Using a biotin switch method we show that proteins are S-nitrosylated in Arabidopsis following challenge with both virulent Pseudomonas syringae pv. tomato strain Pst DC3000 and avirulent Pst carrying (avrB). Purification and MALDI analysis indicates that one of these S-nitrosylated proteins is a carbonic anhydrase. A single copy Arabidopsis GS-FDH was identified which shows 75% homology in amino acid sequence to mouse GS-FDH. The induction of GS-FDH by GSNO in Arabidopsis was confirmed by northern analysis.  GS-FDH expression in Arabidopsis is constitutive at basal levels. During the HR, which was induced by Pst DC3000 (avrB), GS-FDH expression and activity were found to be rapidly suppressed. Whereas no GS-FDH suppression takes place following infection with virulent Pst DC3000. Therefore GS-FDH expression and activity are tightly regulated during the HR. Three GS-FDH T-DNA insertion lines were identified from the SAIL and SIGnAL databases. Two of these lines fdh1-1 and fdh1-2 are allelic and contain a T-DNA insert in the promoter of GS-FDH. Northern analysis and enzyme activity assays found that GS-FDH is over-expressed in these lines. Characterisation of fdhl-1 revealed that it constitutively expresses the defence gene PDF1.2. fdh1-1 exhibits resistance against the virulent biotroph Pst DC3000. Moreover, fdh1-1 has a delayed HR response following Pst DC3000 (avrB) infiltration. In addition, an activation tagged population was screened to isolate mutants resistant to nitrosative stress, from which ten NO resistant mutant candidates were identified.