The interface between haustoria of Puccinia poarum (monokaryon) and Tussilago farfara

Abstract

The haustoria of the monokaryon of Puccinia poarum and associated structures in infected cells of Tussilago farfara leaves were examined by electron microscopy. Specimens were fixed with glutaraldehyde and osmium tetroxide and sections stained with uranyl and lead salts and by methods for the specific localization of carbohydrate (periodic acid, thiocarbohydrazide and silver proteinate (PATCHSP), plasma membranes (periodic acid, chromic acid and phosphotungstate (PACP)), or ATPase activity, all with appropriate controls. Haustoria occurred in mesophyll cells but were more abundant in vascular tissues where all cell types except sieve tubes were infected. They were filamentous and each was enclosed in an extension of the host plasma membrane (the extrahaustorial membrane). Between the haustoria and their extrahaustorial membranes, host wall-like material extended to varying distances from the penetration sites, sometimes covering the haustoria. Where this layer was incomplete, the proximal and distal matrices lay beyond it. The amorphous proximal matrix was distinguished mainly by strong reactions with uranyl acetate, PATCHSP and PACP and the distal matrix by radiating fibrils stained with PACP but unstained by PATCHSP. The plasma membrane lining the host cell wall and the extrahaustorial membrane over cell wall-like material gave positive reactions with PACP and for ATPase. Over proximal and distal matrices, the extrahaustorial membrane usually stained weakly with PACP and always lacked ATPase activity. The transitions in staining and ATPase reactions were usually gradual, extending over a distance of ~1·5 μm but staining indicating the presence of a neckband was never observed. The observations are discussed with reference to the development and physiology of the haustoria and infected cells. It is suggested that the extrahaustorial membrane is less fluid than a normal plasma membrane, that solute transport is less efficient and the nutritional relationship of individual haustoria less permanent than in infections by the dikaryotic phase of the fungus.