Ultrastructure and immunocytochemistry of the host–pathogen interface in poinsettia leaves infected with powdery mildew

Abstract

Transmission electron microscopy of high-pressure frozen and freeze-substituted samples was used to examine details of the host–pathogen interface in poinsettia leaf epidermal cells infected by the powdery mildew fungus Oidium sp. Cryofixation provided excellent preservation of both host cells and fungal haustoria. Each haustorium possessed a slender neck and an expanded body with numerous slender, aseptate, coiled lobes. The extrahaustorial membrane that separated each haustorium from the cytoplasm of an invaded host cell appeared thicker than other cellular membranes and was highly convoluted, particularly near the distal end of the haustorium neck. The neck of each haustorium was encased by a papilla that formed in response to the development of a haustorium. Immunogold labeling was used to elucidate the chemical nature of these papillae. Epitopes of callose, xyloglucan, and arabinogalactan proteins were localized in three separate regions of papillae. The localization sites for these compounds in poinsettia pa pillae differed somewhat from their reported locations in papillae of various other plants infected by either true fungi or oomycetes. In the current study, no labeling with the antibodies used was observed on the extrahaustorial matrix, the extrahaustorial membrane, the fungal plasma membrane, or the host plasma membrane.Key words: Oidium sp., Euphorbia pulcherrima, host–pathogen interactions, papillae, haustoria.