A fine-structural study of the pea downy mildew fungus Peronospora pisi in its host Pisum sativum

Abstract

Downy mildew disease of the cultivated pea Pisum sativum L. caused by the fungus Peronospora pisi Sydow was studied in mature leaves and young shoots of the host plant. Particularly in systemic infections of young shoot tissue, a common occurrence was an extremely electron-opaque membrane-bound, hemispherical deposit extending through the host cell wall into the host cytoplasm. This material which abutted directly onto the intercellular hyphal wall was termed the penetration matrix. Its formation was apparently the result of a specific interaction between the host and obligate fungal parasite. Similar apparently solid or gellike material constituted the matrix surrounding the digitlike intracellular haustorium. This membrane-bound extrahaustorial matrix was present through the penetrated host cell wall and formed a relatively thick layer around haustoria in young shoot tissue, but was much thinner distally around haustoria in mature leaf mesophyll cells. An unusual, regularly arranged, tubular network of ribosome-free endoplasmic reticulum was occasionally found in the host cytoplasm in systemically infected shoot tissue adjacent to haustoria.