Ultrastructural features of the haustorial apparatus of the white blister fungus Albugo candida

Abstract

The small spherical haustorium of the white blister fungus is connected to the much larger haustorial mother cell by a slender cylindrical neck. The haustorium contains mitochondria with tubular cristae as well as ribosomes and occasional cisternae of rough endoplasmic reticulum. Nuclei and perinuclear dictyosomes are found in the mother cells but are absent from haustoria. No discontinuity is found in the fungal cell wall in the haustorial neck. Immediately adjacent to the fungal wall and extending through the penetration site to a point about midway along the neck is a dark-staining layer continuous with the host cell wall. A collar consisting of fibrillar material, also continuous with the host cell wall, is commonly found around the proximal portion of the neck external to this dark-staining layer. An electron-dense sheath surrounds the thin wall of the haustorial body but is absent from the neck region. A series of tubules is continuous with the invaginated host plasmalemma which surrounds the haustorial body. These tubules contain an electron-dense core similar in appearance to, and continuous with, the sheath matrix. No evidence was obtained for the involvement of host dictyosomes and their secretory vesicles in the formation of the haustorial sheath. A constant feature of the haustorial apparatus is the association of flattened cisternae of host endoplasmic reticulum with the distal portion of the haustorial neck. The significance of this finding is discussed in relation to the endomembrane concept.