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 least six peroxidase isozymes. Two isozymes, P8.5 and P5.2, specifically increase in both resistant and susceptible barley following inoculation 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 barley with E. graminis hordei. Accumulation of the P8.5 polypeptide and total peroxidase activity in IWF was monitored over the same time course. These experiments show that enhanced levels of peroxidase activity are correlated with elevated levels of P8.5 polypeptide and mRNA in seedlings following inoculation with the powdery mildew pathogen. Finally, increased deposition of P8.5 was observed in papillae by immunogold labeling experiments. All the data are consistent with the hypothesis that peroxidase P8.5 participates in the deposition of phenolic compounds in papillae and thereby contributes to general, nonspecific defense responses in barley.