants should remember long-term tendency of the environment such as temperature for past certain period. We aim to reveal the molecular mechanism of this “memory” of temperature in a natural environment.

Chemical modifications of histones (histone modifications) function as “memory” mechanism in plants. In annual Arabidopsis thaliana, long-term cold exposure (vernalization) silences a flowering repressor gene, FLOWERING LOCUS C (FLC), associated with deposition of repressive histone modifications allowing the maintenance of the silencing. Through this “epigenetic memory of winter” mechanism, plants can flower after the temperature returns to warm.

We conducted our study in a natural population of perennial Arabidopsis halleri and analyzed seasonal change in FLC RNA expression and histone modifications at FLC. Statistical analysis revealed that RNA expression showed the best correlation with past temperature for 6 weeks whereas repressive histone modifications for much longer periods, i.e., 10 weeks. This indicates that change in RNA expression precedes those of repressive histone modifications, forming long-term memory of the environments.