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EF-hands of
membrane-localized
NADPH oxidase
Activation
NADPH
oxidase
Ca
2+
Activation
Superoxide
radicals
Phosphorylation of N-terminal
of NADPH oxidase by CDPK
CDPK
H
2
O
2
Fig. 4.9
Synergistic action of Ca
2+
binding on EF-hand domains of NADPH oxidase and
phosphorylation of NADPH oxidase by CDPK in ROS generation
4.21.2
NO Generation
Nitric oxide (NO) is a key mediator for rapid induction of plant immune responses
(Bellin et al.
2013
). Rapid NO production has been shown to be dependent on Ca
2+
signaling system (Lamotte et al.
2004
; Ali et al.
2007
; Courtois et al.
2008
; Choi
et al.
2009
; Ma et al.
2008
,
2012
; Vatsa et al.
2011
). It has been demonstrated that
different calcium channels involved in calcium infl ux are also involved in NO pro-
duction. The bacterial PAMP lipopolysaccharide (LPS) activates the Ca
2+
infl ux
through Ca
2+
channels and this Ca
2+
current leads to downstream NO production
(Ali et al.
2007
). Plants without functional CNGC2 lack this cell membrane Ca
2+
current and do not display immune responses (hypersensitive response, HR). The
impaired HR phenotype to an avirulent pathogen in
cngc2
mutant plants can 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
). The
oomycete PAMP elicitor signal cryptogein activates GLR calcium channels trigger-
ing NO production (Vatsa et al.
2011
). The addition of the Ca
2+
channel blocker
Gd
3+
or the Ca
2+
chelator EGTA abolished LPS-induced NO synthesis (Ali et al.
2007
). The results indicate that NO synthesis occurs downstream of cytosolic Ca
2+
elevation.
NO synthase (NOS) is the key enzyme involved in NO production. NOS has
been reported to be a 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
). CaM dependent NOS production has been reported
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