G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 179

3.12.2 G-Proteins Switch on Ca 2+ Infl ux - RBOH-Mediated

ROS Signaling Pathway

Numerous Rboh (for respiratory burst oxidase homolog) genes have been isolated
in plants. All rboh genes identifi ed to date possess a conserved N-terminal exten-
sion that contains two Ca 2+ binding EF-hand motifs. A substantial part of the
N-terminal region of Rboh, including the two EF-hand motifs, is required for the
Rac GTPases and the N-terminal extension interaction. Cytosolic Ca 2+ concentra-
tion has been shown to regulate Rac-Rboh interaction. Transient coexpression of
OsRac1 and rbohB enhanced ROS production in Nicotiana benthamiana , suggest-
ing that direct Rac-Rboh interaction may activate NADPH oxidase activity in plants
(Fig. 3.5 ; Wong et al. 2007 ; Zhao et al. 2010 ). Collectively these results suggest that
cytosolic Ca 2+ concentration may modulate NADPH oxidase activity by regulating
the interaction between Rac GTPase and Rboh.

3.12.3 Interplay Between ROP, RBOH, CDPK, Ca 2+ [cyt] ,

and ROS in G-Protein-Mediated ROS Signaling

An interplay between the monomeric small G-protein Rho-like GTPase of plants
(ROP), the plant orthologs of the respiratory burst NADPH oxidases (RBOH),
calcium-dependent protein kinase (CDPK), cytosolic calcium transients ( Ca 2+ [cyt] ),
and reactive oxygen species (ROS) production has been described (Van Breusegem
et al. 2008 ). It has been reported that activation of the plasma membrane-localized
RBOHs involves phosphorylation of two N-terminal Ser by a calcium-dependent
protein kinase as well as interaction with ROP. RBOH phosphorylation as well as
binding to calcium synergizes its activation, raising the possibility that it may func-
tion as a calcium sensor (Ogasawara et al. 2008 ; Takeda et al. 2008 ). The RBOH/
ROP-GTP interaction is regulated by the binding of calcium to two EF-hand motifs
at the N terminus of the NADPH oxidase (Wong et al. 2007 ). The RBOH activation
results in production of ROS (Van Breusegem et al. 2008 ). The activation of RBOH
promotes calcium channel activation and calcium infl ux, thereby stimulating RBOH
activity and amplifi cation of the initial signal (Takeda et al. 2008 ).

3.12.4 Phosphatidic Acid Activates G-Protein-Mediated

Pathway of ROS Generation

Phosphatidic acid (PA), a second messenger generated in a phospholipid signaling
pathway, has been shown to play an important role in Rho-related small G protein
GTPase-mediated pathway of ROS generation. G proteins activate phospholipase C
(PLC) and phospholipase D (PLD) (Munnik et al. 1995 ; Ritchie and Gilroy 2000 )