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host-associated molecular pattern (HAMP)/endogenous elicitor
Arabidopsis
Pep
peptides (Ma et al.
2009a
,
b
; Qi et al.
2010
). However, DND1 is not required for
fl g22 and elf18 activation of Ca
2+
signaling (Jeworutzki et al.
2010
). Early signaling
through the
Arabidopsis
PRRs FLS2 and EFR has been shown to involve calcium
ion-associated opening of plasma membrane anion channels (Jeworutzki et al.
2010
). Glu-receptor-like-type Ca
2+
channels are involved in cryptogein- and fl g22-
triggered immune responses (Kwaaitaal et al.
2011
; Michard et al.
2011
; Vatsa et al.
2011
). The fl g22/FLS2 signaling show greater requirement for intracellular Ca
2+
stores and inositol phosphate signaling, whereas Pep/PEPR signaling requires
extracellular Ca
2+
to activate the Ca
2+
signaling system (Ma et al.
2012
). Ca
2+
signaling by the
Arabidopsis thaliana
Pep peptides depends on CGMP-activated
Ca
2+
channels (Qi et al.
2010
). These results show the requirement of different Ca
2+
channels for the different PAMPs to activate Ca
2+
signaling system and innate
immune responses.
4.11
Induction of Increases in Concentration, Oscillations
and Waves in Cytoplasmic Calcium Ion ([Ca
2+
]
cyt
)
Various elicitors have been shown to induce biphasic [Ca
2+
]
cyt
perturbations in plant
cells These elicitors elicit an immediate transient increase in [Ca
2+
]
cyt
in plant cells,
which is followed by a more prolonged elevation of [Ca
2+
]
cyt
lasting many minutes
or hours (Lecourieux et al.
2002
). The sustained increase in [Ca
2+
]
cyt
alone is corre-
lated with the induction of defense responses (Cessna and Low
2001
; Rudd and
Franklin-Tong
2001
; Lecourieux et al.
2002
). The elicitor-induced [Ca
2+
]
cyt
eleva-
tions predominantly result from a continuous Ca
2+
infl ux through the plasma
membrane (Hu et al.
2004
; Vandelle et al.
2006
).
The cytoplasmic Ca
2+
spikes (oscillations and waves) result from two opposing
reactions, Ca
2+
infl ux through channels and Ca
2+
effl ux through pumps and transport
systems (Hwang et al.
2000a
,
b
). Different messages can be encoded by changing a
Ca
2+
spike's magnitude, duration, location, or frequency (Sanders et al.
1999
;
McAinsh and Pittman
2009
). Ca
2+
signal is presented by the concentration of Ca
2+
(Trewavas and Malho
1998
; Trewavas
1999
). PAMPs/elicitors may activate Ca
2+
infl ux and the different signals may induce different Ca
2+
concentrations in the cyto-
sol. The different concentrations of Ca
2+
may activate different Ca
2+
-induced pro-
teins (Karita et al.
2004
).
The changes in [Ca
2+
]
cyt
concentrations are monitored by the Ca
2+
sensors, such
as calmodulins. Varying concentration of [Ca
2+
]
cyt
may differentially regulate the
calmodulin (CaM)-stimulated expression of CaM-binding proteins. Three types of
tobacco CaM isoforms have been reported in tobacco. Type I CaM induced NAD
kinase at 1-5
M, which is the increased Ca
2+
concentration in stimulated cells.
Type II CaM activated NAD kinase at lower Ca
2+
concentration of around 0.1
μ
M,
which is the cytosolic concentration in unstimulated cells. The type II CaM is
expressed constitutively and remained unchanged after external stimuli application.
μ
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