Visualization of actin in situ by rhodamine-conjugated phalloin in the fungus Uromyces phaseoli.

Rhodamine-conjugated phalloin, a derivative of phalloidin which binds to F-actin, was associated with three types of structures in uredospore germlings of the bean rust fungus, Uromyces phaseoli. The structures were filaments, peripheral plaques, and intranuclear inclusions. The filaments, located throughout the germ tube but especially in the more basipetal regions, were observed as either barely perceivable, fine elements or as easily detectable, coarser structures. The plaques, which we suggest to be equivalent to filasomes, occurred near the periphery of the cell's cytoplasm. They were most numerous in the hyphal tip regions. Nuclear inclusions occurred within the nucleoplasm subjacent to the spindle pole body. Treatments with KI and phalloidin substantiated that the fluorescently labelled sites were F-actin. Treatment of the germlings with cytochalasin E caused the intranuclear inclusions to become extended often with branched, fine filaments. Similar treatments led to a disappearance of cytoplasmic filaments, but had no perceivable effect on the peripheral plaques.