Function of triterpenes in Arabidopsis thaliana and their anti-inflammatory activities in mammalian hepatoma cells

Plant triterpenes belong to the class of secondary metabolites. They encompass a highly diverse range of ring structures consisting of thirty carbon atoms which can be enzymatically modified. Importantly, genes for their biosynthesis are arranged in epigenetically regulated gene clusters. However, very little is known about the physiological function of triterpenes in plants, but some triterpenes fulfill specialized roles in plant defense and growth. Two examples for triterpenes are marneral and thalianol. Marneral and thalianol are mainly synthesized in the roots of young seedlings. Accumulation of marneral or thalianol leads to a dwarfed phenotype in Arabidopsis thaliana, but their cellular function is unknown. Furthermore, other triterpenes like betulin exhibit anti-inflammatory properties in mammalian cells. However, the pharmacological potential of most triterpenes has not yet been fully investigated. Therefore, the aim of this thesis was to shed a light on the physiological function of triterpenes in plants and to elucidate their potential in triggering the anti-inflammatory Nrf2 EpRE pathway in animal cells. The Nrf2-EpRE pathway has become an important drug target in pharmacological research topics. For elucidating anti-inflammatory activities of triterpenes, a reporter-gene-assay was performed, in which murine hepatoma cells were treated with triterpene containing Rhodobacter capsulatus extracts. Using bacterial extracts containing recombinant synthesized triterpenes instead of fully purified compounds had a time-saving advantage by skipping purification protocols. As a result, no new anti-inflammatory triterpenes in bacterial extracts were identified, probably due to low triterpene concentrations or due to non-bioactive compounds. However, an anti-inflammatory activity of a non-triterpene metabolite, prodigiosin, in bacterial extracts was demonstrated. This proof of concept opens new opportunities for future studies on large-scale compound screenings of recombinant synthesized metabolites in bacterial extracts. The physiological functions of triterpenes in Arabidopsis thaliana were investigated using the example of marneral and thalianol. Arabidopsis marneral synthase 1 (mrn1) and thalianol synthase (thas) knock-out lines were crossed and their growth phenotype and their response to abiotic and biotic stress were analyzed. Furthermore, a putative linkage of marneral/thalianol and growth hormones was investigated. Combinatorial loss of marneral and thalianol in Arabidopsis did not lead to an altered plant growth. Nevertheless, the results provide evidence for a positive correlation between marneral and thalianol and the phytosulfokine-α mediated root growth. Furthermore, lacking marneral and thalianol led to a slightly increased defense against pathogenic root-associated microbiota. Whether slightly increased root defense in mrn1*thas is connected to impaired phytosulfokine-α signaling has to be investigated in future studies. Although it was tested, a function for marneral and thalianol in jasmonate signaling and abiotic stress response could not be proven. This thesis provides a new approach for screenings of anti-inflammatory compounds in the context of complex bacterial extracts. It also delivers first evidence for a function of marneral and thalianol in phytosulfokine-α signaling and a slightly negative influence on plant defense for both triterpenes.