Cellular expression of plant resistance and susceptibility to biotrophic fungal pathogens

The focus of this thesis is the cellular expression of resistance and susceptibility to biotrophic fungal pathogens such as the rust and powdery mildew fungi that obtain their nutrients from living plant cells. In their host plants, both groups of fungi commonly form intracellular feeding structures. However, in nonhost plants fungal growth is often stopped prior to the formation of these structures because of constitutive plant features or following expression of a variety of inducible plant defence responses. A cytological approach was taken to identify which nonhost responses were involved in restricting fungal growth during penetration of epidermal cells by the cowpea rust fungus (monokaryotic stage) and plantain powdery mildew fungus. While the complement and timing of plant responses varied in different plant-fungus combinations, the reactive oxygen species hydrogen peroxide was implicated as a primary determinant of the outcomes of each of these interactions, although not the factor that directly inhibited fungal growth.; Most fungi induce defensive plant responses when they attempt to infect the tissues of either host or nonhost plants. Rust fungi, however, trigger no expression of such responses during infection of their host species apparently due to the ability of the rust fungus to inhibit these responses in some way. The current study shows plant responses to fungal penetration of the epidermal wall are dependent on communication between the plant cell wall and cytoskeleton across the plasma membrane. Data are presented suggesting rust fungi may interfere in this process by causing a localized decrease in plant membrane-wall adhesion under the penetration point effectively eliminating defensive plant responses in host cells.; To understand the genetic basis of nonhost resistance to rust fungi, a combined cytological and genetic approach was performed using defence-compromised mutant and transgenic Arabidopsis thaliana plants during dikaryotic infection by three rust fungi. The data suggest that different signaling pathways affect different infection stages and suggest that A. thaliana can be developed as a system to study compatible, as well as incompatible, interactions with rust fungi.