G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 182

H 2 O 2 production than the wild-type cells. Transgenic plants overexpressing OsMT2b
showed increased susceptibility to the rice bacterial blight pathogen X. oryzae pv.
oryzae and the blast pathogen M . oryzae (Wong et al. 2004 ). The results suggest that
OsRac1 plays a role as a suppressor of ROS scavenging and triggers immune
responses against bacterial and fungal pathogens.

3.12.6 G-Proteins Act as Redox Regulators in ROS Signaling

The G-protein appears to be a key redox regulator in defense signaling (Ono et al.
2001 ; Baxter-Burrell et al. 2002 ). Fujiwara et al. ( 2006 ) detected activation of
several redox regulators in cultured rice cells transformed with OsRac1. The induced
redox regulators included glyceraldehyde-3-P dehydrogenase, NADPH-thioredoxin
reductase, ferredoxin-NADPH reductase, NADPH dependent oxidoreductase,
quinine oxidoreductase, and glutathione-S-transferase (GST1) (Fujiwara et al.
2006 ). Redox signaling is known to play an important role in innate immune system
(Desikan et al. 2005 ; Fedoroff 2006 ). Cytosolic glyceraldehyde-3-phosphate dehy-
drogenases interact with phospholipase D (PLD

) to transduce the ROS hydrogen
peroxide signal in Arabidopsis thaliana (Guo et al. 2012 ).

δ

3.13 G-Proteins Activate Nitric Oxide Signaling System

Nitric oxide (NO) plays a key role in immune signaling transduction system
(Lindermayr et al. 2010 ; Perchepied et al. 2010 ; Wang et al. 2010 ). Heterotrimeric
G-proteins have been shown to be involved in generation of NO (Li et al. 2009 ; He
et al. 2013 ). The calcium ion sensor protein calmodulin activates GPA1, the
G

-subunit of heterotrimeric protein in Arabidopsis . The activated heterotrimeric
G-protein in turn activates NADPH oxidases (Li et al. 2009 ). GPA1 has been
shown to function upstream of the NADPH oxidases AtrbohD and AtrbohF (Zhang
et al. 2011 ). These NADPH oxidases are involved in the production of H 2 O 2
(Li et al. 2009 ). The gpa1 mutants have been shown to be disrupted in production
of ROS (Zhang et al. 2011 ). The modulation of NO production by G

α

protein has
been shown to require NADPH oxidase-dependent H 2 O 2 generation (Fig. 3.7 ; Li
et al. 2009 ).
Stress-induced H 2 O 2 and NO generation were found to be regulated by GPA1,
the G

α

-subunit of heterotrimeric G-protein (He et al. 2013 ). The H 2 O 2 and NO accu-
mulation were nullifi ed in gpa1 knockout mutants but enhanced by overexpression
of a constitutively active form of GPA1 (He et al. 2013 ). The results suggest that the
heterotrimeric protein is involved in NO production. Further it has been demon-
strated that G

α

activation of NO production depends on H 2 O 2 (He et al. 2013 ). It
suggests that the signaling pathway involves G-protein-dependent activation of
H 2 O 2 production and subsequent NO accumulation.

α