Furthermore, NO plays important roles in plant responses to biotic and abiotic Trometamol stresses. Different modes of NO signaling have been reported to mediate this abundance of function in plants. Most NO signaling is accomplished through the posttranslational modification of target proteins, such as the nitration of protein tyrosine moieties, binding to metal centers or the nitrosylation of cysteine residues. S-nitrosylation, the reversible attachment of a NO moiety to thiol groups of selected cysteine residues functions as the most important PTM in the context of NO signaling. Snitrosylation can impact protein functionality, Apoptosis Activator 2 stability and cellular localization. The S-nitrosylation of enzymes regulates their activity either negatively or positively. Several detailed analyses of the S-nitrosylation of specific proteins have used NO donors to promote S-nitrosylation in vitro. In most cases, the NO donor treatment seems to reduce the enzyme activity in plants. Reports showing that the protein S-nitrosylation of specific cysteine residues promotes enzyme activity are scarce in plant science, and only a few hints regarding an activity-enhancing effect have been reported for animals. Although de-nitrosylation represents a less described aspect of NO signaling, the process of de-nitrosylation is also a strictly regulated event involving two recently proposed enzyme systems: the thioredoxin system, which comprises thioredoxin, thioredoxin reductase and NADPH, and the glutathione/GSNO reductase system. Some proteins are constitutively S-nitrosylated, and de-nitrosylation has been observed after stimulation, leading to the activation of enzyme activity or vice versa. S-nitrosylation and de-nitrosylation together generate the S-nitroso-proteome of a cell, a dynamic and rapidly changing regulatory network, especially under stress conditions. More than a decade ago, Jaffrey and colleagues introduced the biotin switch assay, which facilitates the identification of Snitrosylated proteins. By utilizing both the biotin switch assay and mass spectrometry, hundreds of putative S-nitrosylation targets have been identified.