ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 385

probably due to a decreased endogenous sugar concentration (Cao et al. 2007 ).
Additionally, PRR5 and TOC1 bind the DREB1 / CBF promoter to repress
its expression (Nakamichi et al. 2012 ), most likely in collaboration with
PHYTOCHROME-INTERACTING PROTEIN 7 (PIF7) (Maruyama et al.
2004 ). Furthermore, the arrhythmic triple mutant prr5 / prr7 / prr9 displays con-
stant high DREB1/CBF transcription levels and shows increased freezing toler-
ance (Nakamichi et al. 2009 ). These results are in agreement with the diurnal
oscillation of the DREB1 genes and may be indicative of an ABA-independent
transcriptional regulation.
ABA also mediates increases in cADPR and [Ca 2 + ]cyt in response to cold,
and these stimuli are necessary for the induction of a numerous cold-responsive
genes such as RD29A ( RESPONSIVE TO DESICCATION29A ) (Viswanathan
and Zhu 2002 ; Wu et al. 2003 ). Abundance of the RD29A transcripts, [Ca 2 + ]cyt
and [cADPR] has been shown to be circadian-regulated (Dodd et al. 2006 , 2007 ;
Johnson et al. 1995 ). Furthermore, cold response experiments at different times
have shown that RD29A and [Ca 2 + ]cyt are also gated by the circadian clock,
with maximum response during the day (Dodd et al. 2006 ). Reciprocal regulation
between the clock and cADRP suggests that the gating of ABA and the cold sign-
aling might be due to a control of the abundance of the transducing second mes-
sengers by the circadian clock.
Gating experiments showed that cold induction of DREB1 / CBF , [Ca 2 + ]cyt and
RD29A is maximized during the day period. This is indicative that this pathway
may be different to the cold response resulting from the temperature drop during the
night. Together with the important role of the circadian clock in photoperiod recog-
nition in seasonal processes such as flowering (Andres and Coupland 2012 ), these
findings strongly suggest that this cold induction may account for an onset of long-
term responses such as induction of freezing tolerance in preparation for winter.
19.8 Circadian Regulation of Other ABA-related
Physiological Processes
Many aspects of plant growth, development, and stress responses in plants
are regulated by the circadian clock, including diurnal processes such as pho-
tosynthesis, stomatal opening, hypocotyl growth and leaf movement, or sea-
sonal processes, such as floral transition, seed dormancy, and freezing tolerance
(McClung 2006 ; Yakir et al. 2007 ). Some of these processes are also known to
be modulated by ABA (Finkelstein and Rock 2002 ). However, the interconnec-
tions between clock and ABA regulation are in most of the cases still unknown.
The role of ABA in these processes have been extensively reviewed in previ-
ous chapters, the following section briefly describes some of the most relevant
aspects of the circadian regulation and the possible connections with the ABA
signaling.