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Matsui et al. 2008 ), hormone signaling and metabolism (Covington et al. 2008 ),
calcium ion fluxes (Johnson et al. 1995 ), flowering (Song et al. 2013 ), sugars sign-
aling (Blasing et al. 2005 ), nitrogen assimilation (Gutierrez et al. 2008 ) and cyclic
adenosine diphosphate ribose (cADPR) signaling (Dodd et al. 2007a ). Therefore,
it is not surprising that a large proportion of the transcriptome is subjected to cir-
cadian regulation. Indeed, microarray expression analysis estimated that at least
one-third of the transcriptome is under clock control (Covington et al. 2008 ). The
use of tilling arrays also allowed to identify intragenic regions, introns, and natu-
ral antisense transcripts with circadian regulation, extending the clock control far
beyond the protein coding transcripts (Hazen et al. 2009 ).
The functional clustering of clock microarray datasets revealed that circadian-reg-
ulated genes were overrepresented in several hormones and stress-responsive path-
ways (Covington et al. 2008 ). Concretely, clock datasets were found to extensively
overlap with ABA datasets (Mizuno and Yamashino 2008 ). The relationship between
the circadian clock and ABA is interesting because, while the circadian clock antici-
pate environmental changes, ABA controls many environmental stress responses,
such as water use and drought tolerance, as well as cold responses (Finkelstein et al.
2002 ). Indeed, genes under clock regulation were also found to be overrepresented
in microarray datasets from osmotic, salt, and drought stress (Covington et al. 2008 ).
Considering the significant overlap between clock, ABA, and ABA-related stress-
responsive pathways, it is not surprising that ABA levels show circadian rhythms
and the clock is able to modulate plant sensitivity to ABA (Robertson et al. 2009 ).
Furthermore, the regulation between ABA and the circadian clock is bidirectional as
ABA feeds back by modifying the circadian pace (Hanano et al. 2006 ).
19.4 Circadian Regulation of ABA Metabolism
ABA-induced responses are regulated by the amount of biologically active hor-
mone present in the cells or tissues. This amount is a result of rates of biosyn-
thesis, catabolism, transport, and activation/deactivation. Transcriptomical analysis
showed that several key enzymes involved in ABA biosynthesis and its precur-
sors and are clock-regulated. For instance, ABA DEFICIENT 1 ( ABA1 ) and ABA
DEFICIENT 2 ( ABA2 ), CLOROPLASTOS ALTERADOS 1 ( CLA1 ), PHYTOENE
SYNTHASE ( PSY ), 9 - CIS - EPOXYCAROTENOID DIOXYGENASE ( NCED ), which
are involved in ABA biosynthesis, synthesis of isoprenoid precursors and carot-
enoid synthesis, respectively (Covington et al. 2008 ; Seung et al. 2012 ). Similarly,
some of the components in ABA degradation and transport also exert circadian
regulation (Seung et al. 2012 ). Additionally, day/night cycles and probably the
circadian clock regulate the activity of an ABA-specific β -glucosidase, AtBG1,
implicated in the production of bioactive ABA through hydrolysis of glucose-con-
jugated ABA (ABA-GE) in response to stress (Lee et al. 2006 ).
Overall, the circadian clock seems to modulate ABA metabolism at all levels.
Indeed, ABA levels in leaves show diurnal fluctuations, with a major abundance
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