ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 333

17.4 ABA-Dependent Signalling Pathway for Cold
Acclimation
Genetic and physiological analyses indicate that ABA-dependent signalling path-
way plays an essential role in stress-responsive gene expression during cold-
induced osmotic stress. ABA biosynthesis genes contribute to the enrichment of
COR genes, such as RD29A , RD22 , COR15A , COR47 , and P5CS via activity of
their ABRE cis -element (Xiong et al. 2001a , 2002b ). In the study on Arabidopsis ,
four AREB/ABF transcription factors are found to be required for the activation
of ABA-mediated signalling (Uno et al. 2000 ). Among them, ABF1 expression is
significantly induced by cold, but not by osmotic stress (Choi et al. 2000 ), whereas
AREB1 / ABF2 , AREB2 / ABF4 , and ABF3 are specifically induced by ABA and
drought stress (Abdeen et al. 2010 ; Kang et al. 2002 ; Fujita et al. 2005 ). AREB
proteins do not appear to play a role in CBF-dependent cold signalling; however,
one study showed that ABF2, ABF3, and ABF4 interact with DREB2C in vitro
(Lee et al. 2010 ). In addition, approximately 10 % of ABA-responsive genes are
also responsive to cold stress (Kreps et al. 2002 ). Thus, although ABA- and cold
stress-induced transcription factors show distinct regulation, some of the COR
genes appear to be induced by both of these stimuli. Therefore, one could specu-
late that limited, cold-induced ABA production could enhance COR gene activa-
tion through the ABA-dependent pathway during cold acclimation.
In addition to AREBs, other bZIP transcription factors belonging to the ABI5
subfamily have been characterised as ABRE-binding proteins and were shown to
affect seed germination as well as the cold stress response. For example, ectopic
expression of the seed-specific transcriptional activator ABI3 confers the abil-
ity to express COR genes in vegetative tissues and enhances freezing tolerance in
Arabidopsis (Tamminen et al. 2001 ). ABI5 has been shown to act downstream of
and interact with ABI3 (Suzuki and McCarty 2008 ; Finkelstein and Lynch 2000 ,
and it can be sumoylated and degraded by the SUMO E3 ligase SIZ1, which
attenuates ABA signalling (Miura et al. 2009 ). Indeed, SIZ1-mediated SUMO con-
jugation is also required for the expression of CBF3 by modulating the activity
of ICE1 (Miura et al. 2007 ). siz1 mutants are impaired in cold acclimation and
are sensitive to ABA during seed germination (Miura et al. 2007 , 2009 ). A recent
study revealed that SIZ1 is able to sumoylate and stabilise the negative regulator
of ABA signalling MYB30 (Zheng et al. 2012 ). Therefore, SIZ1 appears to be an
integration node that mediates ABA and cold responses through post-transcrip-
tional modifications.
Several MYC and MYB family genes have been reported to be important
regulators of ABA-responsive gene expression under cold stress. For instance,
MYB96 is induced by ABA and drought, and it enhances ABA-mediated
drought and freezing tolerance (Guo et al. 2013 ; Seo et al. 2009 ). Expression of
OsMYB3R - 2 in rice is induced by cold, drought, and salt stresses, and overexpres-
sion of OsMYB3R - 2 increases tolerance to freezing, drought, and salt stresses in