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

item MANNING, VIOLA - Oregon State University
item CHU, ASHLEY - Oregon State University
item Scofield, Steven - Steve
item CIUFETTI, LYNDA - Oregon State University

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2010
Publication Date: 9/1/2010
Citation: Manning, V.A., Chu, A.L., Scofield, S.R., Ciufetti, L.M. 2010. Intracellular expression of a host-selective toxin, ToxA, in diverse plants phenocopies silencing of a ToxA-interacting protein, ToxABP1. New Phytologist. 187:1034-1047.

Interpretive Summary: This paper provides new insight into the mechanism of pathogenesis of the fungal pathogen Pyrenophora tritici-repentis, the causal agent of wheat tan spot disease. This pathogen must produce the toxin Ptr ToxA to cause tan spot disease on susceptible wheat genotypes. Previous work has shown that resistant wheat lines do not internalize ToxA and once inside of susceptible wheat cells ToxA physically interacts with the protein, ToxABP1. This paper shows that if ToxA is engineered to expressed inside of wheat cells, it causes cell death regardless of whether the lines are resistant or susceptible to tan spot. Additionally, it demonstrates that silencing ToxABP1 expression results in a pattern of cell death that is very similar to internal expression of ToxA. Taken together these findings support the model that tan spot resistance operates at the level of preventing entry of ToxA into wheat cells, and once internalized ToxA kills by inferring with the function of ToxABP1. This information will of significance to the disease resistance research community.

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.