Auto-downregulation of the florigen FT production prevents precocious flowering in plants

In many flowering plants, the developmental switch from vegetative growth to reproduction (flowering) is timed by seasonal changes in the length of daylight (photoperiod). Under inductive day lengths, the photoperiod pathway typically generates rhythmic expression of a transcriptional activator for florigen production. In the facultative long-day plant Arabidopsis thaliana, long-day exposure results in increasing buildup of the CONSTANS (CO) protein towards the end of daylight, and CO activates the expression of the major florigen gene FLOWERING LOCUS T (FT) to confer long-day induction of flowering. CO-mediated FT activation must be properly controlled to prevent an excessive florigen production and precocious flowering under inductive long days, 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 in plants, ensuring the floral transition at a proper time to balance vegetative growth with reproductive success and maximize plant production.