Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 305

Chapter 6
Nitric Oxide Signaling System in Plant
Innate Immunity
Abstract Nitric oxide (NO) is a diffusible molecular messenger that plays an impor-
tant role in plant immune response signal transduction. The pathogen-associated
molecular pattern (PAMP) signal molecules trigger a very rapid NO burst in plant
cells. NO is synthesized predominantly by the enzyme nitric oxide synthase (NOS).
NOS contains calmodulin (CaM)-binding motifs and full activation of the enzyme
needs both Ca 2+ and CaM. NO is involved in infl ux of Ca 2+ into the cytosol. It modu-
lates the activity of plasma membrane as well as intracellular Ca 2+ -permeable chan-
nels. NO acts as an important key redox-active signal for the activation of various
defense responses. Both an oxidative and a NO burst have been reported to occur
prior to activation of the signal cascade that eventually activates the transcription of
defense genes. NO and reactive oxygen species (ROS) work in strong partnership
during induction of the defense genes. A tight interrelationship between NO and SA
in plant defense has been reported. NO is required for the full function of SA as a
systemic acquired resistance (SAR) inducer. NO induces the key enzymes of the JA
and ethylene biosynthesis pathways. NO acts substantially in cellular signal trans-
duction through stimulus-coupled S-nitrosylation of cysteine residues. NO rapidly
induces reversible S -nitrosylation of proteins involved in signal transduction . This
redox-based post-translational modifi cation is a key regulator of protein function in
plant immunity. NO reacts rapidly with glutathione (GSH) to yield S-nitrosoglutathione
(GSNO). GSNO is a bioactive, stable, and mobile reservoir of NO. It acts synergisti-
cally with SA in SAR. GSNO reductase (GSNOR) is the main enzyme responsible
for the in vivo control of intracellular levels of GSNO and also the levels of
S -nitrosylated proteins. NO bioactivity is controlled by NO synthesis by the different
routes and by NO degradation, which is mainly performed by the GSNOR.
S-nitrosothiols (SNOs) also play important role in SAR. GSNOR controls SNO in
vivo levels and the SNO content positively regulates plant defense responses.
Keywords Nitric oxide (NO) burst • Nitric oxide synthase (NOS) • Ca 2+ infl ux
• Redox-active signal • Systemic acquired resistance (SAR) • S-nitrosylation •
S-nitrosoglutathione (GSNO) • GSNO reductase (GSNOR)