G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 186

The G-protein-triggered synthesis of polyamines is involved in activation or
suppression of various signaling systems. Spermine treatment elicited biosynthesis
of jasmonic acid in lima bean leaves (Ozawa et al. 2009 , 2010 ). JA treatment
enhanced polyamine biosynthesis in sugarbeet (Haggag et al. 2010 ). Spermine
treatment induced calcium infl ux and ROS production (Ozawa et al. 2010 ).
Polyamines have been found to be a common source of hydrogen peroxide in
host- and nonhost hypersensitive response during pathogen infection (Yoda et al.
2009 ). Polyamine oxidases are H 2 O 2 producing enzymes (Angelini and Federico
1989 ; Angelini et al. 2008 ). Polyamines induce rapid biosynthesis of nitric oxide
(NO) in Arabidopsis thaliana (Tun et al. 2006 ). Polyamines also induced the activi-
ties of various enzymes involved in redox signaling system and the activated
enzymes included ascorbate peroxidase, glutathione peroxidase, glutathione reduc-
tase, superoxide dismutase and catalase (Ozawa et al. 2010 ). Interaction between
polyamines and SA signaling system in tomato has been reported (Szepesi et al.
2011 ). Polyamines induced susceptibility to the necrotrophic pathogen Botrytis
cinerea in tomato. The polyamine-mediated susceptibility to B. cinerea was shown
to be linked to interference with the functions of ethylene in plant defense
(Nambeesan et al. 2012 ).

3.16 G-Proteins Modulate Salicylic Acid Signaling Pathway

G-proteins may trigger salicylic acid signaling system (Beffa et al. 1995 ).
Cholera toxin from Vibrio cholerae is a multimeric protein consisting of A1, A2, and
fi ve B subunits. The A1 subunit catalyses the ADP-ribosylation of G

, which
irreversibly blocks the GTPase activity of G-proteins leading to the sustained
activation of the downstream signaling pathway (Beffa et al. 1995 ). Cholera
toxin does not activate G-proteins directly; it acts to maintain the active state of
G-proteins with bound GTP (Beffa et al. 1995 ). Transgenic tobacco plants
expressing A1 subunit of cholera toxin were developed and tissues of these
transgenic plants showed accumulation of high levels of salicylic acid (Beffa
et al. 1995 ). Sano et al. ( 1994 ) reported that expression of a small G- protein in
transgenic tobacco abnormally induced salicylic acid in response to an external
stimulus. Transgenic tobacco plants expressing a rice gene encoding small
GTPase, rgp1 , showed high accumulation of salicylic acid (Yoda and Sano
2003 ; Sano et al. 1994 ). These studies reveal that G-proteins are involved in SA
biosynthesis.
Tobacco plants transformed with the rgp1 gene showed increase in the mRNA
levels of genes encoding acidic pathogenesis-related proteins, which are inducible
by SA (Sano et al. 1994 ). Cultured rice cells were transformed with the rice
OsRac1 gene encoding a small G-protein. A salicylic acid induced protein, gluco-
syltransferase IS5a, was found to accumulate in these transformed cells (Fujiwara
et al. 2006 ). Engineering the rgp1 gene in tobacco has been shown to increase

α