br S NO signalling during the plant immune
(S)NO signalling during the plant immune response Redox signalling molecules must exhibit specificity for their target substrate. Moreover, a given redox-based modification must be reversible to ensure transient signalling. Oxidative cysteine (Cys) modifications are integral to a variety of redox-based cellular signalling mechanisms. These residues posses a low-pKa sulphahydryl group which supports significant susceptibility to oxidation . These reactive Cys may be subject to an assortment of redox-based post-translational modifications including, Cys thiol (SH), S-nitrosylation (SNO), sulphenic Ro3306 (SOH), disulphide (S–S), S-glutathionylation (SSG), sulphinic acid (SO2H) and irreversible sulphonic acid formation (SO3H) . S-nitrosylation, the addition of an NO moiety to a reactive Cys thiol, is quickly emerging as a key regulatory feature during the establishment of plant disease resistance. The deployment of the biotin-switch technique  has recently lead to the identification of metabolic, structural and regulatory proteins that are specifically S-nitrosylated during the plant defence response , , . However, how this NO-mediated post-translational modification controls the functions of its myriad of protein targets is only just beginning to be uncovered.
GSNO: a global reservoir of NO bioactivity S-nitrosoglutathione (GSNO), formed by the addition of an NO moiety to the highly abundant, antioxidant tripeptide glutathione (GSH) by an O2 dependent reaction, might function as a global reservoir of NO bioactivity. In this context, while the half-life of NO in biological systems is only a few seconds, GSNO is a relatively stable store for NO . Also, an adequate GSH level is required for an effective defence response because the phytoalexin deficient 2-1 (pad2-1) mutant, which is compromised in GSH biosynthesis, exhibits increased susceptibility to pathogens . GSNO is presumably phloem mobile and thus might constitute a vehicle for the long distance transport of this important redox signalling molecule. Significantly, the spontaneous homolytic cleavage of the Cys-based S–NO bond within GSNO releases NO , freeing this highly reactive small molecule to execute its cell signalling functions. Recent evidence has also tentatively suggested the presence of cellular SNO-lyase activity, at least in Saccharomyces cerevisiae, which could catalyze the release of NO from GSNO . Thus, the mobilization of NO bioactivity might be under tight regulatory control. In addition to releasing NO, GSNO can also directly transfer an NO group onto a target Cys residue, a process termed trans-nitrosylation . Thus, the formation of this metabolite may constitute a key step during the transmission of a significant portion of NO bioactivity. Recent findings in the plant reference system, Arabidopsis, have suggested that the levels of GSNO increase during the development of plant disease resistance, implying this small molecule may represent an important store for NO bioactivity following attempted infection . Therefore, mechanisms that may control GSNO turnover might have a profound impact on NO function.
GSNOR controls GSNO turnover The synthesis of GSNO takes place by an O2 dependent reaction of NO with GSH which once formed is more stable than an NO molecule , , . Cellular GSNO homeostasis is controlled by the enzyme GSNO reductase (GSNOR) . The function of this enzyme is conserved between animals, plants and bacteria , . The emerging evidence suggests that GSNOR plays a key role in ameliorating the effects of nitrosative stress manifested by an increase in GSNO levels , , . As cellular redox status is central to normal growth, developmental and environmental interactions, perturbations in GSNOR activity consequently might impact a variety of cellular activities that are only just beginning to be uncovered. The execution of cellular signalling is characteristically a transient process. Thus, mechanisms have evolved not only to activate signal transmission but also to terminate this process. While S-nitrosylation has emerged as a key redox-based post-translational modification we are only just beginning to understand how this Cys modification can be switched off. The first insights into this procedure came with the purification of a single activity from Escherichia coli, S. cerevisiae and mouse macrophages that metabolizes GSNO . An ortholog of this protein, originally designated as a class III alcohol dehydrogenase, was also identified in plants . Loss-of-function mutations in this GSNOR were shown to reduce GSNO-consuming activity and increases the cellular quantity of both GSNO and total protein SNO . Arabidopsis GSNOR activity is encoded by a single gene (AtGSNOR1), with the corresponding protein 379 residues in length and 40.7kDa in size. GSNOR is evolutionary conserved across dicotyledonous and monocotyledonous plant species. Importantly, this enzyme does not directly act on protein SNOs , . Rather, GSNOR is involved in the reduction of GSNO to S-amino-l-glutathione via an S-hydroxylamine intermediate, with the end products of this reaction being glutathione disulphide (GSSG) and ammonia (NH3) . Thus, GSNOR does not directly reverse the S-nitrosylation of a given protein Cys residue but rather turns over GSNO thereby reducing the cells store of NO bioactivity and by extension leading to a depletion of total protein SNO (Fig. 1).