Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 307

L- Citrulline + NO

1. L- Arginine

NO synthase

Unknown
enzyme

2. Arginine

Polyamine

NO

Ornithine decarboxylase

Nitrite NO

3. Nitrate

Nitrate reductase

4. 2NO 2 + 2H +

2HNO 2 NO +NO 2 +H 2 O

Nitrogen
dioxide

Nitrous acid

Fig. 6.1 Four different pathways in NO synthesis in plants

Although studies concerning NO synthesis and signaling in animals are
well-advanced, in plants there are still fundamental questions concerning how NO
is produced and used. There is a range of potential NO-generating enzymes in
plants, but no obvious plant nitric oxide synthase homolog has yet been identifi ed
(Neill et al. 2008 ; Wilson et al. 2008 ). The PAMP/elicitor-induced NO in tobacco
cells has been shown to be reduced by NOS inhibitors, suggesting the occurrence of
a NOS-like enzyme in plants (Lamotte et al. 2004 ). In Arabidopsis thaliana , a NOS-
like enzyme, AtNOS1, has been identifi ed and NO levels were found to be lower in
the Atnos1 mutants impaired in AtNOS1 expression (Guo et al. 2003 ). Inhibitors of
mammalian NOS have been found to suppress NO production in plants subjected to
biotic and abiotic stresses (Zhang et al. 2003 ; Lamotte et al. 2004 ; Zeidler et al.
2004 ; Mur et al. 2005 ; Zhao et al. 2007 ; Zottini et al. 2007 ; Mur et al. 2013 ). These
observations suggest that NOS may be involved in NO generation. However, a clear
homologue of animal NOS has not yet been identifi ed in plants (Guo et al. 2003 ;
Crawford et al. 2006 ; Zemojtel et al. 2006 ; Besson-Bard et al. 2008a , b ; Wilson
et al. 2008 ). The plant enzyme displaying NOS-like activity is structurally different
from classical mammalian NOS (Guo et al. 2003 ). Crawford et al. ( 2006 ) suggested
that the AtNOS1 from Arabidopsis might act as a GTPase.
Polyamines may also be involved in NO synthesis (Fig. 6.1 ; Besson-Bard et al.
2008a ). Rapid production of NO was observed in primary leaves of Arabidopsis
when the polyamines spermidine and spermine were added to the Arabidopsis
seedlings (Tun et al. 2006 ). This conversion might be carried out by unknown
enzymes or by polyamine oxidases (Yamasaki and Cohen 2006 ; Besson-Bard
et al. 2008a , b ).