NPR1 protects young leaves from systemic salicylic acid-induced damage during bacterial infection

The phytohormone salicylic acid (SA) is an important molecular signal that mediates pathogen defence mechanisms, including triggering Arabidopsis immune responses to the hemi-biotroph Pseudomonas syringae pv. tomato ( Pst ). SA induces the expression of a myriad of defence genes via its receptor and transcriptional regulator NONEXPRESSER OF PR GENES 1 (NPR1). Here, we used chlorophyll fluorescence imaging of F _v /F _m , to detect damage to photosystem II before Pst -induced disease symptoms were visible. We observed that the pathogen only induced damage, and subsequent cell death, in mature leaves while developing leaves in the center of the rosette appeared to be protected. However, in the npr1-1 mutant, Pst -infected mature leaves were able to systemically transmit a signal that caused damage to the photosynthetic machinery in uninfected young leaves. Reductions in F _v /F _m could also be induced systemically in developing npr1-1 leaves by high levels of SA in mature leaves, and rescued by SA biosynthesis deficiency in npr1-1sid2-2 mutants. Together, these results indicate that, in addition to its well-known role as a positive regulator of SA responses, NPR1 also acts to suppress SA-dependent immune responses and thereby protects developing leaves from autoimmune damage.