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system (Fig.
5.3
; Desikan et al.
2001
; Vranová et al.
2002
; Gupta and Luan
2003
;
Vandenabeele et al.
2003
; Desikan et al.
2005
; Fedoroff
2006
; Hancock et al.
2006
;
Torres et al.
2006
).
5.7
Interplay Between ROS and Ca
2+
Signaling System
Increase in Ca
2+
infl ux is known to trigger ROS generation (Desikan et al.
2001
;
Choi et al.
2009
). The Ca
2+
sensor protein calmodulin (CaM) has been suggested to
be involved in Ca
2+
-triggered ROS generation. An increase in Ca
2+
amount in cyto-
sol triggered by PAMP elicitor stimuli is perceived by the calmodulin. The calmod-
ulin gene is strongly induced by the Ca
2+
infl ux in
Arabidopsis
(Desikan et al.
2001
).
The Ca
2+
/CaM complex regulates NAD kinase, which generates NADPH for
NADPH oxidase activity (Harding et al.
1997
). Increase in NADPH oxidase activity
results in generation of ROS (Choi et al.
2009
; Pitzschke and Hirt
2006
,
2009
;
Pitzschke et al.
2009a
,
b
; Mazars et al.
2010
).
While Ca
2+
infl ux -dependent activation of calmodulin gene is required for ROS
production, ROS also triggers Ca
2+
infl ux (Levine et al.
1996
). Intracellular Ca
2+
concentrations increase in response to oxidative burst (Price et al.
1994
). A calmod-
ulin gene was strongly induced by H
2
O
2
in
Arabidopsis
(Desikan et al.
2001
),
suggesting that ROS is involved in triggering Ca
2+
signaling system. H
2
O
2
has been
shown to trigger calcium infl ux in tobacco (Kawano and Muto
2000
).
It has been suggested that ROS might have regulated Ca
2+
infl ux through
plasma membrane transport proteins (Laohavisit et al.
2010
,
2012
). The PAMP-
induced H
2
O
2
may trigger [Ca
2+
]
cyt
increase, probably through the activation of
H
2
O
2
- sensitive Ca
2+
channels located in the plasma membrane (Lecourieux
et al.
2002
). The ROS activates hyperpolarization- activated Ca
2+
infl ux current
(Pei et al.
2000
; Foreman et al.
2003
).
The NADPH oxidase-derived ROS stimulates a Ca
2+
infl ux into the cytoplasm
(Takeda et al.
2008
). H
2
O
2
and OH˚ may serve as distinct signals in the regulation
of calcium infl ux, due to the existence of calcium channels that are distinctively
sensitive to the generated H
2
O
2
(Demidchik et al.
2007
). The rise in Ca
2+
level in
turn activates NADPH oxidase to produce ROS (Takeda et al.
2008
), suggesting a
positive feedback regulation of Ca
2+
infl ux - ROS signaling system. These results
suggest that a signifi cant amount of cross-talk occurs between ROS and calcium
signaling systems.
ROS may also act as a second messenger activating Ca
2+
signaling (Kwak et al.
2003
). Abscisic acid (ABA) promotes ROS production, activates plasma membrane
Ca
2+
-permeable channels, and triggers cytosolic Ca
2+
increases in
Arabidopsis thaliana
.
Disruption of NADPH oxidase catalytic subunits genes,
AtrbohD
and
AtrbohF
, has
been shown to impair ABA signaling, ABA promotion of ROS production, ABA-
induced cytosolic Ca
2+
increases and ABA-activation of plasma membrane Ca
2+
-
permeable channels in guard cells (Kwak et al.
2003
). Application of H
2
O
2
rescued
Ca
2+
channel activation in the
atrbohD/F
mutants, suggesting that the ROS acts as a
second messenger activating Ca
2+
signaling (Kwak et al.
2003
).
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