Seasonal shift in timing of vernalization as an adaptation to extreme winter

Many plant species require vernalization, a need to experience prolonged cold before flowering, as a mechanism to align reproduction with favourable spring conditions. In Arabidopsis thaliana vernalization depends on the cold-induced epigenetic silencing of the floral repressor locus FLC and is generally considered to occur as plants overwinter. Here, through analysis of vernalization temperature requirement in several A. thaliana accessions we discover the timing of vernalization alters if winter conditions are extreme. Vernalization was found to be ineffective at 0⁰C, still relatively effective at 14⁰C and differentially effective at intermediate temperatures in a range of A. thaliana accessions. The Lov-1 accession from N. Sweden showed greatest dependence on specific temperature, with vernalization occurring most efficiently at 8⁰C. Comparison of the Lov-1 vernalization temperature profile to the average historical autumn temperature at the Lov-1 collection site predicted vernalization should occur in autumn. This prediction was confirmed using field transplantation experiments in N. Sweden. Vernalization was complete before snowfall, with plants flowering immediately upon snowmelt in spring. The differential temperature requirement for vernalization in Lov-1 requires a few non-coding SNPs in the floral repressor locus, FLC, which alter chromatin silencing. As well as informing mechanisms of adaptive evolution these data have important implications for modeling flowering time and predicting consequences of climate change.