PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 83

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. 2000). Different messages can be encoded by changing a
Ca 2+ spike's magnitude, duration, location, or frequency (Sanders et al. 1999). Ca 2+
signal is presented by the concentration of Ca 2+ (Trewavas 1999). PAMPs may
activate Ca 2+ infl ux and the different signals may induce different Ca 2+ concentra-
tions in the cytosol. The changes in [Ca 2+ ] cyt concentrations are monitored by the
Ca 2+ sensors and the Ca 2+ signals are subsequently decoded and propagated down-
stream to activate plant defense responses.
Activation of Ca 2+ -signaling system by different PAMPs has been demonstrated
(Lecourieux et al. 2006 ; Aslam et al. 2009 ). The PAMP Flg22 recognized in several
bacterial pathogens has been shown to induce Ca 2+ infl ux in Arabidopsis . The PAMP
activates calmodulin-like gene CML41 within 1 h after treatment and the peak
expression of the gene was observed at 12 h (Denoux et al. 2008 ). Cyclic nucleotide-
gated ion channels (CNGCs) are involved in the Ca 2+ -dependent signaling pathways
(Talke et al. 2003; Yoshioka et al. 2003 ). Arabidopsis DND1 codes for a cyclic
nucleotide-gated channel 2 (CNGC2) (Clough et al. 2000) and Flg22 induces the
expression of DND1 in Arabidopsis (Denoux et al. 2008 ). Flg22 also activates
cyclic nucleotide gated channel 4 encoding gene CNGC4 (Denoux et al. 2008 ).
These channels have been found to be calmodulin (CaM)-binding proteins
(Borsics et al. 2007).
The fl agellin upregulated the gene encoding Ca 2+ -dependent protein kinase in
rice cells (Fujiwara et al. 2004 ). Calcium-dependent protein kinases CDPK4,
CDPK5, CDPK6, and CDPK11 were shown to mediate the PAMP fl g22-triggered
defense responses, including defense gene expression and ROS production
(Boudsocq et al. 2010 ). The pathway involving the calcium-dependent protein
kinases (CDPK) 4/5/6/11 has been proposed to act in parallel to the MAPK path-
ways to control fl g22-dependent gene expression (Boudsocq et al. 2010 ). Flg22 also
induces the expression of BON1 gene which encodes a calcium-dependent phos-
pholipid binding protein (Denoux et al. 2008 ). It also activated CCD1 in cultured
rice cells (Fujiwara et al. 2004 ). CCD-1 encodes a Ca 2+ -binding protein that shares
homology with the C-terminal half domain of centrin and centrins are involved in
Ca 2+ signaling (Takezawa 2000).
Several PAMPs are known to trigger Ca 2+ infl ux, as one of the earliest signaling
systems (Aslam et al. 2009 ). The oomycete PAMP CBEL induces calcium ion
fl uxes in tobacco cells (Gaulin et al. 2006 ). Each PAMP may elicit different cal-
cium signatures. A comparison of calcium infl ux patterns revealed that Flg22
induced a rapid (about 2 min) response, usually with two or three minor, decreas-
ing peaks, whereas the elf18-induced calcium infl ux pattern was less defi ned and
broader (Aslam et al. 2009 ). Eighteen transcripts involved in calcium sensing were
up-regulated by Nep1 treatment. These genes encoded calcium-binding EF hand
family proteins, CAM, CAM-binding, CAM-related proteins, and Ca 2+ -ATPases
(Bae et al. 2006 ). The induction of a transcript encoding CAM-related protein