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two main classes, referred to as sensor relays and sensor responders. The information
encoded in transient Ca
2+
changes is decoded by an array of Ca
2+
binding proteins
giving rise to a cascade of downstream effects. Thus the extracellular signals are
transmitted to cellular calcium-dependent effectors to activate the transcription
of immune response-related genes. The activation of calcium signaling system
enhances host defense responses against a wide range of plant pathogens (Lecourieux
et al.
2006
; McAinsh and Pittman
2009
; Abdul Kadar and Lindsberg
2010
; DeFalco
et al.
2010
; Dodd et al.
2010
; Hamada et al.
2012
; Hashimoto et al.
2012
).
Thus, Ca
2+
signaling system involves several ion channels, pumps, transporters,
Ca
2+
sensor proteins, calmodulin-binding proteins, and calcium-dependent protein
kinases. Activation of any one key component in the calcium signaling pathway
triggers defense responses against pathogens, suggesting interplay of the different
components in the signaling pathway. CNGC is an important Ca
2+
channel involved
in Ca
2+
infl ux into cytosol. Application of cAMP results in CNGC2-dependent
elevation of cytosolic Ca
2+
in
Arabidopsis
leaves (Ma et al.
2009a
). Activation of the
Ca
2+
ion channel by cAMP leads to NO generation, ROS generation, and enhanced
defense response gene expression (Ma et al.
2009a
). Glutamate receptor gated chan-
nel is another Ca
2+
channel detected in plant cell plasma membrane. In
Arabidopsis
thaliana
overexpressing a glutamate receptor gene (
RsGluR
) from small radish,
glutamate treatment triggered greater Ca
2+
infl ux in the root cells of transgenic plants
(Kang et al.
2006a
,
b
). The increased Ca
2+
infl ux through the glutamate receptor-
gated channel triggered several defense-related genes including JA-biosynthetic
genes (Kang et al.
2006a
,
b
). The rice two-pore channel1 (OsTPC1) is a putative
voltage-gated Ca
2+
-permeable channel (Kurusu et al.
2005
; Hamada et al.
2012
).
Overexpression of
OsTPC1
induced several defense-related signaling systems,
resulting in induction of oxidative burst and activation of a mitogen-activated
protein kinase and hypersensitive cell death (Kurusu et al.
2005
). Plant annexins
appear capable of mediating passive, channel-like Ca
2+
transport (Mortimer et al.
2008
; Laohavisit et al.
2009
,
2010
; Laohavisit and Davies
2011
). Annexins are Ca
2+
transporter involved in Ca
2+
infl ux (Laohavisit and Davies
2011
). Transgenic
tobacco plants expressing the annexin gene (
AnnBj1
) isolated from
Brassica juncea
showed enhanced defense responses against
Phytophthora parasitica
var.
nicoti-
anae
. The transgenic plants showed increased expression of several defense-related
proteins (Jami et al.
2008
).
Calmodulin (CaM) and calmodulin-like (CML) proteins are the potential Ca
2+
signal sensor proteins. Activation of the expression of these proteins triggers several
immune responses and confers resistance against several pathogens. Transgenic
overexpression of pepper calmodulin gene
CaCaM1
activates ROS and NO genera-
tion, and triggers defense responses against
Xanthomonas campestris
pv.
vesicato-
ria
in pepper leaves (Choi et al.
2009
). Overexpression of the same pepper
CaCaM1
gene in
Arabidopsis
enhanced ROS and NO generation and conferred resistance
against
Pseudomonas syringae
and
Hyaloperonospora parasitica
(Choi et al.
2009
).
Overexpression of the pathogen-inducible tobacco calmodulin gene
NtCaM13
,
which is a component in the Ca
2+
signaling pathway, triggered defense responses
against the oomycete pathogen
Pythium aphanidermatum
, the fungal pathogen
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