Phospholipids Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 375

membrane by activating a Ca 2+ channel (Alexandre et al. 1990 ). It operates through
receptors which resemble ryanodine receptors of human muscle (Berridge et al.
2000 ). This calcium channel is voltage-dependent and opened only on depolariza-
tion of the vacuoles (Alexandre et al. 1990 ). The calcium released through this
channel induces calcium waves and oscillations in the cytosol (Berridge 1993 ;
Hisatsune et al. 2005 ). The calcium ion infl ux into the cytosol may activate Ca 2+ -
dependent protein kinases (Munnik et al. 1995 ) and Ca 2+ signaling system (Huang
et al. 2001 ; Luan et al. 2002 ).

8.3 Phosphatidic Acid in G Proteins-Mediated

Signaling System

The second messenger PA has been shown to be involved in activation of ROS sig-
naling system. PA induces ROS-induced hypersensitive cell death through G
proteins-mediated signaling system (Park et al. 2004 ). The plant Rac-like GTPases,
named ROPs (Rho-related small G proteins) are involved in ROS generation (Yang
2002 ). PA activates Rho-related small G protein GTPase-mediated pathway of ROS
generation (Park et al. 2004 ). The induced ROS generation may be due to activation
of NADPH oxidase by PA (Park et al. 2004 ).

8.4 Phosphatidic Acid in ROS Signaling System

PA has been shown to be able to trigger an oxidative burst (Fig. 8.3 ; Sang et al.
2001 ; de Jong et al. 2004 ; Park et al. 2004 ). PA is involved in the activation of
NADPH oxidase in macrophages (McPhail et al. 1999 ) and it has been suggested
that similar activation of NADPH oxidase may also occur in plants and ROS is gen-
erated through the action of NADPH oxidase (Laxalt et al. 2007 ). PA and DAG
directly activate NADPH oxidase by interacting with enzyme components (Palicz
et al. 2001 ). PA has been shown to induce ROS in tomato cells. Scavenging of NO
or inhibition of either the PLC or the DAG kinase enzyme diminished elicitor-
induced ROS production (Laxalt et al. 2007 ). PA promotes superoxide-generating
activity in plants through the activation of NADPH oxidase (Sang et al. 2001 ). DAG
is also able to generate NADPH-dependent superoxide synthesis (Sang et al. 2001 ).
PA has been shown to bind a protein kinase, 3 -phosphoinositide-dependent

kinase 1 (PDK1) in Arabidopsis and to activate protein kinase AGC2-1 in a PDK-
dependent manner (Deak et al. 1999 ; Anthony et al. 2004 ). PDK1 is specifi cally
activated by PLD-generated PA in Arabidopsis cells treated with a fungal elicitor
(Anthony et al. 2006 ). AGC2-1 is identical to OXI1, a protein kinase implicated in
oxidative burst-mediated signaling in Arabidopsis (Rentel et al. 2004 ).

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