Decoding nitric oxide signals: The S-denitrosation machinery in plants

S-nitrosation of protein cysteines has been recognised as a crucial mechanism mediating the biological activity of nitric oxide (NO). Here, we review the current knowledge on the enzymatic machinery mediating protein S-denitrosation in plants, a key process that modulates S-nitrosothiol levels within NO redox signalling pathways. Three major enzymatic systems are characterised: the NADPH-dependent thioredoxin system, S-nitrosoglutathione reductase (GSNOR), and aldo-keto reductases (AKRs). Protein S-nitrosothiols are reduced via dithiol-disulfide exchange mechanisms catalysed by thioredoxins, which are re-reduced by NADPH-dependent thioredoxin reductases. GSNO, the principal low-molecular-weight S-nitrosothiol, is degraded by GSNOR, indirectly modulating the global S-nitrosation status. This process is tightly regulated via reversible oxidative and nitrosative modifications of GSNOR's cysteine residues. In the absence or impairment of GSNOR activity, compensatory GSNO catabolism is mediated by upregulated AKR isoforms exhibiting NADPH-dependent GSNO reductase activity. The physiological and developmental relevance of protein denitrosation is examined in the context of root morphogenesis, gametophytic development, and immune responses, where S-denitrosation has been demonstrated to modulate the activity, stability, and subcellular localisation of key regulatory proteins. Moreover, pathogen-derived effectors targeting denitrosylases such as GSNOR have been implicated in virulence strategies to disrupt NO homeostasis. Denitrosation represents a critical regulatory node in NO redox signalling, with spatial and temporal specificity yet to be fully elucidated. Further elucidation of the enzymatic substrate specificity, subcellular localisation, and cross-regulatory mechanisms under both physiological and stress conditions is required to fully define the role of denitrosation in plant redox biology.