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Changes in redox status have been observed during immune responses in different organisms, but the associated signaling mechanisms are poorly understood. In plants, these redox changes regulate the conformation of NPR1, a master regulator of salicylic acid (SA)–mediated defense genes. NPR1 is sequestered in the cytoplasm as an oligomer through intermolecular disulfide bonds. We report that S-nitrosylation of NPR1 by S-nitrosoglutathione (GSNO) at cysteine-156 facilitates its oligomerization, which maintains protein homeostasis upon SA induction. Conversely, the SA-induced NPR1 oligomer-to-monomer reaction is catalyzed by thioredoxins (TRXs). Mutations in both NPR1 cysteine-156 and TRX compromised NPR1-mediated disease resistance. Thus, the regulation of NPR1 is through the opposing action of GSNO and TRX. These findings suggest a link between pathogen-triggered redox changes and gene regulation in plant immunity.