Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 315

MAP kinases (Neill et al. 2002 ). Death of host cells during HR results from the
simultaneous, balanced production of NO and ROS (Delledonne et al. 2001 , 2002 ).
The cytotoxic effects of NO and ROS derive from the diffusion-limited reaction of
NO with O 2 − to form the peroxynitrite anion ONOO - . Peroxynitrite causes oxidative
damage and protein modifi cations such as Tyr nitration and oxidation of thiol resi-
dues (Radi 2004 ). These results suggest that NO and H 2 O 2 work in strong partner-
ship during induction of the defense genes (Zago et al. 2006 ). NO and H 2 O 2 can
also act independently in the same signaling pathways with similar downstream
responses as a consequence (Zago et al. 2006 ).

6.7 Role of NO in SA, JA, and Ethylene Signaling Systems

NO is involved in the production of salicylic acid (Chamnongpol et al. 1998 ; Durner
et al. 1998 ). NO triggers UDP-glucose:SA:glucosyltransferase that converts SA to
SA -glucoside, a conjugated and stable form of SA (Zago et al. 2006 ). SA in turn,

activates nitric oxide synthesis in A. thaliana (Zottini et al. 2007 ). A tight interrela-
tionship between NO and SA in plant defense has been reported. NO donors pro-
duce SA accumulation (Durner et al. 1998 ). NO is required for the full function of
SA as a SAR (systemic acquired resistance) inducer (Song and Goodman 2001 ).
Many NO-regulated enzymes, including aconitase or catalase, are regulated by SA
(Durner et al. 1997 ; Clark et al. 2000 ). Thus NO may be involved in SA signaling
system. Both NO and SA activated SIPK in tobacco (Kumar and Klessig 2000 ).
Studies with transgenic NahG tobacco revealed that SA is required in the
NO-mediated induction of SIPK. SIPK may function downstream of SA in the NO
signaling pathway (Kumar and Klessig 2000 ).
NO is involved in induction of biosynthesis of oxylipins and JA. NO induces the
key enzymes of the JA biosynthesis pathway (Fig. 6.4 ; del Rio et al. 2004 ; Palmieri
et al. 2008 ). Transcripts encoding lipoxygenase (LOX), 12-oxophytodienoate
reductase (12-OPR), and diacylglycerol kinase (DAGK), all involved in the biosyn-
thesis of oxylipins and JA, are up-regulated by NO (Grün et al. 2006 ; Zago et al.
2006 ). Interaction between NO and JA signaling has been described (Palmieri et al.
2008 ). NO induces allene oxide synthase gene ( AOS ) and 12-oxophytodienoate
reductase gene OPR3 (Grün et al. 2006 ); both are involved in JA biosynthesis
(Vidhyasekaran 2007a , b ). NO induces the JA-regulated PDF1.2 gene encoding
PR12 protein (Grün et al. 2006 ).
NO induces S-Adenosyl-L-Met synthetase, which catalyzes the conversion of
ATP and L-Met into the ethylene precursor S -adenosyl-L-Met (Fig. 6.5 ; Zago et al.
2006 ). NO induces ACC synthase involved in ethylene biosynthesis (Lamotte et al.
2004 ). Thus, NO is involved in ethylene biosynthesis (Lindermayr et al. 2005 ,
2006 ). NO activates ein3 , a gene involved in ethylene perception and transduction
(Chang and Stadler 2001 ). NO is known to infl uence several ethylene-dependent
processes in the plant (Leshem et al. 1998 ).

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