Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 213

CNGCs are involved in Ca 2+ -dependent signaling pathways (Talke et al. 2003 ;
Yoshioka et al. 2006 ). The CNGC detected in Arabidopsis , AtCNGC2, is a plasma
membrane protein permeable to Ca 2+ (Yu et al. 1998 ). Leng et al. ( 1999 , 2002 ) and
Balague et al. ( 2003 ) showed cyclic nucleotide-dependent conductance of Ca 2+ in
Arabidopsis . The atcngc2 knock-out plants showed an altered Ca 2+ signature in
response to the bacterial PAMP lipopolysaccharide (LPS) (Ma et al. 2009b ). The
unique position of CNGCs as ligand-gated Ca 2+ -permeable channels suggests that
they function at key sites where cyclic nucleotide and Ca 2+ signaling pathways
interact (Talke et al. 2003 ).
The CNGCs contain a C-terminal cyclic nucleotide-binding domain with over-
lapping calmodulin-binding domain. Calmodulin-binding domains are common in
CNG ion channels and calmodulin binding has been demonstrated for the
Arabidopsis CNGC proteins AtCNGC1 and AtCNGC2 (Kohler et al. 1999 ; Kohler
and Neuhaus 2000 ). A tobacco plasma membrane calmodulin-binding channel
protein (designated NtCBP4) had a putative cyclic nucleotide-binding domain. The
NtCBP4 calmodulin-binding domain was found to perfectly coincide with an alpha-
C-helix motif of its putative cyclic nucleotide-binding domain. The coinciding
calmodulin- and cyclic nucleotide-binding domains may serve as a point of com-
munication between calcium and cyclic nucleotide signal transduction pathways in
plants (Arazi et al. 2000 ). CNGCs that are regulated by calmodulin play essential
roles in signal transduction (Borsics et al. 2007 ).
CNGCs play important role in plant innate immunity signaling (Baxter et al.
2008 ; Abdel-Hamid et al. 2011 ; Moeder et al. 2011 ). In Arabidopsis , the CNGCs
AtCNGC2 and AtCNGC4 have been shown to trigger hypersensitive reaction (HR),
an important component of defense signaling pathway. Arabidopsis mutants, dnd1
(without functional CNGC2) and dnd2 / HLM1 (without functional AtCNGC4), fail
to produce HR (Balague et al. 2003 ; Jurkowski et al. 2004 ). AtCNGC11 and
AtCNGC12 are positive regulators of defense signaling in Arabidopsis . These two
AtCNGCs play a role in SA-signaling system dependent on EDS1 and PAD4
(Yoshioka et al. 2006 ). The chimeric gene AtCNGC11/12 induced resistance associ-
ated HR-related cell death. The vacuolar processing enzyme (VPE), a caspase-like
protein, was involved in this process. In VPE-silenced plants, development of cell
death was much slower and weaker compared to control plants, suggesting the
involvement of VPE as a caspase in AtCNGC11/12-induced HR-related cell death
(Urquhart et al. 2011 ). DND1 negatively regulates disease resistance to bacterial
pathogens in Arabidopsis (Ahn 2007 ). The resistance of dnd1 mutant against
Pectobacterium carotovorum was dependent on calmodulin and inhibition of Ca 2+
increment (Ahn 2007 ).
The signaling cascade initiated by the host-associated molecular pattern
(HAMP)/damage-associated molecular pattern (DAMP) AtPep1 leads to expres-
sion of defense genes in a Ca 2+ -dependent manner (Qi et al. 2010 ). The endogenous
elicitor AtPep1 after binding with its pattern recognition receptor (PRR) AtPepR1
activates plant membrane inwardly conducting Ca 2+ permeable channels in meso-
phyll cells, resulting in cytosolic Ca 2+ elevation. This activity is dependent on the
PRR AtPepR1 as well as a cyclic nucleotide-gated channel (CNGC2). The PRR