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Conversion of Exogenous Cholesterol into Glycoalkaloids in Potato Shoots, Using Two Methods for Sterol Solubilisation
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
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2013 (English)In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 8, no 12, e82955Article in journal (Refereed) Published
Abstract [en]

Steroidal glycoalkaloids (SGA) are toxic secondary metabolites naturally occurring in the potato, as well as in certain other Solanaceous plant species, such as tomato, eggplant and pepper. To investigate the steroidal origin of SGA biosynthesis, cut potato shoots were fed cholesterol labelled with deuterium (D) in the sterol ring structure (D-5- or D-6-labelled), or side chain (D-7-labelled), and analysed after three or five weeks. The labelled cholesterol and presence of D-labelled SGA were analysed by GC-MS and LC-MS/MS, respectively. When feeding D-labelled cholesterol solubilised in Tween-80, labelled cholesterol in free form became present in both leaves and stems, although the major part was recovered as steryl esters. Minor amounts of D-labelled SGA (alpha-solanine and alpha-chaconine) were identified in cholesterol-treated shoots, but not in blank controls, or in shoots fed D-6-27-hydroxycholesterol. Solubilising the labelled cholesterol in methyl-beta-cyclodextrin instead of Tween-80 increased the levels of labelled SGA up to 100-fold, and about 1 mole% of the labelled cholesterol was recovered as labelled SGA in potato leaves. Both side chain and ring structure D labels were retained in SGA, showing that the entire cholesterol molecule is converted to SGA. However, feeding side chain D-7-labelled cholesterol resulted in D-5-labelled SGA, indicating that two hydrogen atoms were released during formation of the SGA nitrogen-containing ring system. Feeding with D-7-sitosterol did not produce any labelled SGA, indicating that cholesterol is a specific SGA precursor. In conclusion, we have demonstrated a superior performance of methyl-beta-cyclodextrin for delivery of cholesterol in plant tissue feeding experiments, and given firm evidence for cholesterol as a specific sterol precursor of SGA in potato.

Place, publisher, year, edition, pages
2013. Vol. 8, no 12, e82955
National Category
Biological Sciences
Research subject
Plant Physiology
Identifiers
URN: urn:nbn:se:su:diva-100392DOI: 10.1371/journal.pone.0082955ISI: 000328705200118OAI: oai:DiVA.org:su-100392DiVA: diva2:693343
Available from: 2014-02-04 Created: 2014-02-03 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Analysis of sterol metabolism in the pathogenic oomycetes Saprolegnia parasitica and Phytophthora infestans
Open this publication in new window or tab >>Analysis of sterol metabolism in the pathogenic oomycetes Saprolegnia parasitica and Phytophthora infestans
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The primary objective of this thesis was to investigate the sterol metabolism of two pathogenic oomycetes, specifically the processes of sterol synthesis and sterol acquisition in the fish pathogen Saprolegnia parasitica (Saprolegniales) and the plant pathogen Phytophthora infestans (Peronosporales). Furthermore, the effects of steroidal glycoalkaloids from Solanaceous plants, on P. infestans, were examined. The improved understanding of these processes should help to identify approaches for the identification of new oomycete inhibitors targeting sterol metabolism in agriculture and aquaculture farming systems, and to guide plant-breeding strategies to defend solanaceous plants against oomycetes.

For these reasons, the molecular basis of the metabolic pathways of sterol synthesis and/or sterol acquisition was investigated. Sterols are derived from isoprenoids and indispensable in various biological processes. Our biochemical investigation of an oxidosqualene cyclase revealed that sterol synthesis in S. parasitica begins with the formation of lanosterol (Paper I), and a reconstruction of the complete sterol synthesis pathway to the final compound, fucosterol, in S. parasitica was performed using bioinformatics (Paper II). Complementary to this work, the extent to which P. infestans, which is incapable of de novo sterol synthesis, is able to modify exogenously provided sterols was investigated by determining the growth impact of various sterol supplements in the growth media (Paper II). 

Building on the sterol investigations, the solanaceous sterol derivatives from the glycoalkaloid family were analysed. These compounds contain both a steroidal and a carbohydrate (glycan) moiety. Data obtained by feeding various deuterium-labeled sterols to potato shoots, supported the theory that steroidal glycoalkaloids in Solanum tuberosum are produced from cholesterol (Paper III).  Since these steroidal glycoalkaloids are thought to play a role in plant defense, their physiological effects on P. infestans were investigated (Paper IV). Unexpectedly we found that non-glycosylated steroidal alkaloids had a greater inhibitory effect than steroidal glycoalkaloids.  Steroidal glycoalkaloids derived from other Solanaceous species exhibited different physiological effects on the growth of P. infestans

This research was conducted on two oomycete species belonging to the Saprolegniales and Peronosporales orders, hence the results presented are likely to be representative of each of these two oomycete orders.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2016. 59 p.
National Category
Biochemistry and Molecular Biology
Research subject
Plant Physiology
Identifiers
urn:nbn:se:su:diva-136551 (URN)978-91-7649-652-7 (ISBN)978-91-7649-653-4 (ISBN)
Public defence
2017-01-27, sal P216, Arrheniuslaboratorierna, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript.

Available from: 2017-01-02 Created: 2016-12-11 Last updated: 2017-01-27Bibliographically approved

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