A class III peroxidase specifically expressed in pathogen-attacked barley epidermis contributes to basal resistance

Higher plants possess large multigene families encoding secreted class III peroxidase (Prx) proteins. In barley, two Prx cDNAs encoding HvPrx07 and HvPrx08 have been isolated and characterized to some extent with respect to a resistance-mediating function upon attack by the powdery-mildew fungus Blumeria graminis f.sp. hordei (Bgh). Here we present evidence for the tissue-specific accumulation of a new Prx mRNA, HvPrx40, in Bgh-attacked epidermis of barley (Hordeum vulgare). The encoded protein is predicted to be secreted into the apoplastic space of epidermal cells due to the absence of a C-terminal extension, which distinguishes it from other Prx proteins reported to accumulate in leaf epidermis. Transient overexpression of HvPrx40 enhanced the resistance of wheat (Triticum aestivum) and barley against Blumeria graminis f.sp. tritici (wheat powdery mildew) and Bgh, respectively. These findings were complemented by transient-induced gene silencing showing hypersusceptibility of barley leaf epidermal cells to Bgh. The local accumulation of oxidized 3,3-diaminobenzidine that reflects H(2)O(2) production at sites of attempted fungal penetration was not reduced in HvPrx40-silenced cells, suggesting a role of this peroxidase other than the production of reactive oxygen species.