Intracellular expression of a host-selective toxin, ToxA, in diverse plants phenocopies silencing of a ToxA-interacting protein, ToxABP1.

*ToxA, a host-selective toxin of wheat, can be detected within ToxA-sensitive mesophyll cells, where it localizes to chloroplasts and induces necrosis. Interaction of ToxA with the chloroplast-localized protein ToxABP1 has been implicated in this process. Therefore, we hypothesized that silencing of ToxABP1 in wheat would lead to a necrotic phenotype. Also, because ToxABP1 is highly conserved in plants, internal expression of ToxA in plants that do not normally internalize ToxA should result in cell death. *Reduction of ToxABP1 expression was achieved using Barley stripe mosaic virus (BSMV)-mediated, viral-induced gene silencing. The BSMV system was modified for use as an internal expression vector for ToxA in monocots. Agrobacterium-mediated expression of ToxA in a dicot (tobacco-Nicotiana benthamiana) was also performed. *Viral-induced gene silencing of ToxABP1 partially recapitulates the phenotype of ToxA treatment and wheat plants with reduced ToxABP1 also have reduced sensitivity to ToxA. When ToxA is expressed in ToxA-insensitive wheat, barley (Hordeum vulgare) and tobacco, cell death ensues. *ToxA accumulation in any chloroplast-containing cell is likely to result in cell death. Our data indicate that the ToxA-ToxABP1 interaction alters ToxABP1 function. This interaction is a critical, although not exclusive, component of the ToxA-induced cell death cascade.