Host-parasite relations in a mycoparasite. V. Electron microscopy of Piptocephalis virginiana infection in compatible [Choanephora cucurbitarum] and incompatible [Phascolomyces articulosus] hosts

Host-parasite interaction between Piptocephalis Virginiana and its host, Choanephora cucurbitarum, is compatible when the host hyphae are young, slender, with a thin cell wall of one layer. The parasite appeared to penetrate mechanically by pushing the host-cell wall inwards. The host plasma membrane invaginated along the involuted cell wall. The older hyphae of C. cucurbitarum possessed two distinct layers of cell wall and showed an incompatible interaction when challenged with the parasite. At the point of contact, the outer layer of the host-cell wall dissolved, probably by enzymatic digestion, and the inner layer became thickened and developed a papilla as a result of its response to the parasite. The haustoria of the parasite in the old hyphae were always surrounded by a thick, well-developed sheath, whereas the haustoria of the same age in the young host mycelium were devoid of a sheath during early stages of infection. Instead, they were in direct contact with the host protoplasts. The incompatible interaction between a resistant host, Phascolomyces articulosus, and the parasite showed similar results as with old hyphae of C. cucurbitarum. The cell wall of P. articulosus appeared thick with two or more layers even in the 18-22-h-old hyphae. The role of cell wall in resistance mechanism is discussed.