Integrating long-day flowering signals: A LEAFY binding site is essential for proper photoperiodic activation of APETALA1.

The transition to flowering inArabidopsisis characterized by the sharp and localized upregulation ofAPETALA1(AP1) transcription in the newly formed floral primordia. Both the flower meristem-identity geneLEAFY(LFY) and the photoperiod pathway involving theFLOWERING LOCUS T(FT) andFDgenes contribute tothis upregulation. These pathways have been proposed to act independently but their respective contributionsand mode of interaction have remained elusive. To address these questions, we studied theAP1regulatoryregion. Combiningin vitroandin vivoapproaches, we identified which of the three putative LFY binding sitespresent in theAP1promoter is essential for its activation by LFY. Interestingly, we found that this site is alsoimportant for the correct photoperiodic-dependent upregulation ofAP1. In contrast, a previously proposedputative FD-binding site appears dispensable and unable to bind FD and we found no evidence for FD bindingto other sites in theAP1promoter, suggesting that the FT/FD-dependent activation ofAP1might be indirect.Altogether, our data give new insight into the interaction between the FT and LFY pathways in theupregulation ofAP1transcription under long-day conditions.