STRUCTURE, EXPRESSION AND LOCALIZATION OF A GERMIN-LIKE PROTEIN IN BARLEY (HORDEUM-VULGARE L.) THAT IS INSOLUBILIZED IN STRESSED LEAVES

The primary leaves of young barley seedlings contain two major, extrac ellular, acid-soluble proteins of ca. 22 and 23 kDa apparent molecular mass. These proteins disappeared from the intercellular washing fluid upon stress treatments that enhanced H2O2 levels and that induced res istance to subsequent challenge by the powdery mildew fungus Erysiphe graminis f. sp. hordei. A partial peptide sequence of the 22 kDa prote in was determined, and a cDNA clone was isolated. The 22 kDa protein b elongs the the group of germin-like proteins (GLPs) and was designated HvGLP1. Despite its similarity to germin, i.e. oxalate oxidase, no ox alate oxidase activity of HvGLP1 could be detected. The RNA and solubl e protein of HvGLP1 was highly abundant in young leaves, less abundant in older leaves and absent in roots. HvGLP1 RNA oscillated with a cir cadian rhythm, the minimum and maximum of RNA abundance being at the e nd of the dark and light periods, respectively. Heat and H2O2 treatmen t as well as pathogen infection caused disappearance of HvGLP1 protein from the fraction of soluble proteins of the intercellular space. HvG LP1 protein could be re-solubilized from cell walls of heat- or H2O2-t reated leaves by boiling in SDS suggesting non-covalent cross linking. Although a physiological role of HvGLP1 insolubilization is still ope n, the protein may serve as marker for oxidative stress in cereals.