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the pivotal factors in the decision of cells to respond to different levels of H
2
O
2
?
Answering these questions requires the analysis of mutants deficient in ROS sig-
nalling combined with specific ROS staining to facilitate monitoring of the ROS
dynamics in situ.
ROS signalling is highly integrated with the hormonal signalling pathways that
regulate many aspects of plant growth and development. The ability of ABA to
inhibit the expression of
ʱ
-amylase gene expression is consistent with a role for
the ABA-induced protein kinase ABA-responsive protein kinase (PKABA) as an
intermediate in GA signalling. In aleurone cells, the application of ABA results
in rapid increase in
PKABA
mRNA levels, and enhancement of
PKABA1
tran-
scription can mimic the effect of exogenous ABA in suppressing the expression
of GA-induced genes, such as those that encode GAMyb and
ʱ
-amylase (Gómez-
Cadenas et al.
2001
). H
2
O
2
released suppression of the accumulation of
GAMyb
mRNA by PKABA and consequently promoted the production of
ʱ
-
amylase
mRNA; this suggests that the H
2
O
2
generated by GA in aleurone cells is a signal
molecule that antagonises ABA signalling (Ishibashi et al.
2012
).
10.4.5 Stress Adaptation in the Transcription Levels
There has been considerable progress in recent years in understanding how ABA
and ROS modulate adaptation to stress at the level of transcription. H
2
O
2
has
been implicated in the numerous observations of ABA-mediated induction of
antioxidant gene expression (Sakamoto et al.
1995
; Guan et al.
2000
; Fryer et al.
2003
; Jiang and Zhang
2003
; Yoshida et al.
2003
). To reveal the molecular mech-
anisms that are involved in the crosstalk between the ABA and ROS signalling
pathways, several large-scale studies have analysed changes in gene expression
in response to ROS and/or ABA (Desikan et al.
2001
; Seki et al.
2002
; Leonhardt
et al.
2004
; Takahashi et al.
2004
; Vandenabeele et al.
2004
). The effects of exog-
enous ABA and H
2
O
2
on gene expression in
Arabidopsis
seedlings were investi-
gated by microarray technology using Affymetrix GeneChips (Wang et al.
2006
).
Of the 24,000 genes that were analysed following H
2
O
2
treatment, we found that
the expression of 459 transcripts was significantly increased, whereas the expres-
sion of 221 transcripts was decreased when compared with those in the seedlings
that were not exposed to H
2
O
2
. Following ABA treatment, we also found that the
levels of transcription of 391 and 322 genes were upregulated and downregulated,
respectively. Further analysis indicated that 143 of the upregulated genes and
75 of the downregulated ones responded to both ABA and H
2
O
2
. These results
imply that there is an overlap between ABA- and H
2
O
2
-induced transcription of
genes and that these two signalling molecules regulate many downstream genes
in a coordinated manner (Wang et al.
2006
). These findings are consistent with
previous reports that the responses to ABA and oxidative stress are linked (Guan
et al.
2000
; Zhang et al.
2001a
; Murata et al.
2001
; Kwak et al.
2003
; Wang and
Song
2008
).
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