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the CDPK functions as a calcium sensor and phosphorylates the plasma membrane
bound NADPH oxidase, which is the key enzyme in ROS signaling system.
CDPK activates amino-1-cyclopropane carboxylate synthase, the enzyme
involved in ethylene biosynthesis (Sebastiá et al.
2004
).
Arabidopsis
CDPK
AtCPK32 interacts with ABF4, a transcriptional regulator of ABA-responsive gene
expression, and modulates its activity (Choi et al.
2005
). Expression of a grape
CDPK, ACPK1, in
Arabidopsis thaliana
activates abscisic acid (ABA) signaling
(Yu et al.
2007
). Phenylalanine ammonia-lyase (PAL), the key enzyme in phenyl-
propanoid pathway involved in biosynthesis of phytoalexins, has been shown to be
a substrate of a specifi c constitutively active
Arabidopsis
CDPK expressed in corn
protoplasts (Cheng et al.
2001
). The maize CDPK, ZmCPK10, triggers a rapid tran-
scriptional activation of
PRms
encoding PR proteins in maize (Murillo et al.
2001
).
Collectively these results suggest that CDPKs may target various enzymes involved
in plant defense systems.
4.20
Nuclear Free Calcium Ion ([Ca
2+
]
nuc
) in Ca
2+
Signaling
Increases in nuclear free calcium concentration ([Ca
2+
]
nuc
) have been reported to
occur in plants in response to some external stimuli. The bacterial (harpin) and
oomycete (elicitins) elicitors induced a pronounced and sustainable [Ca
2+
]
nuc
eleva-
tion (Lecourieux et al.
2006
; Mazars et al.
2009
,
2010
). The [Ca
2+
]
nuc
rise depends
on free cytosolic calcium ([Ca
2+
]cyt), 1,4,5-trisphosphate (IP3) and reactive oxygen
species (ROS) (Lecourieux et al.
2006
).
Levy et al. (
2005
) identifi ed an
Arabidopsis
gene,
IQDI
(
IQ-DOMAIN I
), which
encodes a calmodulin-binding nuclear protein.
IQD1
integrates intracellular Ca
2+
signals towards stimulation of plant defenses, including accumulation of glucosino-
lates, the secondary metabolites involved in plant defense. CaM, CaM-binding
proteins (Bouche et al.
2005
), CDPK (Damman et al.
2003
), and Ca
2+
-CaM-
regulated protein phosphatase (Andreeva and Kutuzov
2001
) have been detected in
plant nucleus. Ca
2+
-ATPase (Downie et al.
1998
) and some components of the phos-
phoinositide signaling pathway (Dröbak and Heras
2002
) have been found to be
localized to the plant nucleus. The nuclei isolated from tobacco were capable of
producing H
2
O
2
in a calcium-dependent manner (Astamker et al.
2007
). The
[Ca
2+
]
nuc
rise depends on free cytosolic calcium, 1,4,5-trisphosphate (IP3), and ROS
(Lecourieux et al.
2005
). The interplay between nuclear and cytosolic calcium elab-
orates a global calcium signature and elicits biological responses (Pauly et al.
2001
).
Nuclear calcium may be responsible for the activation of Ca
2+
-dependent proteins in
the nucleus, and may be involved in the regulation of nuclear activities such as gene
expression (White and Broadley
2003
; Mazars et al.
2010
). It has also been reported
that the nucleus exhibits a Ca
2+
signature independently of the cytosol in response
to stresses (Mazars et al.
2010
).
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