Nitric oxide (NO) is a reactive free radical with pleiotropic function that is not only involved in regulation of plant growth and development, but also in the response to biotic and abiotic stressors. It mainly acts by posttranslationally modifying proteins. The most important mode of action of NO is protein S-nitrosylation, the covalent attachment of an NO group to the thiol side of protein cysteine residues. Other major types of NO-dependent modifications are metal nitrosylation and tyrosine nitration. NO can regulate gene expression at different levels. On one side, it can initiate signalling cascades or modify proteins involved in signal transduction pathways. On the other side, redox-sensitive transcription factors can be also target for S-nitrosylation, and NO can also affect redox-dependent nuclear transport of some proteins. This suggests that NO plays a pivotal role in regulating transcription and/or general nuclear metabolism in plants.