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The different calcium ion channels involved in the calcium infl ux have also been
found to be involved in NO production. Cyclic nucleotide-gated ion channels
(CNGCs) have been reported to play important role in NO synthesis. Plants without
functional CNGC2 lack the cell membrane Ca
2+
current and do not display immune
responses induced by NO. The hypersensitive response impaired phenotype to an
avirulent pathogen in
cngc2
mutant plants could be complemented by the addition
of an NO donor (Ali et al.
2007
). The results suggest the importance of the cyclic
nucleotide gated channel in induction of NO in the immune signaling system
(Ali et al.
2007
). Another type of ion channels involved in Ca
2+
infl ux is glutamate
receptor (GLR)-like channels Ma et al.
2012
). An oomycete PAMP elicitor has been
shown to activate GLR calcium channels triggering NO production (Vatsa et al.
2011
). The addition of the Ca
2+
channel blocker Gd
3+
or the Ca
2+
chelator EGTA
abolished the PAMP-induced NO synthesis (Ali et al.
2007
), suggesting that NO
synthesis occurs downstream of cytosolic Ca
2+
elevation.
6.3.2
Role of Calmodulin in NO Production
NOS production in plants has been reported to be dependent on calmodulin (CaM),
a Ca
2+
sensor protein (Delledonne et al.
1998
; Courtois et al.
2008
; Ma and Berkowitz
2011
). The pepper CaM gene
CaCaM1
has been shown to be involved in NO gen-
eration (Choi et al.
2009
). Upon treatment with the CaM antagonist, virulent
Pseudomonas syringae
pv.
tomato -
induced NO generation was also compromised
in
CaCaM1
overexpressing plants (Choi et al.
2009
). The calmodulin antagonist
N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7) blocked NO genera-
tion. The effect of W7 was downstream from the Ca
2+
channel, as the elicitor activa-
tion of the channel was demonstrated in the presence of W7 (Ali et al.
2007
).
NO synthase (NOS) is the key enzyme involved in NO production and NOS is a
calmodulin (CaM)-binding protein. NOS contains CaM-binding motifs and full
activation of the enzyme needs both Ca
2+
and CaM (Guo et al.
2003
; Lamotte et al.
2004
; Zeidler et al.
2004
; Ma and Berkowitz
2007
). The pathogen-induced Ca
2+
signals lead to CaM activation of NOS (Ma et al.
2008
). Application of a CaM
antagonist prevents NO generation and induction of immune responses. Ca
2+
chela-
tion also abolishes NO generation (Ma et al.
2008
). Collectively these results sug-
gest that the initial pathogen recognition signal of Ca
2+
infl ux into the cytosol
activates CaM, which then acts to induce downstream NO synthesis, leading to
innate immune responses (Fig.
6.2
).
6.3.3
ROS and ABA Act Upstream of NO Production
NO has been shown to act downstream of H
2
O
2
in ABA signaling system (Lü et al.
2005
). ABA-mediated NO generation has been shown to be dependent on ABA-induced
H
2
O
2
production in
Vicia faba
and
Arabidopsis
(Lü et al.
2005
; Bright et al.
2006
).
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