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.
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.
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