Ultrastructure of the haustoria or intracellular hyphae in four different fungi

Summary An examination was made of the ultrastructure of haustoria or intracellular hyphae in four fungi: an obligate parasite (Puccinia hordei), a facultative parasite (Exobasidium japonicum) and two facultative saprophytes (Phytophthora palmivora and Sclerotinia fructigena). P. hordei haustoria showed the typical ultrastructure and host-parasite interface of most of the obligate parasites already studied. Connections between the host endoplasmic reticulum and host plasmalemma were observed at the encapsulation site. Tubules connecting the haustorial cytoplasm with the encapsulation, through the haustorial wall were occasionally seen. The host cell remained alive in the presence of the parasite. E. japonicum haustoria lacked a neck and encapsulation and were irregularly shaped, with branches which appeared to be partly surrounded by a sheath. Some of these branches showed cytoplasmic connections between the parasite and the host through the sheath. All the observed haustoria of E. japonicum were anucleate and contained only a few mitochondria and sparse membranes. The host cell was dead and its organelles disorganized. P. palmivora haustoria were simple with nucleus, endoplasmic reticulum, mitochondria and Golgi bodies. Neither sheath nor encapsulation was observed, and the host cell was also dead and disorganized. S. fructigena did not produce haustoria of any kind, the intercellular hyphae became intracellular by the degradation of the host cell walls, and the host cells were killed in advance of the growing hyphae.It is suggested that a new definition of haustorium is required to include all these intermediate haustorial bodies which cannot be included within the present concept of haustorium.