EXPRESSION OF AN EXTRACELLULAR PEROXIDASE THAT IS INDUCED IN BARLEY (HORDEUM-VULGARE) BY THE POWDERY MILDEW PATHOGEN (ERYSIPHE-GRAMINIS F-SP HORDEI)

The intercellular wash fluids (IWF) of barley seedlings contain at lea st six peroxidase isozymes. Two isozymes, P8.5 and P5.2, specifically increase in both resistant and susceptible barley following inoculatio n with the powdery mildew pathogen, Erysiphe graminis f. sp. hordei. A polyclonal antiserum to the cationic P8.5 isozyme was used to recover a clone for this peroxidase from a cDNA expression library made from inoculated barley seedlings. This cDNA was used as a probe to examine P8.5 mRNA levels from 0 to 48 h following inoculation of resistant bar ley with E. graminis hordei. Accumulation of the P8.5 polypeptide and total peroxidase activity in IWF was monitored over the same time cour se. These experiments show that enhanced levels of peroxidase activity are correlated with elevated levels of P8.5 polypeptide and mRNA in s eedlings following inoculation with the powdery mildew pathogen. Final ly, increased deposition of P8.5 was observed in papillae by immunogol d labeling experiments. All the data are consistent with the hypothesi s that peroxidase P8.5 participates in the deposition of phenolic comp ounds in papillae and thereby contributes to general, nonspecific defe nse responses in barley. (C) 1995 Academic Press Limited