Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 317

L-Methionine

ATP

Methionine adenosyl
transferase

NO

Activation

PPi+Pi

S-adenosyl methionine (SAM)

NO ACC synthase

Activation

1-Aminocyclopropane-1-carboxylic acid (ACC)

ACC oxidase

Ethylene

Fig. 6.5 Role of NO in ethylene biosynthesis

activity (Grennan 2007 ). Since SNOs interact with intracellular reducing agents,
such as ascorbic acid or glutathione (GSH), they are highly labile. This lability
results in tissue half-lives of seconds to a few minutes and therefore provides a very
sensitive mechanism for regulating cellular processes. Thus S-nitrosylation is
considered as posttranslational modifi cation which is similar to phosphorylation
(Lindermayr et al. 2005 ). Like phosphorylation, S-nitrosylation is a precisely tar-
geted and rapidly reversible posttranslational modifi cation (Mannick and Schonhoff
2004 ; Astier and Lindermayr 2012 ). S-nitrosylated proteins can be easily denitro-
sylated, as the S-NO bond is labile in a reductive environment. Posttranslational
modifi cation of proteins by S -nitrosylation potentially alters function of the pro-
teins. Rapidly reversible S -nitrosylation of proteins induced by NO is involved in
signal transduction (Grennan 2007 ).
The proteins, which are regulated by nitrosylation, include catalase (Foster and
Stamler 2004 ), superoxide dismutase, glutathione peroxidase and peroxiredoxin
(Lindermayr et al. 2005 ) and these enzymes are involved in redox signaling system.
S-Nitrosylation regulates Met adenosyltransferase involved in ethylene biosynthe-
sis (Lindermayr et al. 2006 ) and metacaspase involved in HR-related cell death
(Belenghi et al. 2007 ). NO reacts rapidly with glutathione (GSH), the major intra-
cellular low-molecular-mass antioxidant to yield S-nitrosoglutathione (GSNO).
GSNO is a bioactive, stable, and mobile reservoir of NO and it is an important
player in plant defense responses against pathogens (Espunya et al. 2012 ). GSNO is
considered to represent a functionally relevant signaling molecule that might act