In vitro formation of haustoria of the cowpea rust fungus Uromyces vignae in the absence of a living plant cell. II. Electron microscopy

The ultrastructure of infection hyphae, haustorial mother cells, and haustoria of Uromyces vignae formed on collodion membranes is described after conventional preparation and after treatment with periodic acid – thiocarbonhydrazide – silver proteinate or periodic acid – chromic acid – phosphotungstic acid treatments. Infection hyphae and haustorial mother cells developed normally in vitro but lacked an outer wall layer present in older infections in the host plant. Haustorium formation in vitro was accompanied by the development of elaborations of the plasmalemma along the infection hypha side of the haustorial mother cell septum, but their contents did not show identical staining responses to the haustorial neck wall as reported for other rust fungus species. Haustorial necks, and to a lesser extent haustorial bodies, were coated with a fibrillar material, the staining characteristics of which were similar to material normally considered part of the extrahaustorial matrix in infected plants. The restriction of this material to the haustorium suggests that it may play an important role in the interaction between the haustorium and the plant cell.