The Arabidopsis NPR1 Protein Is a Receptor for the Plant Defense Hormone Salicylic Acid

SummarySalicylic acid (SA) is an essential hormone in plant immunity, but its receptor has remained elusive for decades. The transcriptional coregulator NPR1 is central to the activation of SA-dependent defense genes, and we previously found that Cys521 and Cys529 of Arabidopsis NPR1's transactivation domain are critical for coactivator function. Here, we demonstrate that NPR1 directly binds SA, but not inactive structural analogs, with an affinity similar to that of other hormone-receptor interactions and consistent with in vivo Arabidopsis SA concentrations. Binding of SA occurs through Cys521/529 via the transition metal copper. Mechanistically, our results suggest that binding of SA causes a conformational change in NPR1 that is accompanied by the release of the C-terminal transactivation domain from the N-terminal autoinhibitory BTB/POZ domain. While NPR1 is already known as a link between the SA signaling molecule and defense-gene activation, we now show that NPR1 is the receptor for SA.

Graphical AbstractFigure optionsDownload as PowerPoint slideHighlights
► NPR1 is a salicylic acid (SA) receptor, binding specifically to SA via Cys521/529
► NPR1 binds copper in vivo via Cys521/529, and metals are required for SA binding
► SA directly regulates the conformation of NPR1 by deoligomerizing NPR1 into a dimer
► The NPR1 BTB/POZ domain autoinhibits the function of the NPR1 transactivation domain