Accumulation and Biosynthesis of Solanapyrone Phytotoxins by Ascochyta rabiei

The biosynthesis of the phytotoxins solanapyrone A , B and C produced by the phytopathogenic fungus Ascochyta rabiei has been investigated. To optimize feeding conditons for the tracer experiments the growth of the fungus and the accumulation of the toxins in submers culture were determined. The accumulation kinetics revealed that formation of the toxins occurs in the stationary phase of the growth indicating that synthesis of solanapyrones follows a typical pattern of secondary metabolism. Incorporation experiments with sodium [1-¹⁴C]- and [2-¹³C]acetate were performed and the NMR-spectroscopically determined labelling pattern of the ¹³C-enriched solanapyrone A compound confirmed that the carbon skeleton of this compound is formed via the polyketide pathway. The biosynthetic route to solanapyrone B is discussed.