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Claviceps purpurea is an ergot fungus known for its neurotropic alkaloids, which have been identified as the main cause of ergotism, a livestock and human disease triggered by ergot consumption. Tetrahydroxanthone dimers, the so-called ergopigments, presumably also contribute to this toxic effect. Overexpression of the cluster-specific transcription factor responsible for the formation of these pigments in C. purpurea led to the isolation of three new metabolites ( 8 - 10 ). The new pigments were characterized utilizing HRMS, NMR techniques, and CD spectroscopy and shown to be xanthone dimers. Secalonic acid A and its 2,4'- and 4,4'-linked isomers were also isolated, and their absolute configuration was investigated. The contribution of secalonic acid A, its isomers, and new metabolites to the toxicity of C. purpurea was investigated in HepG2 and CCF-STTG1 cells. Along with cytotoxic properties, secalonic acid A was found to inhibit topoisomerase I and II activity.

Claviceps purpurea is a plant pathogenic fungus which is still highly relevant in modern agriculture as it infects grasses such as rye and wheat. The disease caused by the consumption of contaminated grain or flour has been known since the Middle Ages and is termed ergotism. The main cause for the toxicity of this fungus is attributed to the ergot alkaloids. Apart from these alkaloids and the ergochromes known as ergot pigments, the secondary metabolism of C. purpurea is not well investigated. This study demonstrated the function of the polyketide synthase PKS7 in C. purpurea by determining the effect of its overexpression on metabolite profiles. For the first time, the depsides lecanoric acid, ethyl lecanorate, gerfelin, and C10-deoxy gerfelin were discovered as secondary metabolites of C. purpurea. Additionally, to estimate the contribution of isolated secondary metabolites to the toxic effects of C. purpurea, lecanoric acid, ethyl lecanorate, and orsellinic acid were tested on HepG2 and CCF-STTG1 cell lines. This study provides the first report on the function of C. purpurea PKS7 responsible for the production of depsides, among which lecanoric acid and ethyl lecanorate were identified as main secondary metabolites., (Copyright © 2020 Elsevier Inc. All rights reserved.)