G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 171

Schulze-Lefert 2003 ; Wong et al. 2004 ; Moeder et al. 2005 ; Nakashima et al. 2008 ).
The small G-protein OsRac1 is involved in basal and R protein-mediated resistance
to the rice blast fungal pathogen M. oryzae and the bacterial blight pathogen
X. oryzae pv. oryzae (Ono et al. 2001 ; Suharsono et al. 2002 ). The constitutively
active OsRac1 greatly reduced blast and bacterial blight disease development
(Ono et al. 2001 ). The role of Medicago truncatula small GTPase MtROP9 , orthol-
ogous to Medicago sativa Rac1 , in the Aphanomyces root rot development in
M . truncatula was studied by silencing the MtROP9 gene using the RNA interfer-
ence (RNAi) vector (Kiirika et al. 2012 ). MtROP9 knockdown promoted the root rot
disease development, suggesting the role of the small G-protein in conferring resis-
tance against the oomycete pathogen Aphanomyces euteiches (Kiirika et al. 2012 ).
Transgenic tobacco plants that expressed a dominant negative form of the small
G-protein OsRac1 from rice showed reduced resistance against Tobacco mosaic
virus (TMV) compared to the wild-type plants, suggesting involvement of OsRac1 in
triggering resistance against the virus disease in tobacco (Moeder et al. 2005 ).

3.7 Small G-Proteins May Be Involved in Susceptible

Interactions

Although small monomeric G-proteins/small GTPases are involved in activation of
immune responses against a wide range of pathogens (Ono et al. 2001 ; Suharsono
et al. 2002 ; Shirasu and Schulze-Lefert 2003 ; Nakashima et al. 2008 ), they may also
be involved in disease development process in some plant-pathogen interactions
(Schultheiss et al. 2003 ). Barley RAC/ROP G-protein family members have been
shown to be involved in susceptibility to the powdery mildew pathogen Blumeria
graminis f. sp. hordei . Five Rac/Rop genes were constitutively expressed in the
barley leaf epidermis and none of these genes showed enhancement of mRNA
abundance after inoculation with B. graminis f. sp. hordei . The small GTPases
HvRACB, HvRAC3 and HvROP6 proteins were found to be potentially involved in
the establishment of susceptibility to the barley powdery mildew fungus B. graminis
f. sp. hordei . These small G-proteins may be involved in processes supporting para-
sitic entry into epidermal host cells (Schultheiss et al. 2003 ). Interaction of plant
RAC homologs with the NADPH oxidase complex appears to regulate activity of
NADPH oxidase that produces O − 2 in response to pathogen attack (Ono et al. 2001 ).
In barley, O − 2 production takes place during attack by B . graminis f. sp. hordei at sites
of successful penetration of epidermal cells, but not at sites where fungal penetration
is prevented (Hückelhoven and Kogel 1998 ). In contrast, H 2 O 2 accumulates in barley
at sites where penetration by B. graminis f. sp. hordei is successfully prevented
(Thordal-Christensen et al. 1997 ; Hückelhoven et al. 2000 ). These results suggest
that RAC small GTP-binding protein might have activated NADPH oxidase-
dependent O − 2 production that would have facilitated the fungal penetration. ROS
has been reported to be involved in both activation and suppression of immune
responses depending on spatial and quantitative differences in the occurrence of ROS