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molecules that are used by plants to encode information and deliver it downstream
to proteins which decode/interpret signals and initiate cellular responses (Snedden
and Fromm 2001 ). Highly complex networks of signaling pathways are involved in
transmission of the PAMP signals to induce the plant immune responses (Koornneef
and Pieterse 2008 ; Gfeller et al. 2010 ; Leon-Reyes et al. 2010 ; Perchepied et al. 2010 ;
Katagiri and Tsuda 2010 ; Choi and Hwang 2011 ; Fernández-Calvo et al. 2011 ).
These signaling pathways are not simple linear and isolated cascades, but can crosstalk
with each other (McGrath et al. 2005 ; Flors et al. 2008 ; Koornneef and Pieterse
2008 ). Both antagonism (Balbi and Devoto 2008 ; Flors et al. 2008 ) and synergism
(Mur et al. 2006 ; De Vos et al. 2006 ; Mao et al. 2007 ) between the signaling systems
have been reported.
2.20.2
Ca 2+ Signaling System
In plant cells, the calcium ion is a ubiquitous intracellular second messenger
involved in numerous signaling pathways (Lecourieux et al. 2006 ; Zhu et al.
2009 ; Ma et al. 2009 ). Calcium ion acts as a signal carrier and the calcium signaling
is modulated by specifi c “calcium signatures” (Lecourieux et al. 2006 ). Spatial
and temporal changes in cytosolic calcium ([Ca 2+ ] cyt ) are called “calcium signatures”
(Luan et al. 2002). These changes may proceed as single calcium transients,
oscillations, or repeated spikes/waves (Lecourieux et al. 2006 ). Specifi c calcium
signatures are recognized by different calcium sensors to transduce calcium-medi-
ated signals into downstream events (Harmon et al. 2000; Sanders et al. 2002;
Reddy and Reddy 2004). The Ca 2+ signature controls diverse cellular processes
via Ca 2+ sensors which include calmodulins (CaM), CaM-like and CaM-related
proteins, calcineurin B-like (CBL) proteins, Ca 2+ -dependent protein kinases
(CDPKs) and Ca 2+ -binding proteins without EF hands (Snedden and Fromm
2001 ; Luan et al. 2002; Tomsig et al. 2003; Kang et al. 2006 ; Kobayashi et al.
2007 ; Takabatake et al. 2007 ).
Several Ca 2+ -permeable channels have been found in plant plasma membranes
and they have been implicated in plant immune signaling system (White and
Broadley 2003). Calcium ion channels are integral membrane proteins that are
involved in transport of solutes across the cell membrane in plants (Maathuis et al.
1997). Cyclic nucleotide-gated ion channels (CNGCs) have been found in plant cell
plasma membrane (Kaplan et al. 2007; Baxter et al. 2008 ). CNGCs are involved in
Ca 2+ -dependent signaling pathways (Talke et al. 2003; Yoshioka et al. 2003 ).
PAMPs elicit calcium ion infl ux within 15-30 min after PAMP treatment in plant
cells, resulting in an immediate increase in Ca 2+ concentration in the cytosol
(Lecourieux-Ouaked et al. 2000 ; Aslam et al. 2008 ). PAMP perception leads to
membrane potential depolarization and an increase in cytoplasmic Ca 2+ concentration
(Lecourieux et al. 2006 ; Aslam et al. 2008 ; Jeworutzki et al. 2010 ). Various PAMPs
elicit an immediate increase in [Ca 2+ ] cyt (cytoplasmic calcium ion) concentration
in plant cells (Lecourieux et al. 2002 ). The PAMP-induced [Ca 2+ ] cyt elevations
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