ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 340

et al. 2006 ; Wolters and Jurgens 2009 ; Peleg and Blumwald 2011 ). The growth
inhibition decreases the capacity for energy utilisation, which in turn, results in
cold acclimation processes.
17.10.1 GA and Cold Stress
As an important plant phytohormone, GA affects plant abiotic stress responses,
including those to salt, oxidative, and cold stresses during germination and seed-
ling development. Acting as GA-signalling repressors, DELLA proteins have
been identified as the central modulators of growth under a variety of stress condi-
tions (Achard et al. 2006 ). ABA promotes the accumulation of DELLA proteins,
which induces growth repression in plant lateral roots under salt stress through
an ABA-dependent pathway (Achard et al. 2006 ). Intriguingly, GA treatment
rescues normal growth and abolishes the late-flowering phenotype of CBF1 - ox
plants (Achard et al. 2008 ). Further investigation revealed that cold stress causes
increased expression of the biosynthetic enzyme genes AtGA2ox3 and AtGA2ox6 ,
which reduce endogenous levels of bioactive GA. Consistently, overexpression of
CBFs represses plant growth via the accumulation of DELLA proteins (Achard
et al. 2008 ). ABA is known to act as an antagonist of GA by inhibiting the expres-
sion of GA20ox and GA3ox to reduce GA levels during seed germination (Razem
et al. 2006 ) (Fig. 17.2 a). It has been demonstrated that the existence of a novel
mechanism through which CBFs may regulate dormancy in parallel to their func-
tions in CBF-mediated cold signalling, by regulation of DOG1 expression, which
is a positive regulator of GA catabolism and ABA biosynthesis that promotes seed
maturation at cool temperatures (Chiang et al. 2011 ; Kendall et al. 2011 ). A recent
study showed that ABI3 and ABI5 interact with DELLA proteins to inhibit seed
germination by repressing the expression of high-temperature-induced genes (Lim
et al. 2013 ). Thus, DELLA-dependent growth restraint is necessary for cold accli-
mation and temperature-mediated seed germination.
17.10.2 Cytokinin and Cold Stress
Cytokinin signalling belongs to the two-component signalling system, which
involves plant adaptation to environmental stress (Argueso et al. 2009 ; Nishimura
et al. 2004 ). The accumulated body of evidence indicates that cytokinin acts as
an antagonist of ABA during environmental stress responses (Peleg and Blumwald
2011 ; Hwang et al. 2012 ). Based on studies involving mutants or transgenic plants
with altered cytokinin biosynthesis, it has been postulated that cytokinin acts as a
negative regulator to modulate abiotic stress signalling (Nishiyama et al. 2011 ).
In Arabidopsis , the cytokinin receptor histidine kinases AHK2, AHK3, and
CRE1 have been shown to play important roles in the regulation of plant abiotic