G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 163

of polyamines, biosynthesis of phosphatidic acid and programmed cell death.
G-proteins also activate various plant hormone signaling systems including salicylic
acid-, jasmonic acid-, ethylene-, abscisic acid-, auxin-, brassinosteroid-, and gibber-
ellic acid- mediated signaling systems. The different subunits in heterotrimeric
G-proteins and the monomeric small G-proteins may behave differently in activating
defense responses against various pathogens. Ability of G-proteins to trigger immune
responses also varies depending upon the type of invading pathogen.

Keywords GTPases • Molecular switches • PAMP signal • G-proteins • GTPase
activating protein • GDI • Heterotrimeric G-proteins • Small G-proteins

3.1 G-Proteins Switch on Plant Innate Immunity

Signaling Systems

Guanosine triphosphate (GTP)-binding proteins (G-proteins) are the regulatory
GTPases, which act as molecular switches in signal transduction system (Gilman
1987 ; Cabrera-Vera et al. 2003 ; Nibau et al. 2006 ; Zeng et al. 2007 ; Fujiwara et al.
2009 ; Yalovsky et al. 2010 ; Zhang et al. 2011 , 2012 ). Plant cells contain many
G-proteins (Xing et al. 1997 ; Roos et al. 1999 ; Suharsono et al. 2002 ; Morel et al.
2004 ; Zeng et al. 2007 ; Yong et al. 2010 ; Zhang et al. 2012 ). Two classes of signal-
ing G-proteins have been reported. These include heterotrimeric G-proteins and
small monomeric G-proteins (Ras/Ras-like small GTPases) (Gu et al. 2004 ; Perfus-
Barboch et al. 2004 ). In the Ras superfamily of small GTPases, only the Ras and
Rho families have been shown to transmit extracellular signals (Gu et al. 2004 ). Ras
superfamily is named the Ras superfamily because the founding members are
encoded by human Ras genes initially discovered as cellular homologs of the viral
ras oncogene. Plants do not possess a true Ras GTPase such as those that are pivotal
signaling in animals. Instead, they have a unique subfamily of Rho-family GTPases,
called ROPs (Rho-related GTPase of plants). ROP is the sole subfamily of Rho
GTPase in plants. ROPs are also referred to as RAC (for Ras [rat sarcoma oncogene
product] related C3 botulinum toxin substrate) proteins (Gu et al. 2004 ; Kiirika
et al. 2012 ). RAC/ROP small GTPases share a common ancestor with Rho, cdc42
and Rac and they are the only Rho-like GTPases in plants (Gu et al. 2004 ).
Many studies using inhibitors and agonists of G-proteins in different plant spe-
cies have suggested that G-proteins are involved in defense signaling initiated by
pathogen-associated molecular patterns (PAMPs)/elicitors (Legendre et al. 1992 ;
Beffa et al. 1995 ; Gelli et al. 1997 ; Ono et al. 2001 ; Park et al. 2000 ; Zhang et al.
2011 , 2012 ). Transgenic tobacco plants that expressed an antisense construct
derived from a Medicago sativa cDNA encoding a G-protein failed to show disease
resistance-related hypersensitive reaction when infi ltrated with an elicitor (Schiene
et al. 2000 ). It suggests that the G-protein is involved in defense signaling.
Both heteromeric G-proteins and small G-proteins (RAC/ROP small GTPases)
play an important role in activating various signal transduction systems initiated by
PAMPs (Fig. 3.1 ; Gao et al. 2010a , b ; Kiirika et al. 2012 ). Ca 2+ is a master regulator