Technical Abstract: Pyrenophora tritici-repentis is a necrotophic, fungal pathogen whose ability to cause tan spot of wheat (Triticum aestivum) is dependent on the production of host-selective toxins. One of these toxins, Ptr ToxA, is a protein that is only toxic to genotypes of wheat carrying the Tsn1 locus. The protein is internalized only in ToxA-sensitive wheat cells where it localizes to chloroplasts and induces cell death through induced production of chloroplastic reactive oxygen species. Ptr ToxA has been shown to interact with a chloroplast-localized protein, ToxABP1. ToxABP1 has been implicated in photosystem biogenesis and thylakoid formation and is highly conserved throughout the plant kingdom. If the interaction of ToxA with ToxABP1 is important for necrosis, we would expect that internal expression of ToxA in wide range of plants, including both monocots and dicots, would lead to cell death. Here we show that ToxA internal expression in wheat (regardless of ToxA sensitivity), barley (Hordeum vulgare) and tobacco (Nicotiana benthamiana) leads to cell death. Furthermore, we show that barley stripe mosaic (BSMV)-mediated, viral-induced gene silencing of ToxABP1 and internal expression of ToxA in wheat produces a similar phenotype. Finally, we show that wheat plants with reduced ToxABP1 expression also have reduced sensitivity to ToxA. These studies suggest that ToxABP1 is required for ToxA activity and that the mode-of-action of ToxA is to impair the function of ToxABP1.