Plant immunity relies on the conformational changes of NPR1 through S-nitrosylation and thioredoxins (TRXs). NPR1, a key regulator of salicylic acid (SA)-mediated defense genes, exists as an oligomer in the cytoplasm, maintained by intermolecular disulfide bonds. S-nitrosylation of NPR1 by S-nitrosoglutathione (GSNO) at cysteine-156 promotes oligomerization, maintaining protein homeostasis during SA induction. Conversely, SA-induced oligomer-to-monomer reaction is catalyzed by TRXs. Mutations in both NPR1 cysteine-156 and TRX compromise NPR1-mediated disease resistance, indicating that GSNO and TRX regulate NPR1 through opposing actions. These findings suggest a link between pathogen-triggered redox changes and gene regulation in plant immunity. Innate immune responses are conserved in plants and animals, often associated with changes in cellular oxidative and reductive states. In plants, these redox changes are sensed by NPR1, a master regulator of defense gene expression. In unchallenged plants, NPR1 resides in the cytoplasm as an oligomer maintained through redox-sensitive intermolecular disulfide bonds. Upon pathogen challenge, SA increases and changes the cellular redox state, leading to reduction of the disulfide bonds in NPR1. This releases monomer that translocates to the nucleus, activating pathogenesis-related (PR) genes. Mutations in NPR1 residues Cys82 and Cys216 result in increased monomer accumulation, constitutive nuclear localization, and NPR1-mediated gene expression in the absence of pathogen challenge. Oligomerization of proteins through intermolecular disulfide bonds is unusual under reductive cytosolic conditions. However, treatment with SA induces NPR1 monomer release and facilitates oligomerization. GSNO, a natural NO donor, markedly facilitates NPR1 oligomerization. This is consistent with the finding that GSNO treatment causes protein S-nitrosylation. The Cys156 of NPR1 is the principal site of S-nitrosylation. Mutation of Cys156 abolishes GSNO-triggered S-nitrosylation and oligomerization. These findings indicate that GSNO S-nitrosylates NPR1 at Cys156, directly facilitating disulfide linkage between NPR1 monomers. TRXs are redox mediators of NPR1. TRX-h5 is substantially up-regulated upon infection with P. syringae. TRX-h5 is required in vivo for SA-induced monomer release and full induction of PR genes. TRX mutants show impaired systemic acquired resistance (SAR) against pathogens. These findings suggest that redox signals are conveyed through SNO and cytosolic TRXs, whichPlant immunity relies on the conformational changes of NPR1 through S-nitrosylation and thioredoxins (TRXs). NPR1, a key regulator of salicylic acid (SA)-mediated defense genes, exists as an oligomer in the cytoplasm, maintained by intermolecular disulfide bonds. S-nitrosylation of NPR1 by S-nitrosoglutathione (GSNO) at cysteine-156 promotes oligomerization, maintaining protein homeostasis during SA induction. Conversely, SA-induced oligomer-to-monomer reaction is catalyzed by TRXs. Mutations in both NPR1 cysteine-156 and TRX compromise NPR1-mediated disease resistance, indicating that GSNO and TRX regulate NPR1 through opposing actions. These findings suggest a link between pathogen-triggered redox changes and gene regulation in plant immunity. Innate immune responses are conserved in plants and animals, often associated with changes in cellular oxidative and reductive states. In plants, these redox changes are sensed by NPR1, a master regulator of defense gene expression. In unchallenged plants, NPR1 resides in the cytoplasm as an oligomer maintained through redox-sensitive intermolecular disulfide bonds. Upon pathogen challenge, SA increases and changes the cellular redox state, leading to reduction of the disulfide bonds in NPR1. This releases monomer that translocates to the nucleus, activating pathogenesis-related (PR) genes. Mutations in NPR1 residues Cys82 and Cys216 result in increased monomer accumulation, constitutive nuclear localization, and NPR1-mediated gene expression in the absence of pathogen challenge. Oligomerization of proteins through intermolecular disulfide bonds is unusual under reductive cytosolic conditions. However, treatment with SA induces NPR1 monomer release and facilitates oligomerization. GSNO, a natural NO donor, markedly facilitates NPR1 oligomerization. This is consistent with the finding that GSNO treatment causes protein S-nitrosylation. The Cys156 of NPR1 is the principal site of S-nitrosylation. Mutation of Cys156 abolishes GSNO-triggered S-nitrosylation and oligomerization. These findings indicate that GSNO S-nitrosylates NPR1 at Cys156, directly facilitating disulfide linkage between NPR1 monomers. TRXs are redox mediators of NPR1. TRX-h5 is substantially up-regulated upon infection with P. syringae. TRX-h5 is required in vivo for SA-induced monomer release and full induction of PR genes. TRX mutants show impaired systemic acquired resistance (SAR) against pathogens. These findings suggest that redox signals are conveyed through SNO and cytosolic TRXs, which