Plant cell responses to infection by rust fungi

grantor: University of TorontoThe cowpea rust fungus ('Uromyces vignae' Barclay)-cowpea ('Vigna unguiculata' [L.] Walp.) interaction was examined by cytological and molecular techniques to determine cellular responses during the monokaryotic stage of infection and changes in gene expression during the dikaryotic stage. Chapter one describes callose deposition around intracellular fungal structures, the main resistance response in cultivar Queen Anne. Ultrastructural examination revealed that the callose encasement was developed from the site where the fungus encountered the inside of the plant cell wall and was separated from the fungus by the extrahyphal membrane and an extension of plant plasma membrane. Treatment of plants with chemical inhibitors indicated that, similar to what is observed during the dikaryotic stage of infection, callose deposition in response to the monokaryotic stage is dependent on transcription, protein synthesis and glycosylation and microfilament polymerization. Experiments in which the fungus was killed by heat treatment indicated that callose deposition is normally suppressed in susceptible plants and that, unlike the extrahaustorial membrane surrounding the dikaryotic haustoria of this fungus, the extrahyphal membrane is capable of generating callose. In chapter two, the role of the plant cytoskeleton in infection-induced nuclear movements and resistance-associated cell death, the hypersensitive response (HR) was investigated by fluorescent microscopy and treatments with anticytoskeletal drugs. The nuclear movements, which differ between the susceptible cultivar California Blackeye (CB) and the resistant cultivars Dixie Cream and Calico Crowder, were dependent on actin microfilaments. Observations of microtubule organization during the HR revealed that the sequence of events leading to protoplast collapse differed between the two resistant cultivars, suggesting a possibility of multiple pathways for cellular degradation during the HR. The microfilament network remained visible even at later stages of the HR and was necessary for this process to occur. Chapter three describes the use of mRNA differential display to identify two genes that are upregulated by infection in the susceptible cultivar CB. Both genes may code for proteins that were previously uncharacterized in plants. One is induced during early stages of infection and may encode a reverse transcriptase and the other is turned on only during haustorium formation and may encode a lipase-like protein.Ph.D