Ultrastructural observations of the rust fungus Physopella zeae in Zea mays

Abstract

Examination of sporulating uredia of the tropical corn rust fungus, Physopella zeae, with the transmission electron microscope revealed that this fungus possessed most of the distinguishing ultrastructural characteristics reported for temperate zone inhabiting members of the Uredinales. Physopella zeae differed from these latter fungi in the repeated production of urediospores from the same site on the sporogenous cell, and in the intracellular location of most of the mycelium. The intracellular mycelium was differentiated into structurally unspecialized hyphae and structurally distinct haustoria arising from differentiated haustorial mother cells which were located in the neighbouring host cell. Both types of intracellular structures were covered with a matrix, and that around the intracellular hyphae was continuous with similar material lining the plant cell wall. The matrix was thicker and more compact around intracellular hyphae than around the bodies of haustoria. The matrix lining the host walls was thinnest in sections of mesophyll cells containing a ratio of haustoria – intracellular hyphae profiles greater than one. These data suggest that the haustorium may actively inhibit matrix formation in invaded plant cells.