Photoperiodic flowering in Arabidopsis: Multilayered regulatory mechanisms of CONSTANS and the florigen FLOWERING LOCUS T.

The timing of flowering affects the success of sexual reproduction. This developmental event also determines crop yield, biomass, and longevity. Therefore, this mechanism has been targeted for improvement along with crop domestication. The underlying mechanisms of flowering are highly conserved in angiosperms. Central to these mechanisms is how environmental and endogenous conditions control transcriptional regulation of the FLOWERING LOCUS T (FT) gene, which initiates floral development under long-day conditions in Arabidopsis. Since the identification of FT as florigen, efforts have been made to understand the regulatory mechanisms of FT expression. Although many transcriptional regulators have been shown to directly influence FT, the question of how they coordinately control the spatiotemporal expression patterns of FT still requires further investigation. Among FT regulators, CONSTANS (CO) is the primary one whose protein stability is tightly controlled by phosphorylation and ubiquitination/proteasome-mediated mechanisms. In addition, various CO interaction partners, some of them previously identified as FT transcriptional regulators, positively or negatively modulate CO protein activity. The FT promoter possesses several transcriptional regulatory "blocks," highly conserved regions among Brassicaceae plants. Different transcription factors bind to specific blocks and affect FT expression, often causing topological changes in FT chromatin structure, such as the formation of DNA loops. We discuss the current understanding of the regulation of FT expression mainly in Arabidopsis and propose future directions related to this topic.