Vernalization-triggered expression of the antisense transcript COOLAIR is mediated by CBF genes.

To synchronize flowering time with spring, many plants undergo vernalization, a floral-promotion process triggered by exposure to long-term winter cold. In , this is achieved through cold-mediated epigenetic silencing of the floral repressor, (). , a cold-induced antisense RNA transcribed from the locus, has been proposed to facilitate silencing. Here, we show that C-repeat (CRT)/dehydration-responsive elements (DREs) at the 3'-end of and CRT/DRE-binding factors (CBFs) are required for cold-mediated expression of . CBFs bind to CRT/DREs at the 3'-end of , both in vitro and in vivo, and CBF levels increase gradually during vernalization. Cold-induced expression is severely impaired in mutants in which all genes are knocked-out. Conversely, -overexpressing plants show increased levels even at warm temperatures. We show that is induced by CBFs during early stages of vernalization but levels decrease in later phases as chromatin transitions to an inactive state to which CBFs can no longer bind. We also demonstrate that and mutants exhibit a normal vernalization response despite their inability to activate expression during cold, revealing that is not required for the vernalization process.