There is increasing evidence that nitric oxide (NO), which was first identified as a unique diffusible molecular messenger in animals, plays an important role in diverse physiological processes in plants. Recent progress that has deepened our understanding of NO signalling functions in plants, with special emphasis on defence signalling, is discussed here. Several studies, based on plants with altered NO-levels, have recently provided genetic evidence for the importance of NO in gene induction. For a general overview of which gene expression levels are altered by NO, two studies, involving large-scale transcriptional analyses of Arabidopsis thaliana using custom-made or commercial DNA-microarrays, were performed. Furthermore, a comprehensive transcript profiling by cDNA-amplification fragment length polymorphism (AFLP) revealed a number of Arabidopsis thaliana genes that are involved in signal transduction, disease resistance and stress response, photosynthesis, cellular transport, and basic metabolism. In addition, NO affects the expression of numerous genes in other plant species such as tobacco or soybean. The NO-dependent intracellular signalling pathway(s) that lead to the activation or suppression of these genes have not yet been defined. Several lines of evidence point to an interrelationship between NO and salicylic acid (SA) in plant defence. Recent evidence suggests that NO also plays a role in the wounding/jasmonic acid (JA) signalling pathway. NO donors affect both wounding-induced H2O2 synthesis and wounding- or JA-induced expression of defence genes. One of the major challenges ahead is to determine how the correct specific response is evoked, despite shared use of the NO signal and, in some cases, its downstream second messengers.