Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 5

and Pittman 2009 : Abdul Kadar and Lindsberg 2010 ; DeFalco et al. 2010 ; Hamada
et al. 2012 ; Stael et al. 2012 ).
Guanosine triphosphate (GTP)-binding proteins (G-proteins) are the regulatory
GTPases, which act as molecular switches in signal transduction system (Yalowsky
et al. 2010 ; Zhang et al. 2011 , 2012 ). Two classes of signaling G-proteins, het-
erotrimeric G-proteins and small monomeric G-proteins (Ras/Ras-like small
GTPases), have been reported. 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 ).
Ca 2+ is a master regulator of gene expression in plants (Galon et al. 2010 ).
Calcium ion acts as a signal carrier (Allen et al. 2000 ). Calcium signaling is modu-
lated by specifi c calcium signatures. Ca 2+ signatures are generated in the cytosol,
and in noncytosolic locations including the nucleus and chloroplast through the
coordinated action of Ca 2+ infl ux and effl ux pathways (McAinsh and Pittman 2009 ).
Specifi c calcium signatures are recognized by different calcium sensors to transduce
calcium-mediated signals into downstream events (Reddy et al. 2011 ; Wang et al.
2012 ; Hashimoto et al. 2012 ).
Mitogen-activated protein kinase (MAPK) cascades are major pathways
downstream of sensors/receptors that transduce extracellular stimuli into intra-
cellular responses in plants (Hettenhausen et al. 2012 ; Zhang et al. 2012 ). A typi-
cal MAPK signaling module consists of three interconnected protein kinases: a
MAP kinase kinase kinase (MAPKKK or MEKK [for MAPK/Extracellular
signal-regulated kinase Kinase Kinase]), a MAP kinase kinase (MAPKK or
MKK), and a MAP kinase (MAPK or MPK) (Mészáros et al. 2006 ). MAP kinase
cascade involves sequence of phosphorylation events (Hirt 2000 ). MAPKs func-
tion at the bottom of the three-kinase cascade and are activated by MAPKKs
through phosphorylation on the Thr and Tyr residues in their activation motif
between the kinase subdomain VII and VIII. The activity of MAPKKs is, in turn,
regulated by MAPKKKs via phosphorylation of two Ser/Thr residues in the
activation loop of MAPKKs. MAPKKKs receive signals from upstream receptors/
sensors (Ichimura et al. 2002 ; Li et al. 2012 ).
The oxidative burst involving rapid and transient production of reactive oxygen
species (ROS) is a very rapid response, occurring within seconds (Bolwell et al.
1995 ) or within a few minutes (Arnott and Murphy 1991 ) of PAMP treatment, suggest-
ing that the oxidative burst may not require de novo protein synthesis but involves
the activation of pre-existing enzymes. NADPH oxidase (Bae et al. 2006 ), peroxi-
dases (Halliwell 1978 ; Lehtonen et al. 2012 ), and xanthine oxidase (Allan and Fluhr