G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 178

Both small G-proteins (Yang 2002 ; Schultheiss et al. 2003 ; Morel et al. 2004 ;
Wong et al. 2007 ; Kiirika et al. 2012 ) and heterotrimeric G-proteins (Zhu
et al. 2009 ; Zhao et al. 2010 ; Zhang et al. 2011 ) are known to trigger generation
of ROS.
Small monomeric G-proteins (small GTPases) are involved in the regulation of
ROS generation in the innate immune responses via the activation of NADPH oxi-
dase homologs of plants termed respiratory burst oxidase homolog (RBOH)
(Agrawal et al. 2003 ; Kiirika et al. 2012 ). A small G-protein from rice, OsRac1, has
been shown to induce ROS production in rice cells (Kawasaki et al. 1999 ; Ono et al.
2001 ; Wong et al. 2007 ). OsRac1 was found to be a positive regulator of disease
resistance. It activates RBOH-mediated ROS signaling through direct binding of
Rac1 to the catalytic subunits of the RBOH protein's N-terminal extension, which
is specifi c for the plant RBOH proteins (Kawasaki et al. 2006 ; Wong et al. 2007 ;
Nakashima et al. 2008 ).
The small GTPase MtROP9 triggered the expression of MtRBOH gene
involved in ROS generation and conferred resistance against root rot of Medicago
truncatula caused by 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. The dominant-negative OsRac1 gene
in tobacco suppressed ROS accumulation (Moeder et al. 2005 ), suggesting that
OsRac1 is involved in ROS production. Collectively these results suggest that the
small G-protein is involved in activating immune responses by activating ROS
signaling.
Heterotrimeric G-proteins are also involved in ROS production. The
Arabidopsis G subunit, GPA1, is involved in ROS production (Zhao et al.

2010 ). The Arabidopsis gpa1 mutants have been shown to be disrupted in pro-
duction of ROS (Zhang et al. 2011 ). It has been reported that different subunits
of a heterotrimeric G-protein may signal ROS production (oxidative burst) in
different manner. The Arabidopsis heterotrimeric G-protein is known to contain
three subunits, G

α

proteins were shown to be nec-
essary component of the biphasic oxidative burst, while only the G

α , G β , and G γ . The G α and G β

protein was
found to be required for the late phase of the oxidative burst in Arabidopsis
thaliana induced by an external signal. The early endogenous ROS production
induced by the elicitor signal was mediated by both the G

α

proteins,
while the extracellular ROS signal induced by the elicitor was generated by
membrane-bound NADPH oxidases mediated by only the G

α and G β

α protein (Joo et al.

2005 ). In addition to a single prototypical G α protein (GPA1), Arabidopsis has

three unique G -like proteins, known as Extra Large G-proteins (XLG). The

transgenic Arabidopsis plants overexpressing one of the genes encoding XLG
proteins, XLG2 , showed constitutive accumulation of transcripts from RbohC
and the NADPH oxidase RbohC is known to be involved in ROS production
(Zhu et al. 2009 ). These studies suggest that both the groups of G-proteins,
small G-proteins and heterotrimeric G-proteins, trigger immune responses by
activating ROS production.

α