Ultrastructure of the host-parasite interaction in leaves of Duchesnea indica infected by the rust fungus Frommeëla mexicana var. indicae as revealed by high pressure freezing

Abstract

A combination of scanning and transmission electron microscopy was used to examine the host-pathogen relationship in leaves of Duchesnea indica (Andrz) Focke infected by the rust fungus Frommeëla mexicana var. indicae McCain & Hennen. Samples for transmission electron microscopy were prepared using high pressure freezing followed by freeze substitution. This protocol provided excellent preservation of both host cells and fungal haustoria. Each haustorium of F. mexicana var. indicae possessed a long slender neck with a neck band and an expanded body that contained two nuclei positioned close together. The haustorial body was lobed and sometimes even branched but lacked septa. Details of the extrahaustorial membrane that separated each haustorium from the cytoplasm of its host cell were particularly well preserved. Extensive labyrinth cell wall ingrowths developed around haustorial necks, as well as elsewhere, in infected cells. These ingrowths appeared to be identical to those present in plant transfer cells. Transfer cells are thought to be involved in intensive solute transfer over short distances. This appears to be the first report of the development of transfer cells in response to infection by a plant pathogenic fungus.Key words: haustoria, transfer cells, freeze substitution, electron microscopy.