Distribution of hydrolytic enzymes at barley powdery mildew encounter sites: implications for resistance associated with papilla formation in a compatible system

Distributions of the hydrolytic enzymes acid phosphatase and non-specific esterase in compatible barley coleoptiles inoculated with Erysiphe graminis fsp. hordei were investigated histochemically by light and electron microscopy. The four different enzymes investigated-acid β-glycerophos-phatase, acid naphthol AS-TR phosphatase, indoxyl acetate esterase and naphthol AS-D acetate esterase—had similar localization patterns. Reaction products were distributed generally in the fungal and host cells, but distinct accumulations were found in certain regions and structures at the encounter sites, including appressoria, penetration pegs, haustoria, papillae, and host cytoplasm. Most papillae showed conspicuous amounts of reaction products, suggesting that papilla formation includes depositions from secretory vesicles derived from the lysosomal system in host cells. There was a marked difference in the apparent concentration of hydrolytic enzyme activity between normal-size papillae accompanied by haustoria (sites of successful penetration) and oversize papillae (sites of unsuccessful penetration). Hydrolytic enzymes, localized most intensively in a lAyer of the oversize papillae, may confer resistance to fungal ingress by degrading components of the penetration pegs.