The plant hormone salicylic acid (SA) plays a prominent role in modulating plant immune responses against diverse pathogens.SA also influences other physiological processes in plants,such as senescence-associated gene expression,basal thermogenesis,and seed germination (Vlot et al.,2009).Because of the critical role of SA in regulating plant immunity,growth,and development,there has been immense research about SA,which has resulted in the discovery of numerous plant genes involved in SA biosynthesis or signal transduction.One of the most notable findings was the identification of NPR1 (non-expressor of pathogenesis protein 1) (Cao et al.,1994;Delaney et al.,1995),a gene that encodes a master regulatory protein of SA-dependent defense responses and is a transcriptional co-activator of the TGA clade of bZIP transcription factors (transcription factors that contain basic region/leucine zipper motif).NPR1 exists in at least two forms in the cell.When the SA level is low in the cell (e.g.in the absence of pathogen infection),NPR1 is localized in the cytoplasm as an oligomer through intermolecular disulphide bonds.However,when the SA level is high (e.g.after pathogen infection),redox changes in the cytosol trigger the reduction of disulphide bonds,and monomeric NPR1 enters the nucleus and functions as a transcriptional co-activator at the target gene promoter (Mou et al.,2003).The npr1 mutant could not initiate the SA-associated global transcriptional response program and is defective inall major SA-dependent defense responses,suggesting a central role in SA signal transduction (Cao et al.,1994;Delaney et al.,1995).Despite these impressive advances,however,one of the most fundamental questions-the identity of the SA receptor-has remained unanswered.