Agriculture Reference
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Improvement of multi-stress tolerance was reported for transgenic tomato over-
expressing
SlAREB1
, which showed increased tolerance to salt and water stress
(Orellana et al.
2010
). These reports suggest that the roles of ABA-dependent bZIP
transcription factors in the modulation of stress tolerance are conserved in plants,
and some of the bZIP genes are promising candidates for genetic engineering for
developing stress-resistant crops in the future.
22.4.2 WRKYs
WRKY transcription factors, originally isolated from plants containing one or
two conserved WRKY domains, are key regulators of many processes includ-
ing the responses to biotic and abiotic stresses (Zou et al.
2004
; Eulgem and
Somssich
2007
). Recent studies on the diverse ABA signaling pathways sug-
gest that WRKY transcription factors may also play important roles in regulat-
ing plant responses to ABA. The magnesium-protoporphyrin IX chelatase H
subunit (CHLH/ABAR) functions as a receptor of ABA in Arabidopsis (Shen
et al.
2006
). ABAR spans the chloroplast envelope, and the cytosolic C termi-
nus of ABAR interacts with a group of WRKY transcription factors (WRKY40,
WRKY18, and WRKY60) which function as negative regulators of ABA sign-
aling in seed germination and post-germination growth (Shang et al.
2010
).
WRKY40 binds in vivo to W box-containing fragments of the promoters of many
ABA-dependent genes such as
ABI4
,
ABI5
,
ABF4
, and
RAB18
, which places
these WRKYs upstream of other known ABA-responsive transcription factors
(Shang et al.
2010
).
In crops, a few WRKYs were reported for their involvement in ABA and stress
response regulation. Tao et al. (
2011
) reported that two alleles of
OsWRKY45
play
different roles in ABA signaling and salt stress adaptation in rice.
OsWRKY45
-
1
-over-expressing lines showed reduced ABA sensitivity, but no obvious differ-
ences in response to salt stress. In contrast,
OsWRKY45
-
2
-over-expressing lines
showed increased ABA sensitivity and reduced salt stress tolerance. In addition,
OsWRKY45
-
1
and
OsWRKY45
-
2
transgenic plants showed differential expres-
sion of a set of ABA- and abiotic stress-responsive genes, although they showed
similar responses to cold and drought stresses. The different roles of the two
alleles in ABA signaling and salt stress may be due to their transcriptional media-
tion of different signaling pathways (Tao et al.
2011
). Over-expression of another
WRKY gene
OsWRKY11
caused significant desiccation tolerance and induction
of genes encoding raffinose synthase and galactinol synthase (Wu et al.
2009
).
Recently, OsWRKY13 was reported to selectively bind to the DNA segments
harboring known
cis
-elements in the promoters of
SNAC1
and
OsWRKY45
-
1
in
vivo during abiotic stresses, thus directly suppressing the transcription of
SNAC1
and
WRKY45
-
1
in vascular tissue and guard cells (Xiao et al.
2013
).
OsWRKY13
-
suppressing plants showed an increased
SNAC1
transcript level and enhanced tol-
erances to drought stress (Xiao et al.
2013
). These results together suggest that
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