Abstract: The fungus Pyrenophora tritici-repentis causes tan spot disease in wheat. Typical symptoms include tan-colored necrotic lesions surrounding small infection sites. The necrosis is induced by the host-selective proteinaceous toxin, ToxA. The ability of ToxA to be internalized into plant cells is required for the toxin to cause cell death. Using yeast two-hybrid (Y2H) analyses, we detected the oligomerization of ToxA and we identified amino acid E145 and D149 residues that were critical for this ToxA-ToxA oligomerization. However, a G141A mutation in the RGD motif, which is important for necrosis, still can form the oligomer. We have developed a transient assay system to provide direct evidence that intracellular expression of ToxA causes cell death in wheat. By screening a cDNA library, we identified wheat plastocyanin as a host target protein of ToxA. Plastocyanin is a component of the electron transport chain of photosynthesis. Virus-induced gene silencing of wheat plastocyanin produced a similar phenotype of ToxA-induced cell death. In Y2H analyses, the mutant G141A does not interact with plastocyanin, but the E145R and D149K mutants do interact with plastocyanin. However, all G141, E145 and D149 residues are required for ToxA-induced necrotic symptom in planta, which indicates that both oligomerization and plastocyanin association are important for ToxA activity.