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phosphorylation/dephosphorylation system in the plasma membrane in tomato
cells (Vera-Estrella et al. 1994b ).
An elicitor-induced increase in ATPase activity was shown to be activated by the
G-proteins in tomato (Vera-Estrella et al. 1994a ). G-protein has been shown to acti-
vate a membrane-bound phosphatase that mediates the dephosphorylation of the
host plasma membrane H + -ATPase in tomato (Xing et al. 1997 ). The dephosphory-
lation of H + -ATPase was followed by rephosphorylation by protein kinase and Ca 2+ -
dependent kinase (Xing et al. 1996 ).
3.11
G-Proteins May Trigger Effl ux of Vacuolar Protons
into Cytoplasm to Activate pH-Dependent Signaling
Pathway
Transient shifts of intracellular and apoplastic pH appear to be essential steps in
signal transduction processes (Viehweger et al. 2002 ). Cytoplasmic acidifi cation
induced by biotic stress is considered as a plant-specifi c trigger for the synthesis of
defense-related compounds including phytoalexins and other secondary metabolites
(Sakano 2001 ). Acidifi cation of the cytoplasm increases the expression of mRNAs
encoding several defense-related enzymes (Lapous et al. 1998 ; He et al. 1998 ). The
G-protein may activate phospholipase and pH-dependent signal path (Viehweger
et al. 2006 ). The function of a G
protein in defense signal transduction system was
studied by developing transgenic cell cultures of California poppy ( Eschscholzia
californica ) in which the G
α
content was decreased via antisense transformation or
by the expression of recombinant anti-G
α
single-chain antibodies. All transgenic
cell types were defi cient in two elicitor-triggered early signal events: activation of
phospholipase A 2 (PLA 2 ), and effl ux of vacuolar protons. The lacking H + effl ux
could be restored by adding lysophosphatidylcholine (LPC), a product of PLA 2
activity, to vacuoles in situ. The results suggest that G
α
mediates the stimulation of
PLA 2 by the elicitor and the resulting peak of LPC initiates a transient effl ux of
vacuolar protons into the cytoplasm. This results in acidic peak of the cytoplasmic
pH which may induce the expression of enzymes of phytoalexin production and
induce resistance against pathogens (Viehweger et al. 2006 ).
α
3.12
G-Proteins Switch on ROS Signaling System
3.12.1
G-Proteins Trigger Generation of ROS to Induce
Immune Responses
G-proteins activate primarily reactive oxygen species (ROS) production and
ROS-mediated signaling systems (Park et al. 2000 ; Bokoch and Diebold 2002 ;
Suharsono et al. 2002 ; Wong et al. 2007 ) to trigger innate immune responses.
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