ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 337

In vivo assays data revealed that the MKK2-MPK4/MPK6 complex functions
downstream of MEKK1 during salt and cold stress. MPK4 and MPK6 are strongly
phosphorylated by MKK2 in response to cold and salt treatment, and null allele of
mkk2 is impaired in ability of cold acclimation (Teige et al. 2004 ). Direct evidence
of MAPK activation by ABA was observed in guard cells. MPK9 and MPK12
function downstream of ROS as positive regulators of ABA signalling, and mpk9
mpk12 double mutants are partially but specifically impaired in cold-induced
stomatal closure, suggesting that they play a role in the cold-mediated ABA sig-
nalling cascade (Jammes et al. 2009 ). Arabidopsis NDP kinase 2 (NDPK2) was
shown to interact with two oxidative stress-related MAPKs, MPK3, and MPK6,
and overexpression of NDPK2 enhances cold tolerance (Moon et al. 2003 ). Taken
together, these findings raise the possibility that the MAPK cascade acts down-
stream of ROS in cold and ABA signalling.
17.7 The Effects of Circadian Signals on Cold and ABA
Signalling
The circadian clock is regulated by a central oscillator consisting of transcription/
translation feedback loops that are entrained to light and temperature cues. The
MYB transcription factors circadian clock associated1 (CCA1) and late elongated
hypocotyl (LHY) can directly bind to the promoters of the evening-loop com-
ponents timing of CAB expression1 ( TOC1 ), pseudo response regulator ( PRR7 ),
and PRR9 to repress their expression after dawn; conversely, PRR7 and PRR9
act as negative regulators of CCA1 and LHY expression in the morning (Harmer
2009 ). Interestingly, CBF gene expression is under circadian regulation, and the
cold induction of CBFs is gated by the circadian clock (Fowler et al. 2005 ). It has
been shown that CCA1 and LHY are positive regulators of CBF expression that
directly bind to CBF promoters (Lee and Thomashow 2012 ; Dong et al. 2011 ).
In the morning, CBF expression increases following induction of CCA1 and
LHY , and CBF transcript levels peak shortly after those of CCA1 and LHY . It has
also been suggested that CAMTA3 and ICE1 act synergistically with CCA1 and
LHY to induce CBF expression during the day (Dong et al. 2011 ). During the
evening, the evening element TOC1 interacts with phytochrome interacting fac-
tor 7 (PIF7) to repress expression of CBF2 by directly binding to its promoter.
Therefore, the circadian-gated regulation of the CBF genes appears to involve
the action of CCA/LHY complex and PIF7/TOC1 evening complex (Dong et al.
2011 ). A recent study showed that TOC1 functions as a molecular switch that con-
nects the circadian clock to the ABA-mediated drought stress response (Legnaioli
et al. 2009 ). ABA signalling is closely related with circadian oscillator period,
for example, ABA-mediated stomatal movement varies during day and night
(Tallman 2004 ). It was proposed that the circadian clock may modulate stoma-
tal closure under high-temperature conditions when water supplies are limited in
order to optimise water-usage efficiency (Robertson et al. 2009 ). TOC1 has been