Early H2O2 accumulation in mesophyll cells leads to induction of glutathione during the hyper-sensitive response in the barley-powdery mildew interaction

H2O2 production and changes in glutathione, catalase, and peroxidase were f ollowed in whole-leaf extracts from the susceptible (AlgS [Algerian/4* (F14 ) Man.(S)]; ml-a1 allele) and resistant (AlgR [Algerian/4* (F14) Man.(R)]; MI-nl allele) barley (Hordeum vulgarc) isolines between 12 and 24 h after i noculation with powdery mildew (Blumeria graminis [DC]. Speer [syn. Erysiph e graminis DC] f.sp hordei Marchal). Localized papilla responses and cell d eath hypersensitive responses were not observed within the same cell. In hy persensitive response sites, H2O2 accumulation first occurred in the mesoph yll underlying the attacked epidermal cell. Subsequently, H2O2 disappeared from the mesophyll and accumulated around attacked epidermal cells. In AlgR , transient glutathione oxidation coincided with H2O2 accumulation in the m esophyll. Subsequently, total foliar glutathione and catalase activities tr ansiently increased in AlgR. These changes, absent from AlgS, preceded inoc ulation-dependent increases in peroxidase activity that were observed in bo th AlgR and AlgS at 18 h. An early intercellular signal precedes H2O2 and t his elicits anti-oxidant responses in leaves prior to events leading to dea th of attacked cells.