The bZIP transcription factor AREB3 mediates FT signalling and floral transition at the Arabidopsis shoot apical meristem.

The floral transition occurs at the shoot apical meristem (SAM) in response to favourableexternal and internal signals. Among these signals, variations in daylength (photoperiod) actas robust seasonal cues to activate flowering. In Arabidopsis, long-day photoperiods stimulate production in the leaf vasculature of a systemic florigenic signal that is translocated tothe SAM. According to the current model, FLOWERING LOCUS T (FT), the main Arabidopsis florigen, causes transcriptional reprogramming at the SAM, so that lateral primordiaeventually acquire floral identity. FT functions as a transcriptional coregulator with the bZIPtranscription factor FD, which binds DNA at specific promoters. FD can also interact withTERMINAL FLOWER 1 (TFL1), a protein related to FT that acts as a floral repressor. Thus,the balance between FT-TFL1 at the SAM influences the expression levels of floral genestargeted by FD. Here, we show that the FD-related bZIP transcription factor AREB3, whichwas previously studied in the context of phytohormone abscisic acid signalling, is expressedat the SAM in a spatio-temporal pattern that strongly overlaps with FD and contributes to FTsignalling. Mutant analyses demonstrate that AREB3 relays FT signals redundantly withFD, and the presence of a conserved carboxy-terminal SAP motif is required for downstream signalling. AREB3 shows unique and common patterns of expression with FD, andAREB3 expression levels are negatively regulated by FD thus forming a compensatoryfeedback loop. Mutations in another bZIP, FDP, further aggravate the late flowering phenotypes of fd areb3 mutants. Therefore, multiple florigen-interacting bZIP transcription factorshave redundant functions in flowering at the SAM.