A full-length cDNA (Hv-GR) whose transcript accumulation increased in response to infection by Blumeria graminis DC.f.sp. tritici (Bgt) was isolated from Haynaldia villosa. Southern analysis revealed a single copy of Hv-GR present in H. villosa. This gene encodes a glutathione reductase (GR) with high similarity to chloroplastic GRs from other plant species. Chloroplastic localization of Hv-GR was confirmed by targeting of the green fluorescent protein (GFP)-Hv-GR fusion protein to chloroplasts of epidermal guard cells. Following inoculation with Bgt, transcript accumulation of Hv-GR increased in a resistant line of wheat, but no significant change was observed in a susceptible line. In vivo function of Hv-GR in converting oxidized glutathione (GSSG) to the reduced form (GSH) was verified through heterologous expression of Hv-GR in a yeast GR-deficient mutant. As expected, overexpression of this gene resulted in increased resistance of the mutant to H(2)O(2), indicating a critical role for Hv-GR in protecting cells against oxidative stress. Moreover, overexpression of Hv-GR in a susceptible wheat variety, Triticum aestivum cv. Yangmai 158, enhanced resistance to powdery mildew and induced transcript accumulation of other pathogenesis-related genes, PR-1a and PR-5, through increasing the foliar GSH/GSSG ratio. Therefore, we concluded that a high ratio of GSH to GSSG is required for wheat defense against Bgt, and that chloroplastic GR enzymes might serve as a redox mediator for NPR1 activation.