Early events in living epidermal cells of cowpea and broad bean during infection with basidiospores of the cowpea rust fungus

The infection process of basidiospores of the cowpea rust fungus (Uromyces vignae) was studied on living host (Vigna sinensis) and nonhost (Vicia faba) leaves using light microscopy with differential interference contrast optics. During the first 8 h, fungal development was similar on host and on nonhost leaves. Penetration and production of intraepidermal vesicles occurred in nonhost cells 4 h earlier than in host cells. Penetration frequency was also higher in nonhost leaves. These results suggest that the cuticle of the cowpea plant delays basidiospore infection. Both host and nonhost cells produced cytoplasmic aggregates during appressorium formation and again, a few hours later, during penetration of the epidermal cell wall. Cytoplasmic aggregates were also observed in cells adjacent to a collapsing cell. Papillae were observed at most infection sites in both host and nonhost cells. The nuclei of infected cells migrated towards the penetration site in both plant–pathogen combinations. Nuclear size increased considerably only in the nonhost epidermis and decreased again markedly after cell collapse. In the nonhost cells, three types of defence reactions occurred during or after formation of the intraepidermal vesicle. First, following the halt of cytoplasmic streaming, the cytoplasm of the invaded cell either contracted or disintegrated into granules. Alternatively, the cytoplasm continued streaming but darkly pigmented material encased the fungal infection structure. Key words: basidiospore, broad bean (Vicia faba), cowpea (Vigna sinensis), cowpea rust fungus (Uromyces vignae), hypersensitivity, nonhost resistance.