Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 308

NO may be formed also from nitrite by the action of nitrate reductase, which
catalyzes the reduction of nitrate to nitrite using NAD(P)H as an electron donor and
also generates NO from nitrite (Fig. 6.1 ; Yamasaki and Sakihama 2000 ; Desikan et al.
2002 ; Morot -Gaudry-Talarmain et al. 2002 ; Rockel et al. 2002 ; Bethke et al. 2004 ;
Planchet and Kaiser 2006 ; Wilson et al. 2008 ). NO generation was reduced in ammo-
nium -fed tobacco plants compared to NO 3 − -fed plants and NO 3 − -fed plants showed
enhanced disease resistance against Pseudomonas syringae pv. phaseolicola (Gupta
et al. 2013 ). NO production was completely absent in ammonium-grown tobacco cell
suspensions totally devoid of nitrate (Planchet et al. 2005 , 2006 ). It indicates that NO
synthesis is catalyzed through a nitrate/nitrite-dependent pathway rather than an
L-arginine-dependent NOS mediated pathway in these tobacco cells. Nitrate reductase
transcript and protein levels increase in response to an elicitor in potato tubers, suggest-
ing a role for nitrate reductase in the synthesis of NO during the defense response
(Delledonne 2005 ). Treatment of protoplasts prepared from Nicotiana benthamiana
leaves with the PAMP elicitin INF1 secreted by the oomycete pathogen Phytophthora
infestans elevated NO production. INF1-induced NO generation was suppressed by an
NO-specifi c scavenger. Silencing of nitrate reductase (NR) genes signifi cantly
decreased INF1-induced NO production (Yamamoto-Katou et al. 2006 ). These results
suggest that nitrate reductase is involved in the PAMP-triggered NO generation.
NO may also be synthesized from nitrite in a nonenzymatic manner (Fig. 6.1 ;
Yamasaki 2000 ). In this process nitrite is protonated to form nitrous acid (HNO 2 ).
Two molecules of HNO 2 interact through a series of reactions and give rise to NO
(Yamasaki and Sakihama 2000 ). The non-enzymatic conversion of nitrite to NO
occurs in the apoplast at acidic pH in the presence of reductants such as ascorbic
acid (Bethke et al. 2004 ).

6.3 Upstream Events in NO Production

6.3.1 Ca 2+ Infl ux into Cytosol May Be an Early Upstream

Event in NO Production

PAMP elicitor signals trigger a NO burst within minutes in plant cells after the elici-
tor treatment (Foissner et al. 2000 ; Lamotte et al. 2004 ; Tischner et al. 2010 ). The
rapid NO production has been shown to be dependent on Ca 2+ signaling system
(Lamotte et al. 2004 ; Ali et al. 2007 ; Courtois et al. 2008 ; Ma et al. 2008 ; Choi et al.
2009 ; Vatsa et al. 2011 ; Ma et al. 2012 ). Infl ux of extracellular Ca 2+ through the cell
membrane appears to be one of the earliest events triggered by the PAMPs
(Laohavisit et al. 2009 ; Vadassery and Oelm ller 2009 ; Kwaaitaal et al. 2011 ;

Vincill et al. 2012 ). The massive infl ux of Ca 2+ occurs via different calcium ion
channels within 15-30 min after PAMP/elicitor treatment (Lecourieux-Ouaked
et al. 2000 ; Kwaaitaal et al. 2011 ). The generated Ca 2+ current leads to downstream
NO production (Ali et al. 2007 ).

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