PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 87

polyamines (Yamasaki and Cohen 2006; Besson-Bard et al. 2008). NO may be
formed also from nitrite by the action of nitrate reductase (Rockel et al. 2002; Bethke
et al. 2004; Wilson et al. 2008 ). Nitrate reductase transcript and protein levels increase
in response to a PAMP in potato tubers, suggesting a role for nitrate reductase in the
synthesis of NO during the plant immune response (Delledonne 2005). NO may also
be synthesized from nitrite in a nonenzymatic manner (Yamasaki 2000).
Ca 2+ signaling system may be involved in activation of NOS-dependent NO
generation (Lamotte et al. 2004 ). Application of a bacterial PAMP induced NO
generation that was downstream from an infl ux of extracellular Ca 2+ (Ali et al.
2007 ). NO synthesis is regulated by the signaling cascade including cyclic nucleo-
tide gated channel (CNGC)-mediated Ca 2+ currents with a concomitant increase in
calmodulin (CaM) or calmodulin-like proteins (CML), and phosphorylation events
(Ali et al. 2007 ; Ma and Berkowitz 2007). MAP kinase signaling cascades may also
participate in NO production (Asai et al. 2008 ). NO acts in signal transduction
through stimulus-coupled S-nitrosylation of cysteine residues (Benhar et al. 2008).
NO plays an important role in redox signaling system. It induced increased
expression of catalase, peroxidase, glutathione S-transferase, glutathione-S-
reductase, glutathione peroxidase, superoxide dismutase, thioredoxin, and glutare-
doxin, which are involved in redox signaling system (Clarke et al. 2000 ; Polverari
et al. 2003 ; Lindermayr et al. 2005) NO and ROS signaling systems appear to oper-
ate together in triggering innate immune responses (Grennan 2007 ; Asai and
Yoshioka 2009 ). NO also acts in SA, ethylene, and jasmonate signaling systems.
NO induces ACC synthase involved in ethylene biosynthesis (Lamotte et al. 2004 ).
It induces the key enzymes of the JA biosynthesis pathway (del Rio et al. 2004;
Palmieri et al. 2008). NO also triggers production of salicylic acid (Chamnongpol
et al. 1998; Durner et al. 1998; Zago et al. 2006 ). SA in turn, activates nitric oxide
synthesis in A. thaliana (Zottini et al. 2007). The role of NO in innate immune
responses is further described in Chap. 6 .

2.20.7 Mitogen-Activated Protein Kinase (MAPK) Cascades

Mitogen-activated protein kinase (MAPK) cascades are major pathways down-
stream of PAMP/PRR signaling complex that transduce extracellular stimuli into
intracellular responses in plants (Liu et al. 2003; Pedley and Martin 2005).
Different PAMPs, including bacterial fl agellin, elongation factor Tu, peptidoglycan,
lipopolysaccharide, HrpZ1 harpin, and fungal chitin activate MAP kinase signal-
ing system (Wu et al. 2011 ; Bethke et al. 2012 ). A typical MAPK signaling module
consists of three protein kinases: a MAP kinase kinase kinase (MAPKKK or
MEKK [for M APK/ E xtracellular signal-regulated kinase K inase K inase]), a MAP
kinase kinase (MAPKK or MKK), and a MAP kinase (MAPK or MPK) (Mészáros
et al. 2006 ). MAPKKKs can be activated by various PAMPs (Teige et al. 2004).
MAPKKK phosphorylates MAPKK, and MAPKK phosphorylates MAPK (Teige
et al. 2004; Mészáros et al. 2006 ).