ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 389

FLOWERING LOCUS T (FLC) (Wang et al. 2013 ), a potent flowering repressor.
ABA promotes SnRK2-mediated phosphorylation of the transcription factor ABI5,
and phosphorylated ABI5 is able to bind FLC promoter and induce its expression,
thus extending vegetative growth (Wang et al. 2013 ). Conversely, FLC expres-
sion is repressed by vernalization and autonomous pathways through modulation
of the chromatin structure (Michaels 2009 ). Furthermore, some studies indicated
that FLC might have circadian regulation (Fujiwara et al. 2010 ), indicating that
FLC might act by integrating ABA, vernalization, autonomous pathway, and circa-
dian signals to regulate flowering. Interestingly, vernalization also inputs the circa-
dian clock by modifying its pace provably through FLC expression (Salathia et al.
2006 ). Consistently, FLC was shown to lengthen the circadian period at higher
temperatures, contributing to temperature compensation, most likely through regu-
lation of LUX expression (Edwards et al. 2005 ). Altogether, these results impli-
cate a complex cross talk among ABA, circadian clock, and flowering pathways
in which FLC plays a central role. Also, FLC emerges as another possible link
through which ABA signaling might turn to regulate clock progression.
19.9 Concluding Remarks
The circadian clock involves a complex signaling network that is able to coordi-
nate external stimuli and an internal time mechanism to optimize multiple pro-
cesses all along the developmental process of plants. Given the important role of
ABA in regulating many physiological and developmental processes in response
to the changing environment, the integration of ABA signaling with the circadian
clock network increases the robustness and the plasticity of the response, thus
enhancing plant fitness and survival.
During the last few years, a series of exiting discoveries have been contributed
substantially to a better understanding of the molecular mechanisms implicated
in ABA signaling and the circadian clock organization. However, most of these
studies have considered these pathways independently. Multiple lines of emerg-
ing evidence indicate an intricate network connecting ABA and the circadian clock
involving multiple interactions at all levels. Thus, extending our knowledge on the
links between these two essential signaling pathways will be crucial to understand
at deeper level how plants cope with their changing environment.
References
Achard P, Cheng H, De Grauwe L, Decat J, Schoutteten H, Moritz T, Van Der Straeten D, Peng
J, Harberd NP. Integration of plant responses to environmentally activated phytohormonal
signals. Science. 2006;311(5757):91-4.
Alabadi D, Oyama T, Yanovsky MJ, Harmon FG, Mas P, Kay SA. Reciprocal regula-
tion between TOC1 and LHY/CCA1 within the Arabidopsis circadian clock. Science.
2001;293(5531):880-3.