Evidence for an important role of WRKY DNA binding proteins in the regulation of NPR1 gene express ion

The Arabidopsis NPR1 gene is a positive regulator of inducible plant diseas e resistance. Expression of NPR1 is induced by pathogen infection or treatm ent with defense-inducing compounds such as salicylic acid (SA). Transgenic plants overexpressing NPR1 exhibit enhanced resistance to a broad spectrum of microbial pathogens, whereas plants underexpressing the gene are more s usceptible to pathogen infection. These results suggest that regulation of NPR1 gene expression is important for the activation of plant defense respo nses. In the present study, we report the identification of W-box sequences in the promoter region of the NPR1 gene that are recognized specifically b y SA-induced WRKY DNA binding proteins from Arabidopsis. Mutations in these W-box sequences abolished their recognition by WRKY DNA binding proteins, rendered the promoter unable to activate a downstream reporter gene, and co mpromised the ability of NPR1 to complement npr1 mutants for SA-induced def ense gene expression and disease resistance. These results provide strong e vidence that certain WRKY genes act upstream of NPR1 and positively regulat e its expression during the activation of plant defense responses. Consiste nt with this model, we found that SA-induced expression of a number of WRKY genes was independent of NPR1.