Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 207

et al. 2010 ), calcineurin B-like (CBL) proteins (Luan et al. 2002 ; Hashimoto et al.
2012 ), Ca 2+ -dependent protein kinases (Harmon et al. 2001 ; Harmon 2003 ; Ludwig
et al. 2004 : Kobayashi et al. 2007 ; Batisti and Kudla 2009 ; Luan 2009 ; Hashimoto

et al. 2012 ), and Ca 2+ -binding proteins without EF hands (Clark and Roux 1995 ;
Reddy 2001 ; Tomsig and Cruetz 2002 ; Tomsig et al. 2003 ).
The calcium sensor proteins fall into two main classes, referred to as sensor
relays and sensor responders (Sanders et al. 2002 ; Reddy and Reddy 2004 ). Sensor
relays include calmodulin (CaM), CaM-related proteins, and calcineurin B-like
(CBL) proteins. They function through bimolecular interactions. They undergo a
conformational change induced by Ca 2+ before interacting with and changing the
activity or structure of the target protein (Lecourieux et al. 2006 ). Sensor responders
are Ca 2+ -dependent protein kinases (CDPKs) (Cheng et al. 2002 ). They function at
fi rst through intramolecular interactions and undergo a Ca 2+ -induced conforma-
tional change that alters the protein's own activity or structure (Harmon et al. 2000 ;
Harper et al. 2004 ). These two groups of proteins are involved in decoding calcium
signals (Lecourieux et al. 2006 ; Kudla et al. 2010 ).
Changes and oscillations in cytosolic free Ca 2+ concentration are associated with
transduction of signals in plant cells (Sanders et al. 1999 , 2002 ). Many biotic and
abiotic signals elicit transient increases in cytoplasmic free Ca 2+ ([Ca 2+ ] cyt ) concentra-
tion in plants (Luan et al. 2002 ). Increases in Ca 2+ ] cyt may be mostly due to infl ux of
external Ca 2+ . However, internal Ca 2+ release may also contribute for increases in
[Ca 2+ ] cyt (Staxen et al. 1999 ; Hwang et al. 2000a , b ; Karita et al. 2004 ). Cellular Ca 2+
levels are tightly regulated in plant cells and hence small changes in intracellular Ca 2+
can provide information for the modifi cation of enzyme activity and gene expression
(Gong et al. 2004 ). Signals may induce an oscillation in the cytosolic free Ca 2+ con-
centration. 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, including
altered phosphorylation and gene expression patterns (Sanders et al. 2002 ; Hashimoto
et al. 2012 ). Thus the extracellular signals are transmitted to cellular calcium-depen-
dent effectors to activate the transcription of immune response-related genes (Luan
et al. 2002 ; Sanders et al. 2002 ; Gong et al. 2004 ; Lecourieux et al. 2006 ; Ma and
Berkowitz 2007 ; Dodd et al. 2010 ; Reddy et al. 2011a ).

ć

4.2 Upstream Events Leading to Activation

of Ca 2+ - Permeable Channels

4.2.1 PAMP-Triggered Ca 2+ Infl ux and Elevations

in Cytosolic Free Calcium

Transient changes in permeability of the plasma membrane to Ca 2+ and infl ux of
extracellular Ca 2+ through the membrane appear to be one of the earliest events in
defense signaling system (Atkinson et al. 1996 ; Wendehenne et al. 2002 ; Garcia-
Brugger et al. 2006 ; Laohavisit et al. 2009 ; Vadassery and Oelm ū

ller 2009 ;