Vernalization-triggered expression of the antisense transcriptCOOLAIRis mediated byCBFgenes

AbstractTo synchronize flowering time with spring, many plants undergo vernalization, a floral-promotion process triggered by exposure to long-term winter cold. InArabidopsis thaliana, this is achieved through cold-mediated epigenetic silencing of the floral repressor,FLOWERING LOCUS C(FLC).COOLAIR, a cold-induced antisense RNA transcribed from theFLClocus, has been proposed to facilitateFLCsilencing. Here, we show that C-repeat (CRT)/dehydration-responsive elements (DREs) at the 3′-end ofFLCand CRT/DRE-binding factors (CBFs) are required for cold-mediated expression ofCOOLAIR. CBFs bind to CRT/DREs at the 3′-end ofFLC, bothin vitroandin vivo, and CBF levels increase gradually during vernalization. Cold-inducedCOOLAIRexpression is severely impaired incbfsmutants in which allCBFgenes are knocked-out. Conversely,CBF-overexpressing plants show increasedCOOLAIRlevels even at warm temperatures. We show thatCOOLAIRis induced by CBFs during early stages of vernalization but COOLAIR levels decrease in later phases asFLCchromatin transitions to an inactive state to which CBFs can no longer bind. We also demonstrate thatcbfsandFLCΔCOOLAIRmutants exhibit a normal vernalization response despite their inability to activateCOOLAIRexpression during cold, revealing thatCOOLAIRis not required for the vernalization process.