Changes in the metabolism of sterols and pentacyclic triterpenoids in plant response to selected abiotic and biotic stress factors

One of the plant responses to stress is the reconfiguration of metabolic pathways, leading to the inhibition or enhancement of the biosynthesis of specialized defense metabolites. Under stress conditions, the inhibition of typical processes of the primary (general) metabolism often appears, accompanied with a redirection of metabolic flux to secondary (specialized) metabolic pathways. The biosynthetic pathways of general sterols and specialized pentacyclic triterpenoids might become competitive and undergo various modifications. The competition of the pathways might have a particular significance for the biotechnological trials of the enhancement of bioactive specialized metabolites in plant in vitro cultures, as well as for the induction of natural plant resistance in sustainable agriculture. The aim of the present thesis was to investigate the influence of selected stress factors, i.e. cadmium ions, jasmonic acid and chitosan, on steroid and triterpenoid metabolism in two experimental models, Calendula officinalis hairy root cultures and plants grown in a greenhouse (pot plants). The qualitative and quantitative analysis of steroids and triterpenoids was made with the use of gas chromatography-mass spectrometry (GC-MS) method; determination of the cadmium concentration was made with the use of the flame atomic absorption spectroscopy (FAAS), and anatomical, histological and morphological analyses of hairy roots with the use of light and fluorescence microscopy. The obtained results revealed unambiguously that steroid and triterpenoid metabolic pathways were significantly altered in response to stress factors applied in this study, both in pot-plants and in hairy root cultures. This confirmed the central research hypothesis. The observed influence of each applied factor was not uniform; for instance, the pattern of modifications in steroid and triterpenoid metabolism, induced as a part of the plant defense reaction, varied depending on the type of stress. The observed modifications involved not only remarkable changes in the composition and content of the analyzed groups of compounds, but also the proportions among individual compounds and their conjugated forms, i.e., esters and glycosides. Increased glycosylation (of both sterols and triterpenoids) was noted after treatment with jasmonic acid, whereas esterification of sterols was markedly induced after chitosan treatment. The competition between steroid and triterpenoid biosynthetic pathways was the most striking in terms of response to jasmonic acid treatment, and resulted in remarkable inhibition of sterol biosynthesis, parallel to the enhancement of the biosynthesis of pentacyclic triterpenoids and their derivatives, such as oleanolic acid and its saponins. Jasmonic acid was the most effective elicitor of triterpenoids, while the chitosan treatment inhibited the biosynthesis of both groups of analyzed compounds; thus calling into question its status as a universal biostimulant of plant growth and resistance. The dynamics of the observed changes was higher in hairy root cultures than in pot plants. Metabolic modifications occurring during the plant stress response were not constant or equally intensive over time; they eventually tended to return to normal physiological levels. The effects exerted by jasmonic acid were the most long-lasting, confirming its efficiency as a priming agent for inducing prolonged pest and pathogen resistance. Hairy root cultures may be a valuable model to track metabolic changes under stress conditions; however, direct extrapolation of study results to plants under field conditions can be misleading. Therefore, the investigation performed on in vitro cultures should represent an initial approach for evaluation of the general tendency of expected changes, e.g., stimulation or inhibition of the biosynthesis of the analyzed groups of compounds.