Discovery of Two Novel Phthalide Phytotoxins from the Wheat Tan Spot Fungal Pathogen <i>Pyrenophora tritici-repentis</i>

The Dothideomycete fungal pathogen <i>Pyrenophora tritici-repentis</i> (Ptr) is the causal agent of the tan spot disease of wheat. The proteinaceous necrotrophic effectors ToxA and ToxB are well characterized. A nonproteinaceous effector called ToxC has also been partially characterized. Ptr produces a number of other small molecular weight compounds, but these remain poorly characterized. In this study, two novel compounds, designated ToxE1 and ToxE2, capable of inducing chlorotic symptoms on wheat leaves in a cultivar-specific manner, were purified from Ptr liquid cultures. There is no evidence that these compounds correspond to ToxC. Most isolates produced ToxE1, ToxE2, or both, and both compounds were detected in infected wheat leaves. The structures of both analogues were elucidated by NMR spectroscopy and comprise a phthalide core structure with an amide moiety. We postulate that these compounds have a general phytotoxic effect and may have an ancillary role in disease development.