MOLECULAR CHARACTERIZATION OF THE OXALATE OXIDASE INVOLVED IN THE RESPONSE OF BARLEY TO THE POWDERY MILDEW FUNGUS

Previously we reported that oxalate oxidase activity increases in extr acts of barley (Hordeum vulgare) leaves in response to the powdery mil dew fungus (Blumeria [syn. Erysiphe] graminis f.sp. hordei) and propos ed this as a source of H2O2 during plant-pathogen interactions. In thi s paper we show that the N terminus of the major pathogen-response oxa late oxidase has a high degree of sequence identity to previously char acterized germin-like oxalate oxidases. Two cDNAs were isolated, pHvOx Oa, which represents this major enzyme, and pHvOxOb', representing a c losely related enzyme. Our data suggest the presence of only two oxala te oxidase genes in the barley genome, i.e. a gene encoding HvOxOa, wh ich possibly exists in several copies, and a single-copy gene encoding HvOxOb. The use of 3' end gene-specific probes has allowed us to demo nstrate that the HvOxOa transcript accumulates to 6 times the level of the HvOxOb transcript in response to the powdery mildew fungus. The t ranscripts were detected in both compatible and incompatible interacti ons with a similar accumulation pattern. The oxalate oxidase is found exclusively in the leaf mesophyll, where it is cell wall located. A mo del for a signal transduction pathway in which oxalate oxidase plays a central role is proposed for the regulation of the hypersensitive res ponse.