br Molecular mechanisms underpinning GSNOR
Molecular mechanisms underpinning GSNOR1 function in the defence response Recently, Arabidopsis GSNOR1 has been shown to govern the extent of S-nitrosylation of two key regulatory proteins, Non-Expresser of Pathogenesis-Related Genes 1 (NPR1) and SA binding protein 3 (SABP3), which are integral to the disease resistance ,  (Fig. 1). Loss-of-function mutations in the transcriptional co-activator, NPR1, compromise SA signalling . In unchallenged plants, conserved Cys residues in NPR1 form intermolecular disulphide bonds, facilitating the formation of a cytosolic NPR1 oligomer . As only NPR1 monomer can translocate to the nucleus, the formation of these disulphide bonds blunt its co-activator function, leading to a reduction in SA-dependent defence gene expression. Following pathogen recognition, SA-mediated redox changes reduce the NPR1 oligomer to monomers, facilitating the movement of NPR1 into the nucleus where it can subsequently promote transcription of target genes. S-nitrosylation of NPR1 at Cys156, located at a predicted multimerization interface, supports disulphide linkage formation between NPR1 monomers leading to the development of NPR1 oligomers. Thus, in the absence of GSNOR1 function, excessive S-nitrosylation of NPR1 disables SA signalling by promoting NPR1 oligomer formation. However, in wild-type plants, S-nitrosylation of NPR1 might not suppress immunity in the long term, rather SNO-NPR1 formation may serve to maintain NPR1 homeostasis, ensuring a sustained supply of NPR1 monomer to transcriptionally activate defence gene 5-Methyl-CTP , . SABP3 exhibits a high affinity for SA and possesses carbonic anhydrase activity . Lipid-based molecules are key players in the plant defence response and their functions have been linked with SA signalling. Significantly, carbonic anhydrase activity is thought to underpin lipid biosynthesis . The S-nitrosylation of SABP3 during the later stages of disease resistance has recently been reported . Moreover, the magnitude of this Cys modification was found to be governed by GSNOR1 activity. S-nitrosylation of SABP3 at Cys280 inhibited both SA binding and the carbonic anhydrase activity of this enzyme and compromised resistance against an avirulent isolate of PstDC3000 . As the kinetics of SABP3 S-nitrosylation is delayed and the function of this modification is to blunt the activity of a positive regulator, SNO-SABP3 formation might therefore govern a negative feedback loop that serves to dampen defence signalling.
Conclusions S-nitrosylation has now emerged as a key post-translational modification integral to plant immune function. The presence of mechanisms to switch-off signal transmission is fundamental to the operation of cellular signalling networks. Thus, the process of de-nitrosylation is an important regulatory feature of NO bioactivity. Despite this, we are only just beginning to uncover the molecular machinery that controls this procedure. GSNOR has emerged as an important player in plant de-nitrosylation, especially during the development of disease resistance. However, there is currently little information on what regulates this regulator. Stress responses are not thought to modulate GSNOR1 protein levels , implying that this enzyme might be subject to post-translational modification. In this context, GSNOR-mediated GSNO reduction has been shown to respond to ambient energy and redox state . Exploring the nature of these possible post-translational modifications, with an emphasis on Cys oxidations, might prove fruitful. Currently, information is also lacking on the enzymatic mechanism of GSNOR1 function. Therefore some detail linking structure to function would also be informative. Finally, GSNOR1, which operates indirectly, is likely to be only one component utilized by the plant cell to de-nitrosylate protein SNOs. Additional, perhaps more direct, mechanisms may also have evolved for the widespread regulation of this critical process.