Transcription factor VRT2 reinitiates vernalization when interrupted by warm temperatures in a temperate grass model

Vernalization-responsive plants use cold weather, or low temperature, as a cue to monitoring the passing of winter. Winter cereals can remember the extent of coldness they have experienced, even when winter is punctuated by warm days. However, in a seemingly unnatural process called "devernalization," hot temperatures can erase winter memory. Previous studies in bread wheat (Triticum aestivum) have implicated the MADS-box transcription factor VEGETATIVE TO REPRODUCTIVE TRANSITION 2 (VRT2) in vernalization based on transcriptional behavior and ectopic expression. Here, we characterized 3 BdVRT2 loss-of-function alleles in the temperate model grass Brachypodium distachyon. In addition to extended vernalization requirements, mutants showed delayed flowering relative to wild-type plants when exposed only briefly to warm temperatures after partial vernalization, with flowering being unaffected when vernalization was saturating. Together, these data suggest a role for BdVRT2 in both vernalization and in its reinitiation when interrupted by warm temperatures. In controlled constant conditions, BdVRT2 transcription was not strongly affected by vernalization or devernalization. Yet, by monitoring BdVRT2 expression in seasonally varying and fluctuating conditions in an unheated greenhouse, we observed strong upregulation, suggesting that its transcription is regulated by fluctuating vernalizing-devernalizing conditions. Our data suggest that devernalization by hot temperatures is not a peculiarity of domesticated cereal crops but is the extreme of the reversibility of vernalization by warm temperatures and has broader biological relevance across temperate grasses.