Reactive Oxygen Species and Cognate Redox Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 285

The early signaling events induced by PAMPs include the increased expression
of genes encoding G-proteins. The gene AGB1 , encoding the -subunit of G-protein

in Arabidopsis , is highly induced after fl g22 treatment (Zipfel et al. 2004 ). The
PAMP-activated small G-proteins (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 ) have been shown to trigger generation of ROS. The G-proteins are
involved in the regulation of ROS generation via the activation of NADPH oxidase
(RBOH) (Fig. 5.1 ; Agrawal et al. 2003 ; Kiirika et al. 2012 ). The small GTP-binding
protein Rac2 homologs (called Rho-like proteins) regulate the production of ROS
by the NADPH oxidase (Kawasaki et al. 1999 ; Moeder et al. 2005 ; Wong et al.
2007 ). Different plant Rac proteins appear to act as either positive or negative regu-
lators of ROS production. Osrac1 is a positive regulator of ROS production in rice
(Ono et al. 2001 ), whereas Ntrac5 acts as negative regulator of ROS production in
tobacco (Morel et al. 2004 ). In Arabidopsis, heterotrimeric G protein signaling
mediates the oxidative burst (Joo et al. 2005 ). The Arabidopsis agb1 mutants are
impaired in the oxidative burst triggered by fl g22, suggesting the importance of
G-proteins in ROS production (Ishikawa 2009 ).
MAP kinases may be involved in generation of ROS by activating the NADPH
oxidase (Asai et al. 2008 ). Two different MAPK cascades have been shown to be
involved in induction of ROS in Nicotiana benthamiana . The MAPK cascades
NPK1-MEK2-SIPK/NTF4 and NPK1-MEK1-and NTF6 are involved in activation
of NADPH oxidase which is involved in production of ROS (Asai et al. 2008 ). The
MAP kinases may induce the NADPH oxidase at the gene transcriptional level and
also by post-translational level (Yoshioka et al. 2003 ). N. benthamiana MAPK
kinase induced the NADPH oxidase gene NbrbohB at the transcriptional level. At
the post-translational level, the NADPH oxidase-induced oxidative burst is controlled
through phosphorylation activation by its upstream MAP kinase and dephosphory-
lation inactivation by its negative regulator, phosphatase (Yoshioka et al. 2003 ).
Calcium-dependent protein kinase (CDPK) has also been shown to phosphorylate
NADPH oxidase (Xing et al. 1997 ; Blumwald et al. 1998 ). Accumulation of ROS
requires both Ca 2+ infl ux and protein kinase activity (Romeis et al. 1999 ).

β

5.2.3 Cell Wall Peroxidases Are Involved in ROS Production

in Some Plant Systems

Cell wall peroxidases have been shown to be involved in ROS production in some
plant systems (Daudi et al. 2012 ; O'Brein et al. 2012 ). In horseradish ( Armoracia
lapathifolia ), the accumulation of H 2 O 2 has been suggested to be due to the action
of peroxidase producing phenolic and NAD radicals which reduce O 2 to superoxide
(Halliwell 1978 ). In this model, the source of electrons in the apoplast is said to be
malate, exported across the plasma membrane by a malate/oxalacetate carrier and
used to reduce NAD + by apoplastic malate dehydrogenase. H 2 O 2 is formed by the
dismutation of superoxide (Bolwell et al. 1998 ).