In animals, protein S-nitrosylation, the covalent attachment of NO to the thiol group of cysteine residues, is an intensively investigated posttranslational modification, which regulates many different processes. A growing body of evidence suggests that this type of redox-based regulation mechanism plays a pivotal role in plants, too. Here we report the molecular mechanism for S-nitrosylation of methionine adenosyltransferase (MAT) of Arabidopsis thaliana, thereby presenting the first detailed characterization of S-nitrosylation in plants. We cloned three MAT isoforms of Arabidopsis and tested the effect of NO on the activity of the purified, recombinant proteins. Our data showed that incubation with GSNO resulted in blunt, reversible inhibition of MAT1, whereas MAT2 and MAT3 were not significantly affected. Cys-114 of MAT1 was identified as the most promising target of NO-induced inhibition of MAT1, because this residue is absent in MAT2 and MAT3. Structural analysis of MAT1 revealed that Cys-114 is located nearby the putative substrate binding site of this enzyme. Furthermore, Cys-114 is flanked by S-nitrosylation-promoting amino acids. The inhibitory effect of GSNO was drastically reduced when Cys-114 of MAT1 was replaced by arginine, and mass spectrometric analyses of Cys-114-containing peptides obtained after chymotryptic digestion demonstrated that Cys-114 of MAT1 is indeed S-nitrosylated. Because MAT catalyzes the synthesis of the ethylene precursor S-adenosylmethionine and NO is known to influence ethylene production in plants, this enzyme probably mediates the cross-talk between ethylene and NO signaling.