Ultrastructure of freeze-substituted appressoria produced by aeciospore germlings of the rust fungus Arthuriomyces peckianus

Abstract

Aeciospores of Arthuriomyces peckianus germinated readily on moist strips of dialysis membrane and developing appressoria were observed within 3 to 6 h after spores were deposited on membranes. A single germ tube typically emerged from each binucleate spore and grew until its tip contacted the dialysis membrane. The germ tube tip was then transformed into a swollen appressorium that adhered tightly to the membrane, apparently as a result of an extracellular material that surrounded the appressorium base. Virtually all the spore cytoplasm and both nuclei moved into the germ tube and developing appressorium. Following a synchronous mitotic division of the two nuclei, a septum formed to delimit the now tetranucleate appressorium from the germ tube. As the appressorium matured, an apparently wall-less region developed in the central portion of the appressorium appressed against the dialysis membrane. In this region the fungus plasma membrane appeared to make direct contact with the underlying dialysis membrane. A funnel-like or cone-like structure referred to as the appressorial cone then developed in the wall-less region. The appressorial cone extended up into the cytoplasm of the appressorium and was lined by the fungal plasma membrane. Numerous branched elaborations of the plasma membrane were associated with the inner portion of the cone. Key words: rust fungus, infection structures, electron microscopy.