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rise in [Ca
2+
]
cyt
in
Vicia faba
guard cells (Garcia-Mata et al.
2003
) and in tobacco
suspension cells (Gould et al.
2003
; Lamotte et al.
2004
,
2006
). Treatment of trans-
genic
Nicotiana plumbaginifolia
cells expressing the Ca
2+
reporter aequorin
addressed in the cytosol with a NO donor resulted in a rapid and transient elevation
in [Ca
2+
]
cyt
(Besson-Bard et al.
2008a
). NO scavengers and mammalian NOS inhibi-
tors reduced the increase in [Ca
2+
]
cyt
triggered by elicitors (Lamotte et al.
2006
;
Vandelle et al.
2006
; Besson-Bard et al.
2008a
). These observations suggest that NO
is involved in infl ux of Ca
2+
into the cytosol. Inhibitors of plasma membrane and
intracellular Ca
2+
permeable channels have been found to inhibit NO-induced
increases in [Ca
2+
]
cyt
(Gould et al.
2003
; Lamotte et al.
2006
; Vandelle et al.
2006
),
suggesting that NO might promote an infl ux of Ca
2+
from the extracellular space and/
or mobilization of Ca
2+
sequestered in intracellular Ca
2+
stores. Ryanodine receptors
(RYR) may be the main targets for NO (Durner et al.
1998
; Lamotte et al.
2004
).
The activation of intracellular Ca
2+
channels by NO may be due to the second
messenger cGMP (guanosine-3
-cyclic monophosphate), produced following the
activation of soluble guanylate cyclase (GC) (Willmott et al.
1996
; Hanafy et al.
2001
). cGMP generation activated by NO in plants activates CNGCs, cytosolic Ca
2+
elevation, and downstream signaling (Ma and Berkowitz
2007
). NO posttranslation-
ally activates GC (Klessig et al.
2000
). cGMP activates ADP-ribosylcyclase
(ADPRC), through a cGMP-dependent protein kinase This results in elevated levels
of another second messenger, cyclic ADP-ribose (cADPR) (Willmott et al.
1996
;
Durner et al.
1998
; Klessig et al.
2000
; Minorski
2003
).
cADPR is a Ca
2+
mobilizing metabolite and activates intracellular Ca
2+
release
channels (Ma and Berkowitz
2007
). It has been suggested that cADPR mediates
Ca
2+
release by activating the intracellular Ca
2+
channels ryanodine receptors (RYR)
in animals and also in plants (Allen et al.
1995
; Fliegert et al.
2007
). The NO-mediated
Ca
2+
transient infl ux is reduced by almost 40 % by the cADPR antagonist 8-bromo-
cADPR (Besson-Bard et al.
2008a
). cADPR is involved in NO signaling of various
defense genes. NO-induced accumulation of
PR-1
transcripts in tobacco leaves was
suppressed in the presence of the cADPR-selective antagonist 8-bromo-cADPR
(Klessig et al.
2000
). Vacuum infi ltration of nanomolar concentrations of cADPR in
tobacco leaf disks triggered the expression of the PR-1 gene, which was suppressed
by RYR inhibitors (Durner et al.
1998
). These results suggest that NO-induced
cADPR is involved in downstream defense signaling system.
Together with cADPR, protein kinases may also be involved in mediating
NO-induced changes in [Ca
2+
]
cyt
. Inhibitors of protein kinases, such as staurospo-
rine and K252a reduced the [Ca
2+
]
cyt
triggered by NO in
Vicia faba
guard cells and
tobacco cell suspensions (Sokolovski et al.
2005
; Lamotte et al.
2006
). Treatment
of
Nicotiana plumbaginifolia
with NO resulted in the activation of a protein kinase
belonging to SNF1-related protein kinase type 2 (SnRK2) family and the mitogen-
activated protein kinase SIPK (Besson-Bard et al.
2008a
,
b
). Several other studies
have demonstrated that artifi cially generated NO stimulated MAPKs including
SIPK (Clarke et al.
2000
; Pagnussat et al.
2004
; Zhang et al.
2007
). These observa-
tions suggest that NO triggers cellular events in plant cells by causing an increase
in [Ca
2+
]
cyt
.
′
,5
′
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