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against cold-induced oxidative damage (Burdon et al. 1996 ; O'Kane et al. 1996 ).
For example, overexpression of antioxidant-defensive genes enhances chill-
ing tolerance in maize and soybean (Van Breusegem et al. 1999 ; Kocsy et al.
2001 ), whereas repression of catalase gene expression reduces chilling toler-
ance (Kerdnaimongkol and Woodson 1999 ). Arabidopsis chs ( chilling - sensitive )
mutants display increased sensitivity to chilling temperature, which is at least
partially due to the excessive accumulation of H 2 O 2 in chs mutant seedlings
(Huang et al. 2010a , b ; Wang et al. 2013 ; Yang et al. 2010 ). Recent studies also
show that ROS act as signal molecules mediating cold stress-signal transduc-
tion. For instance, the Arabidopsis mutant frostbite1 ( fro1 ) has a defective form
of mitochondrial complex I NADH dehydrogenase, leading to constitutively
lower ROS levels compared with the wild type, and this mutant shows reduced
expression of cold stress-responsive genes and decreased cold acclimation (Lee
et al. 2002 ). ZAT12 encodes a ROS-responsive zinc-finger transcription factor that
functions as a positive regulator of the cold-response pathway. Overexpression
of ZAT12 results in enhanced freezing and oxidative tolerance (Davletova et al.
2005 ), which suggests that ROS signalling plays a beneficial role in mediating
cold acclimation in plants.
It has been shown that the application of exogenous ABA effectively alleviates
the symptoms of cold injury in many species. One explanation for this phenom-
enon is that ABA signalling is able to induce the transcription of ROS-scavenging
enzymes. Indeed, one study showed that stress-induced ABA accumulation
stimulates ROS scavenging to help maintain the cellular redox state (Guan et al.
2000 ). Furthermore, ABA-dependent proline accumulation under abiotic stress
is regulated by the ROS-scavenging-mediated cellular redox state, which is also
important for the cold response (Kishor et al. 2005 ). In particular, ABA treatment
enhances ROS accumulation by activating plasma membrane-bound NADPH oxi-
dases (Kwak et al. 2003 ). ABO5 and ABO6 , which encode a PPR (pentatricopep-
tide repeat) protein and DEXH box RNA helicase, respectively, are required for
ABA-induced ROS production in mitochondria (He et al. 2012 ; Liu et al. 2010 ).
The activities of ABI1 and ABI2 are inhibited by H 2 O 2 in vitro (Meinhard and
Grill 2001 ; Meinhard et al. 2002 ), and further studies demonstrated that ABI2
interacts with glutathione peroxidase 3 (GPX3), which is an ROS-scavenging
enzyme that functions as a redox transducer and scavenger to modulate ABA sig-
nalling (Miao et al. 2006 ). Glucosamine (GlcN) is a naturally occurring amino
sugar that inhibits plant growth by significantly increasing the production of ROS.
Ectopic overexpression of GlcN induces cell death, whereas scavenging of endog-
enous GlcN can enhance tolerance to oxidative, drought, and cold stresses in
Arabidopsis (Chu et al. 2010 ).
As well-known signalling components downstream of ROS, several mitogen-
activated protein kinases (MAPKs) have been shown to be activated by cold and
ABA. ABA application induces MAPK activation within only a few minutes
(Knetsch et al. 1996 ). The MAPK cascade is an important signalling pathway that
enables the transmission of environmental and hormone signals to activate regula-
tory components within the cytoplasm and initiate cellular-responsive processes.
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