Precursors of epi-/shamixanthone formed in Hülle cells cause oxidative stress sensitivity and repress sexual development of the filamentous fungus Aspergillus nidulans

Abstract

Fungal secondary metabolites are small molecular products that are not directly involved in survival but play a role as a reaction to environmental changes including multiple chemical structures and bioactivities. Fungal secondary metabolism and developmental programs are interconnected. The aim of this thesis was to analyze the biological activity of the mdp/xpt clusters derived metabolites in the sexual development of the filamentous soil fungus Aspergillus nidulans. This work firstly elucidated the negative effect of secondary metabolites derived by the polyketide synthase (PKS) encoding mdp/xpt gene clusters on sexual fruiting body maturation in Aspergillus nidulans. The mdp/xpt gene clusters derived intermediates such as emodins and benzophenones as well as the final products epi-/shamixanthone are produced during sexual development. The PKS MdpG and the other four members MdpH, MdpL, XptB and XptC are localized in sexual mycelia and Hülle cells. To obtain more insights about the metabolites derived by this clusters and their functions, the intact epi-/shamixanthone biosynthetic pathway was disrupted by deleting the PKS encoding gene mdpG and the biosynthetically following seven genes mdpF, mdpC, mdpL, mdpD, xptA, xptB and xptC separately. Deletion of the genes mdpG and mdpF, encoding the first two enzymes in the biosynthetic pathway, lost the yellowish products present in wild type during sexual developement, resulting in pale Hülle cells. Deletion of the genes mdpC, mdpL, mdpD, xptA and xptB resulted in the accumulation of various precursors of epi-/shamixanthone in Hülle cells. This led to smaller Hülle cells with reduced activity and a delayed maturation of sexual fruiting bodies. All five deletion strains were more sensitive to abiotic oxidative and weak acidic stress. All of them showed no remarkable changes in vegetative growth and the production and viability of asexual spores. The accumulated intermediates mainly consist of emodin and its derivatives, benzophenone-derived compounds, and xanthone derivatives. Therein, the most abundant and active ingredients are emodin and its derivatives particularly accumulated in ∆mdpC and ∆mdpL strains resulting in the strongest effect on Hülle cells and sexual fruiting bodies. They exhibited a broad bioactivity on other organisms, suppressing the resting structure formation of Verticillum sp., the fruiting body formation of Sordaria macrospora, and the egg-laying activity of Drosophila melanogaster. In summary, increased amounts of precursors of epi-/shamixanthone derived by mdp/xpt gene clusters, in particular emodin and its derivatives, repressed the development of cleistothecia and Hülle cells of Aspergillus nidulans and the formation of resting and reproductive structures of other fungi and insects.