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17.10.4 JA and Cold Stress
JA has recently been found to positively modulate cold tolerance. In
Arabidopsis
, exogenous application of JA significantly improves freezing tol-
erance, whereas blockage of JA biosynthesis and signalling decreases freez-
ing tolerance. Further study revealed that JAZ1 and JAZ4 proteins physically
interact with the ICE1-proteins to repress their transcriptional activity, thereby
repressing expression of
CBF
s and their regulons (Hu et al.
2013
) (Fig.
17.2
d).
Therefore, it appears that JA modulates cold stress primarily through an ABA-
independent pathway. However, an antagonistic interaction between ABA and JA
signalling has also been reported. In particular, the transcription factor MYC2
was shown to act as a node for the integration of ABA and JA signalling. MYC2
can directly interfere with defence signalling through JAZ-mediated repression,
whereas MYC2 acts as an important positive regulator of the ABA-dependent
signalling pathway (Abe et al.
2003
; Fernandez-Calvo et al.
2011
) (Fig.
17.2
d).
Furthermore, recent studies showed that an ABA-inducible bHLH transcrip-
tion factor, JAM1, acts as a repressor that negatively regulates JA signalling in
Arabidopsis
(Nakata et al.
2013
).
17.10.5 SA and Cold Stress
Salicylic acid (SA) is important for the activation of plant defence responses
(Raskin
1992
). Multiple studies have demonstrated a link between cold sig-
nalling and the SA-mediated defence response. Cold stress induces the accu-
mulation of SA in
Arabidopsis
, whereas SA-deficient mutants show increased
growth in response to cold (Scott et al.
2004
). Gain of function of R/R-like pro-
tein mutants that overproduce SA, such as
chs1
,
chs2
, and
chs3
, display chilling-
sensitive phenotypes that are partially dependent upon SA (Huang et al.
2010a
;
Yang et al.
2010
; Wang et al.
2013
). Indeed, there appears to be an optimal
threshold of endogenous SA that is crucial for the chilling-dependent inhibition
of plant growth. Nevertheless, the exact role of SA in freezing tolerance remains
unclear. For example, several SA-overproducing mutants, including
cpr1
,
cpr5
,
and
slh1
, show enhanced freezing tolerance (Yang et al.
2010
), whereas overex-
pression of DEAR1 (DREB and EAR motif protein) induces SA accumulation
and freezing sensitivity in
Arabidopsis
(Tsutsui et al.
2009
). The master regu-
lator of cold signalling, ICE1, was shown to directly regulate the expression of
BON1
-
associated protein
(
BAP1
), which encodes a C2-domain protein that nega-
tively mediates the SA-dependent defence responses (Zhu et al.
2011
; Yang et al.
2006
). CAMTA3/AtSR1 recognises the
CBF2
promoter to positively regulate
CBF2
expression during cold stress, but it also interacts with the
EDS1
promoter
to repress SA-dependent disease resistance (Kim et al.
2013
; Doherty et al.
2009
;
Du et al.
2009
).
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