PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 85

2.20.4 G-Proteins

G-proteins (guanosine triphosphate-binding proteins) act as molecular switches in
signal transduction system (Cabrera-Vera et al. 2003; Zeng et al. 2007 ). Several
studies using inhibitors and agonists of G-proteins in different plant species have
suggested that G-proteins are involved in activation of various defense signaling
systems initiated by PAMPs (Beffa et al. 1995; Gelli et al. 1997; Ono et al. 2001;
Park et al. 2000). G-proteins trigger changes in cytosolic Ca 2+ concentrations
(Schultheiss et al. 2003). The G-proteins induce Ca 2+ channel opening in plants
through the action of PAMPs (Gelli et al. 1997). G-proteins are involved in PAMP-
activated ROS-mediated signaling system (Park et al. 2000; Suharsono et al. 2002).
The PAMP fl g22 induces G-protein-activated ROS signaling systems. The gene
AGB1 , encoding the -subunit of G protein in Arabidopsis , is highly induced after

fl g22 treatment (Zipfel et al. 2006 ). The agb1 mutants are impaired in the oxidative
burst triggered by fl g22, suggesting the importance of G-proteins in ROS signaling
system (Ishikawa 2009). G-proteins are also involved in salicylate signaling system
(Beffa et al. 1995), jasmonate signaling system (Zhao and Sakai 2003), ethylene
signaling system (Fujiwara et al. 2006 ), and abscisic acid signaling system (Liu
et al. 2007 ). G-protein OsRac1 induces biosynthesis of the important second mes-
senger polyamine (Fujiwara et al. 2006 ). The G-protein may also be involved in
generation of phospholipid second messengers (Viehweger et al. 2006). G-proteins
may be involved in Ca 2+ channel opening (Gelli et al. 1997). Protein phosphoryla-
tion precedes Ca 2+ infl ux in tobacco cells treated with a PAMP isolated from the
oomycete pathogen Phytophthora cryptogea (Tavernier et al. 1995). The G-proteins
modulate the phosphorylation/dephosphorylation system in the plasma membrane
of tomato cells and transduce the signal (Vera-Estrella et al. 1994a). Phosphorylation
of proteins involved in G-protein coupled signaling has been reported in tobacco
cells treated with a bacterial elicitor (Gerber et al. 2006 ). Heterotrimeric G proteins
are involved in many diverse physiological processes in plants (Temple and Jones
2007 ; Chen 2008 ; Gao et al. 2008b ; Oki et al. 2009). The role of G-proteins in sig-
naling system in plant immune responses is further described in Chap. 3 .

β

2.20.5 ROS Signaling System

The oxidative burst involving rapid and transient production of reactive oxygen
species (ROS) is one of the most rapid defense responses observed in plants (Faize
et al. 2004; Asada 2006; Sagi and Fluhr 2006; Vidhyasekaran 2007a , b ). The oxida-
tive burst is a very rapid response, occurring within seconds (Bolwell et al. 1995) or
within a few minutes (Arnott and Murphy 1991) of PAMP treatment, suggesting
that the oxidative burst may not require de novo protein synthesis but involves the
activation of pre-existing enzymes. NADPH oxidase (Bae et al. 2006 ), peroxidases
(Halliwell 1978; Lehtonen et al. 2012 ), and xanthine oxidase (Allan and Fluhr 1997;