Cytology of basidiospore germination, penetration, and early colonization of Phaseolus vulgaris by Uromyces appendiculatus var. appendiculatus

Abstract

Cytological studies were done on a compatible interaction between Phaseolus vulgaris and Uromyces appendiculatus var. appendiculatus. The cytoplasm of germinated basidiospores contained numerous microbodies in close association with lipid bodies, vacuoles, and endoplasmic reticulum. A single, polar germ tube commonly developed from a germinating basidiospore. The germ tubes tended to grow toward or along the junction lines between host epidermal cells. Appressorium formation in ca. 75% of all observations occurred less than 3.5 μm away from the lines of juncture. A mucilaginous exudate was associated with germ tubes and appressoria along areas of contact between host and fungus. Epidermal wall papillae were often observed beneath basidiospores and appressoria but did not restrict entrance of the fungus. The fungus directly penetrated the epidermal cell and invaginated the host plasma membrane. The apex of the penetration peg expanded to form an ovate intraepidermal vesicle. Electron-opaque material occluded the penetration pore after migration of the usually binucleate protoplast into the vesicle. The intraepidermal vesicle developed apically and formed a multicellular primary hypha that initially was multinucleate and later became uninucleate or binucleate. The primary hypha then grew into adjacent epidermal cells, subtending palisade parenchyma cells and underlying intercellular spaces.