Synthesis of the solanapyrone phytotoxins by Ascochyta rabiei in response to metal cations and development of a defined medium for toxin production

The phytotoxic solanapyrones A, B and C, were produced by Ascochyta rabiei when grown on Czapek Dox nutrients supplemented with an aqueous extract of chickpea seed but not when grown on Czapek Dox nutrients alone. Addition of amino acids, vitamins and inorganic ions to Czapek Dox nutrients allowed the production of the solanapyrones in good yields. By systematic elimination of components of this medium, the constituents essential for toxin production were identified as the divalent metal cations Zn²⁺, Ca²⁺, Mn²⁺ and Cu²⁺. When these ions were added to Czapek Dox nutrients in the same concentrations as those found in chickpea extract, concentrations of the phytotoxins similar to those found in Czapek Dox nutrients supplemented with chickpea extract were detected. Removal of cations from the chickpea supplement by a cation exchange resin did not affect the growth of the fungus compared with growth on the medium containing the complete supplement, but no toxin was produced. A defined medium and cultural conditions suitable for solanapyrone production by A. rabiei are described.