Antioxidant defences of the apoplast

The apoplast of barley and oat leaves contained superoxide dismutase (SOD), catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroasc orbate reductase, and glutathione reductase activities. The activities of t hese enzymes in the apoplastic extracts were greatly modified 24 h after in oculation with the biotrophic fungal pathogen Blumeria graminis. The quantu m efficiency of photosystem II, which is related to photosynthetic electron transport flux, was comparable in inoculated and healthy leaves during thi s period. Apoplastic soluble acid invertase activity was also modified in i noculated leaves. Inoculation-dependent increases in apoplastic SOD activit y were observed in all lines. Major bands of SOD activity, observed in apop lastic protein extracts by activity staining of gets following isoelectric focusing, were similar to those observed in whole leaves but two additional minor bands were found in the apoplastic fraction. The apoplastic extracts contained substantial amounts of dehydroascorbate (DHA) but little or no g lutathione (GSH). Biotic stress decreased apoplastic ascorbate and DHA but increased apoplastic GSH in resistant lines. The antioxidant cycle enzymes may function to remove apoplastic H2O2 with ascorbate and GSH derived from the cytoplasm. DHA and oxidized glutathione may be reduced in the apoplast or returned to the cytosol for rereduction.