ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 388

Concretely, Arabidopsis is a facultative long-day plant capable of flowering
in both long-day and short-day conditions, but the floral transition occurs much
earlier in long-day conditions (Hayama and Coupland 2004 ). The interactions
between the light signals and the circadian clock converge in CONSTANS ( CO ),
a key gene in the photoperiodic regulation of the flowering transition. CO gene
expression, protein stability, and activity are regulated by light/dark cycles and
the circadian clock (Suarez-Lopez et al. 2001 ; Yanovsky and Kay 2002 ). In long-
day conditions, the expression of CO is restricted to the afternoon, whereas under
short-day conditions its expression is mainly shifted to the dark period (Suarez-
Lopez et al. 2001 ). This CO oscillatory expression has been proven to be essen-
tial for the photoperiodic discrimination (Yanovsky and Kay 2002 ) and therefore
extensively studied. To precisely control the diurnal CO expression, in the morning,
CO is repressed by CYCLING DOF FACTORs (CDFs) through direct binding to
its promoter region (Fornara et al. 2009 ; Imaizumi et al. 2005 ). At the same time,
CCA1/LHY represses the CO activators FLAVIN - BINDING, KELCH REPEAT,
F - BOX 1 ( FKF1 ) and GI . In the afternoon, repression of FKF1 and GI is released,
which form a blue light-dependent complex. The FKF1-GI complex removes CDF
repressors via proteasomal degradation (Sawa et al. 2007 ). At the same time, the
CDF expression is downregulated sequentially by PRR9, PRR7, PRR5, and TOC1
(Nakamichi et al. 2009 , 2012 ; Ito et al. 2008 ), thus ensuring CO expression.
Functionally, CO directly promotes FLOWERING LOCUS T ( FT ) expression,
one of the major florigens. FT is synthesized in the leaf vascular tissue and trans-
ported through the phloem to the meristem where, together with other factors, it
promotes the transcription reprogramming that is going to allow the floral transi-
tion (Kobayashi and Weigel 2007 ; Turck et al. 2008 ). CO activation of FT depends
at least in part on the action of the photoreceptors, PHYA, CRY1, and CRY2
(Kardailsky et al. 1999 ; Kobayashi et al. 1999 ; Samach et al. 2000 ). These photo-
receptors promote the stabilization of CO protein and stability, whereas PHYB and
SUPRESSOR OF PHYA-105 (SPA1) stimulate CO degradation (Ishikawa et al.
2006 ; Laubinger et al. 2006 ; Valverde et al. 2004 ).
Preventing photoperiodic flowering induction under non-favorable environ-
mental conditions is essential for the reproductive success. Therefore, ABA, being
the major stress-response hormone, might play an important role. Indeed, ABA
is considered a flowering repressor. External addition of ABA delays flowering,
whereas ABA-deficient mutants show early flowering phenotype (Achard et al.
2006 ; Barrero et al. 2005 ). Considering the pervasiveness of ABA and circadian
clock interaction, it is most likely that ABA may play also a role in the photo-
periodic flowering pathway. In this sense, exogenous ABA lengthens the clock
period (Hanano et al. 2006 ) and therefore is possible that, under photoperiodic
inductive conditions, ABA might delay CO expression by shifting its peak to the
dark period, thus inhibiting flowering. This is also consistent with the ABA acute
induction of TOC1 because its precise expression has been shown to be essential
to sense variations on the photoperiod (Perales and Mas 2007 ).
Although ABA may delay flowering partially by its interaction with the
photoperiodic pathway, its inhibitory role has been majorly associated with