Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 227

surface in each domain. These hydrophobic surfaces are largely responsible for the
binding of CaM to its targets. The unique fl exibility and high polarizability of the
Met residues located at the entrance of each hydrophobic pocket together with other
hydrophobic amino acid residues create adjustable, sticky interaction surface areas
that can accommodate CaM targets, which have various sizes and shapes (Zhang
and Yuan 1998 ). The binding of calcium to calmodulin induces a conformational
change that exposes hydrophobic binding sites that interact with target proteins,
altering the activity of those proteins (Harmon 2003 ).
Plant cells contain multiple CaM isoforms with varying degrees of sequence
homology to the single CaM reported in mammals. Several CaM genes have been
isolated from plants (Lee et al. 1995 , 1999 ; Heo et al. 1999 ; Kim et al. 2009 ).
Thirteen CaM genes have been detected in tobacco (Takabatake et al. 2007 ). In
soybean fi ve CaM isoforms with varying degrees of sequence homology to the
single mammalian CaM have been identifi ed (Lee et al. 1995 ). Not all, but specifi c
CaM genes are involved in defense signaling. The tobacco CaM gene, NtCaM13 ,
was found to induce resistance against the bacterial pathogen Ralstonia sola-
nacearum , the fungal pathogen Rhizoctonia solani , the oomycete pathogen Pythium
aphanidermatum , and not against Tobacco mosaic virus in tobacco, while NtCaM1
did not have any role in inducing resistance against the pathogens (Takabatake et al.
2007 ). The CaM isoforms have different expression patterns in various plant tissue
types, suggesting that they play unique roles in the many different Ca 2+ signaling
pathways of plants (Lee et al. 1995 , 1999 ; Cho et al. 1998 ; Takabatake et al. 2007 ).
Elicitors activate CaM isoforms which participate in Ca 2+ -mediated induction of
defense response (Heo et al. 1999 ). Upon increase of Ca 2+ to submicromolar levels,
all CaM molecules are activated. Full activation of the CaM occurs in a narrow
region of calcium concentration during a signaling event (Luan et al. 2002 ).
Induction of CaM genes, SCaM-4 and SCaM-5 genes in soybean depended on the
increase of intracellular Ca 2+ level (Heo et al. 1999 ). The constitutive expression of
these soybeans genes in transgenic tobacco plants constitutively expressed genes
encoding PR-1a, PR-1b, PR2, PR3, PR4, PR5, class III acidic chitinase and class III
basic chitinase (Heo et al. 1999 ). The expression of tobacco NtCaM13, which is
closely related to SCaM4 and SCaM5, was elevated both at the RNA and protein
level in TMV-infected leaves (Yamakawa et al. 2001 ).
Ca 2+ -CaM binds and regulates the activity of a wide range of proteins. Three
types of tobacco calmodulin (CaM) isoforms originated from 13 genes. These CaMs
differentially activate target enzymes. Plant NAD kinase was activated most effec-
tively by type II (NtCaM3 - NtCaM12), moderately by type I (NtCaM1 and
NtCaM2), and weakly by type III (NtCaM13) CaMs. By contrast, NO synthase was
activated most effectively by type III, moderately by I, and weakly by type II CaMs
(Karita et al. 2004 ). In soybean, SCaM-4 activates cyclic nucleotide phosphodies-
terase while it is unable to activate the CaM-dependent NAD kinase (Lee et al.
1995 ). By contrast, SCaM-1 activates NAD kinase (Lee et al. 1997 ). SCaM-1
activates the protein phosphatase calcineurin, while SCaM-4 antagonizes its activa-
tion (Cho et al. 1998 ).