Nitric Oxide Signaling System in Plant Innate Immunity

Abstract

Nitric oxide (NO) is a diffusible molecular messenger that plays an important 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 Ca2+ and CaM. NO is involved in influx of Ca2+ into the cytosol. It modulates the activity of plasma membrane as well as intracellular Ca2+ -permeable channels. 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 transduction 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 modification 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 synergistically 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.