Fatty acid peroxidation by rubellin B, C and D, phytotoxins produced by Ramularia collo-cygni (Sutton et Waller)

Rubellin D is a toxic anthraquinone derivative produced by Ramularia collo-cygni, the fungus causing Ramularia leaf spot disease on barley and some other cereals. In this study, we were able to show that rubellin D induces the peroxidation of α-linolenic acid, measured as ethane formation, in a biochemical model system as well as in barley and tobacco leaves. Rubellin-induced fatty acid peroxidation was strictly light-dependent in all systems tested. In parallel to ethane formation, chlorophyll bleaching occurred in toxin-treated leaves. Inhibitor experiments indicate that singlet oxygen produced in a photodynamic reaction plays the predominant role in rubellin D-induced fatty acid peroxidation. In this study, we further describe the identification of rubellin C as an additional anthraquinone derivative of R. collo-cygni besides rubellin B and D. Comparison of rubellin D to rubellin B and C in the biochemical model system indicates a similar activity of the three toxins. Cercosporin, a photodynamically active phytotoxin from Cercospora species, was more effective in inducing ethane formation from α-linolenic acid. The addition of ferrous ions stimulated the reaction induced by the rubellins but inhibited α-linolenic acid peroxidation triggered by cercosporin. The possible role of rubellin production in symptom development after R. collo-cygni infection is discussed.