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Fig. 19.4
Simplified model
defining the regulation of the
stomatal aperture by ABA
and circadian clock (adapted
from Pokhilko et al.
2013
).
Red arrows
and
blue broken
lines
represent activation
and repression, respectively.
Transcriptional and protein
regulation are shown by
solid
and
dotted lines
, respectively
first postulated after a series of wide cold-responsive transcriptomal analysis.
These studies showed significant differences depending on the sampling time
(Bieniawska et al.
2008
). More detailed analysis including the promoter region of
cold-induced genes showed a significantly overrepresentation of evening elements
(EE), CCA1/LHY binding elements commonly associated with genes expressed
during the evening.
Molecularly, ABA and cold induce the transcription factors
DEHYDRATION
-
RESPONSIVE ELEMENT
-
BINDING PROTEIN 1b
(
DREB1b
),
DREB1c
and
DREB1a
(also known as
CBF
s genes), which are partly responsible for cold
response and the induction of freezing tolerance (Vogel et al.
2005
). Analysis
of their promoter regions revealed the presence of EEs often coupled to ABA-
responsive elements (ABRE), and both elements might be necessary for cold
induction (Mikkelsen and Thomashow
2009
). Indeed, DREB1/CBF expression
shows diurnal oscillation and its cold induction exert circadian gating (Fowler et al.
2005
). In this sense, cold induction studies at different circadian times showed a
time-dependent induction with its maximum 4 h after dawn. Similar studies using
CCA1
-overexpressing plants showed no diurnal variation in the cold response,
clearly in agreement with the circadian gating of
DREB1
/
CBF
cold induction
(Fowler et al.
2005
). The importance of CCA1 and LHY was further studied by
using double
cca1
/
lhy
mutants that exhibit significantly low levels of
DREB1
/
CBF
expression and reduced freezing tolerance (Dodd et al.
2007b
). Furthermore,
CCA1
and
LHY
exhibit temperature-dependent splicing; while the functional splicing vari-
ants of
CCA1
are produced preferentially at low temperatures and are important for
cold induction of
DREB1
/CBF, LHY active variant is preferentially transcribed at
high temperatures (James et al.
2012
; Seo et al.
2012
).
Other clock-related genes also have been described to participate in the
cold-induced freezing tolerance. The
gi
-
3
mutant is susceptible to freezing,
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