G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 174

(Fu et al. 2009 ), suggesting the role of these genes in disease development. The RAR1
and SGT1 genes are required for SA accumulation in Arabidopsis and both are required
in a genetically additive manner for induction of disease resistance (Zhou et al. 2008 ).
Overexpression of OsRAR1 and OsSGT1 in rice signifi cantly induced basal resistance
to both the bacterial pathogen X. oryzae pv. oryzae and the fungal pathogen M. oryzae
(Wang et al. 2008 ). RAR1 and SGT1 together contribute to basal resistance in
Arabidopsis . Both RAR1 and SGT1 are required for inducing disease resistance, SA
accumulation, and lesion formation after pathogen infection. RAR1 and SGT1 trigger
expression of various SA-regulated defense-related genes including PR-1 , PR-2 , PR-5 ,
RPW8.1 , RPW8.2 , WRKY6 , WRKY29 , and EDS1 (Zhou et al. 2008 ).
RAR1 and SGT1 have also been shown to be involved in JA-mediated signaling
system. Up-regulation of JA-inducible PDF1.2 and JR2 expression was found to be
compromised in rar1 and sgt1 mutants, suggesting that both RAR1 and SGT1 play
important role in JA signaling system (Kawamura et al. 2009 ). SGT1 is also required
for the activation of the SCF COI1 -mediated JA response (Gray et al. 2003 ; Lorenzo
and Solano 2005 ). The Arabidopsis COI1 gene is required for the JA-mediated
defense response against pathogens (Xie et al. 1998 ). COI1 protein contains an
F-box motif and associates physically with AtCUL1, AtRbx1 and the Skp1-like
proteins ASK1 and ASK2 to assemble SCF COI1 ubiquitin-ligase complexes (Skp1-
cdc53-F-box protein) (Xu et al. 2002 ). SCF COI1 targets key regulators of JA signal-
ing pathway for ubiquitination and subsequent degradation by the 26S proteasome
(Kawamura et al. 2009 ).
SGT1 and HSP90 are highly expressed in plants infected with pathogens
(Azevedo et al. 2006 ; Takahashi et al. 2003a ). HSP90 and HSP70 have a tight func-
tional link (Thao et al. 2007 ). SGT1, which associates with HSP70, is required for
its nuclear localization (Shirasu 2009 ). Small monomeric G-protein Rac1 forms a
complex with RAR1, SGT1, HSP90 and HSP70 in rice cells and these proteins play
important roles in plant innate immunity.

3.9 PAMP Signal May Convert the G-Proteins

from Their Inactive State to Their Active State
to Trigger Immune Responses

G-proteins have the ability to bind guanosine-5 -triphosphate (GTP) and hydrolyze

it to guanosine diphosphate (GDP). GDP locks G proteins into their inactive state,
while GTP locks G- proteins into their activated state (Gelli et al. 1997 ; Oki et al.
2009 ; Pandey et al. 2010 ). Active or inactive states of G-proteins depend on the
binding of GTP or GDP, respectively (Xing et al. 1997 ; Cabrera-Vera et al. 2003 ).
Both classes of G-proteins, heterotrimeric G-proteins and small G-proteins, use the
GTP/GDP cycle as a molecular switch for signal transduction (Xing et al. 1997 ;
Pandey et al. 2010 ).
Heterotrimeric G-proteins act as the specifi c reaction partners of G-protein-
coupled receptors. The GTPase is normally inactive. In the basic state, the G

′

α