Oxicam-type nonsteroidal anti-inflammatory drugs inhibit NPR1-mediated salicylic acid pathway

Salicylic acid (SA) and its structural analogs are nonsteroidal anti-inflammatory drugs (NSAIDs) that target mammalian cyclooxygenases. In plants, SA acts as a defense hormone that regulates NON-EXPRESSOR OF PATHOGENESIS RELATED GENES 1 (NPR1), the master transcriptional regulator of immunity-related genes. We identified a number of NSAIDs that enhance bacterial effector-induced cell death. Among them, the oxicam-type NSAIDs tenoxicam (TNX), meloxicam, and piroxicam, but not other types of NSAIDs, exhibit an inhibitory effect on immunity to bacteria and SA-dependent immune responses in plants. TNX treatment reduces NPR1 levels, independently from the proposed SA receptors NPR3 and NPR4. Instead, TNX induces oxidation of cytosolic redox status, which is also affected by SA and regulates NPR1 homeostasis. Surprisingly, however, cysteine modification associated with NPR1 oligomerization via intermolecular disulfide bonds is not affected by either SA or TNX. Therefore, oxicam-type NSAIDs highlight importance of SA effects on the cytosolic redox status, but not on cysteine modification or oligomerization of NPR1.