Nuclear localization of NPR1 is required for activation of PR gene expression

Systemic acquired resistance (SAR) is a broad-spectrum resistance in plants that involves the upregulation of a battery of pathogenesis-related (PR) g enes. NPR1 is a key regulator in the signal transduction pathway that leads to SAR. Mutations in NPR1 result in a failure to induce PR genes in system ic tissues and a heightened susceptibility to pathogen infection, whereas o verexpression of the NPR1 protein leads to increased induction of the PR ge nes and enhanced disease resistance. We analyzed the subcellular localizati on of NPR1 to gain insight into the mechanism by which this protein regulat es SAR. An NPR1-green fluorescent protein fusion protein, which functions t he same as the endogenous NPR1 protein, was shown to accumulate in the nucl eus in response to activators of SAR. To control the nuclear transport of N PR1, we made a fusion of NPR1 with the glucocorticoid receptor hormone bind ing domain. Using this steroid-inducible system, we clearly demonstrate tha t nuclear localization of NPR1 is essential for its activity in inducing PR genes.