Auto-Downregulation of the Florigen FT Production Prevents Precocious Flowering in Plants

In many plants, flowering is timed by seasonal changes in the length of daylight (photoperiod). In Arabidopsis thaliana, long-day exposure results in an increasing buildup of the CONSTANS (CO) protein toward the end of daylight, which activates the expression of the major florigen gene FLOWERING LOCUS T (FT) to induce flowering. CO-mediated FT activation must be properly controlled to prevent an excessive florigen production and precocious flowering under inductive photoperiods, but the underlying mechanism remains elusive. Here, we report an auto-repression mechanism to prevent excessive FT production in inductive photoperiods. We show that the transcription factor FD is expressed in leaf veins and complexes with FT to recognize several cis-regulatory DNA motifs in FT promoter. FT-FD antagonizes CO-mediated FT activation to feedback down-regulate FT expression and thus prevent its excessive induction by long-day signals, thereby precluding precocious transition to flowering. Furthermore, we found that in the facultative short-day plant soybean, an FT homolog directly represses its own expression. Thus, the auto-repression of FT or an FT homolog is a conserved mechanism to prevent excessive production of this potent floral regulator, ensuring the floral transition at a proper time to balance vegetative growth with reproductive success and maximize plant production.