Seasonal changes in day length influence flowering time in
many plant species. In Arabidopsis, flowering is accelerated
by exposure to long day (LD). Those inductive photoperiods
are perceived in leaves [1] and initiate a long-distance signaling
mediated by CO and FT. CO is expressed in the phloem
according to a circadian rhythm [2–4]. Only under LD does
CO induce FT expression as high levels of CO in the evening
coincide with the external light that stabilizes CO protein [4,
5]. Subsequently, FT protein travels through the phloem to
the shoot apex where, together with FD, it initiates flowering
[6–12]. Despite the photoperiodic induction, a mechanism of
floral repression is needed to avoid precocious flowering.
We show that TEMPRANILLO genes (TEM1 and TEM2) act
as novel direct FT repressors. Molecular and genetic analyses
suggest that a quantitative balance between the activator
CO and the repressor TEM determines FT levels. Moreover,
developmental TEM downregulation marks the timing
of flowering, as it shifts the CO/TEM balance in favor of CO
activity, allowing FT transcript to reach the threshold level
required to trigger flowering. We envision that this might
be a general mechanism between long-day plants to ensure
a tight regulation of flowering time.