ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 329

Lipid composition of membranes, especially the portion of galactolipids contain-
ing unsaturated fatty acids, is crucial for high plant species during cold acclima-
tion. In Arabidopsis , the expression of FAD2 , which encodes an enzyme that is
essential for polyunsaturated lipid synthesis. fad2 mutants show irregular mem-
brane composition and cannot survive at low temperature (Miquel et al. 1993 ).
Similar chilling-sensitive phenotypes are also observed in loss-of-function fad5
(Hugly and Somerville 1992 ). A recent study showed that a lipid desaturase,
acyl-lipid desaturase2 (ADS2), is required for chilling and freezing tolerance in
Arabidopsis and functions by altering membrane lipid composition (Chen and
Thelen 2013 ). Furthermore, the Arabidopsis sensitive to freezing2 (SFR2) pro-
tein was identified as a galactolipid-remodelling enzyme that is localised to the
outer chloroplast membrane, and it is essential for membrane lipid remodelling
of chloroplast envelope under freezing stress (Moellering et al. 2010 ; Fourrier
et al. 2008 ).
Under cold stress, membrane rigidity triggers second-messenger molecules
such as Ca 2 + and to activate complex signaling pathways involved in protein
kinases or transcription-factor cascades (Viswanathan and Zhu 2002 ). However,
the underlying mechanisms that plants use to perceive and transduce cold signals
remain elusive. Currently, the most thoroughly understood cold-signalling path-
way is the ICE-CBF-COR transcriptional cascade, which plays a crucial role in
the activation of multiple downstream cold - regulated ( COR ) genes (Thomashow
2010 ) (Fig. 17.1 ). Acting as central nodes in the cold acclimation pathway, the
C-repeat binding factors (CBFs) gene family is conserved in many plant spe-
cies. In Arabidopsis , three CBF transcription factors belonging to the AP2/ERF
(apetala 2/ethylene-responsive factor) superfamily have been identified. CBFs are
also known as dehydration-responsive element binding factors (DREBs), which
recognise the conserved cold- and dehydration-responsive C-repeat/DRE DNA
motifs found within the promoter regions of COR genes (Liu et al. 1998 ). CBF1
overexpression triggers constitutive expression of the COR genes and induces
freezing tolerance in Arabidopsis (Jaglo-Ottosen et al. 1998 ). Expression of the
CBFs is rapidly induced by cold temperatures, and a growing number of stud-
ies have demonstrated that transcriptional regulation of the CBF genes is con-
trolled by multiple mechanisms. In Arabidopsis , CBF2 is a negative regulator of
the expression of CBF1 and CBF3 , and it plays an important role in freezing tol-
erance (Novillo et al. 2004 ; Novillo et al. 2007 ). To date, several key upstream
regulators of the CBF genes have been identified and characterised. CAMTA3
encodes a calmodulin-binding protein and acts upstream to activate CBF2
expression by binding to the CG-element in its promoter region in response to
cold signals (Doherty et al. 2009 ). The MYC-type, basic helix-loop-helix tran-
scriptional activator inducer of CBF expression 1 (ICE1) has been identified
as a transcriptional activator that binds to MYC cis -elements within the CBF3
promoter, and the expression of CBF3 and its target COR genes is impaired in
ice1 -mutant plants during cold acclimation. In addition, overexpression of ICE1
in Arabidopsis results in increased freezing tolerance, supporting a pivotal role
for ICE1 in the cold stress response (Chinnusamy et al. 2003 ). The transcription