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transcription factor AtMYB15 (MYB domain protein 15) was found to physically
interact with ICE1. MYB15 overexpression leads to reduced expression of the
CBF genes, whereas a myb15 mutant shows increased CBF expression. MYB15
binds to the CBF3 promoter to repress its CBF expression and negatively regu-
lates freezing tolerance (Agarwal et al. 2006 ). Subsequently, a SUMO E3 ligase,
SAP and Miz (SIZ1), was identified as a positive regulator of the ICE1 protein.
SIZ1 can sumoylate ICE1 and repress the polyubiquitination of ICE1, which, in
turn, enhances ICE1 stability (Miura et al. 2007 ). Therefore, regulation of the
ICE1-CBF cascade at both transcriptional and post-translational levels demon-
strates the existence of a complex network of CBF-dependent cold-signalling
pathways.
17.3 The Role of ABA Biosynthesis and Signalling
in Cold Stress
Investigations into the role of ABA in cold stress were originally based on obser-
vations made in the 1960s in woody species, showing that application of the gib-
berellic acid (GA) inhibitor dormin—which was later identified as ABA-resulted
in increased freezing tolerance in trees, equivalent to that observed in plants
that had undergone cold acclimation (Chrispeels and Varner 1967 ; Thomas
et al. 1965 ). It has been shown that cold tolerance is usually accompanied by
increased endogenous ABA levels in various plant species (Daie and Campbell
1981 ; Lang et al. 1994 ; Mantyla et al. 1995 ). Continuous application of ABA
induces chilling tolerance in chilling-sensitive plant species, such as maize, rice,
cucumber, and pepper. Furthermore, exogenous ABA application in temperate
plants such as poplar, barely, wheat, and Arabidopsis can partially mimic cold
acclimation and enhance freezing tolerance (Kadlecová et al. 2000 ; Smoleᄡska-
Sym et al. 1995 ; Zhu et al. 2000 ; Thomashow 1999 ). Transcriptome analyses
in Arabidopsis showed that a number of ABA-responsive genes can be induced
by cold treatment (Zeevaart and Creelman 1988 ). However, it has also been
shown that ABA application in several plant species has little effect on freezing
tolerance, or if there are any changes, they are much less significant than those
induced by cold acclimation (Gusta et al. 1982 ; Fayyaz et al. 1978 ; Holubowicz
et al. 1982 ), suggesting the natural diversity of ABA responses in different plant
species.
The primary source of increased ABA levels under stress conditions is a de
novo biosynthesis pathway that converts carotenoids into bioactive ABA. ABA
biosynthesis pathway may be required for full development of the cold response,
as defects in both basal and acquired freezing tolerance have been observed in
ABA-deficient mutants. For instance, ABA1 and ABA3 are identified as genes
encoding enzymes involved in ABA biosynthetic pathway. Cold induction of the
COR genes is reduced in the aba3 mutants aba3/los5/frs1 (Llorente et al. 2000 ;
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