Lisa Arnqvist

Swedish University of Agricultural Sciences, Uppsala, Uppsala, Sweden

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Publications (3)9.9 Total impact

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    ABSTRACT: Sitosterol and stigmasterol are major sterols in vascular plants. An altered stigmasterol:sitosterol ratio has been proposed to influence the properties of cell membranes, particularly in relation to various stresses, but biosynthesis of stigmasterol is poorly understood. Recently, however, Morikawa et al. (Plant Cell 18:1008-1022, 2006) showed in Arabidopsis thaliana that synthesis of stigmasterol and brassicasterol is catalyzed by two separate sterol C-22 desaturases, encoded by the genes CYP710A1 and CYP710A2, respectively. The proteins belong to a small cytochrome P450 subfamily having four members, denoted by CYP710A1-A4, and are related to the yeast sterol C-22 desaturase Erg5p acting in ergosterol synthesis. Here, we report on our parallel investigation of the Arabidopsis CYP710A family. To elucidate the function of CYP710A proteins, transgenic Arabidopsis plants were generated overexpressing CYP710A1 and CYP710A4. Compared to wild-type plants, both types of transformant displayed a normal phenotype, but contained increased levels of free stigmasterol and a concomitant decrease in the level of free sitosterol. CYP710A1 transformants also displayed higher levels of esterified forms of stigmasterol, cholesterol, 24-methylcholesterol and isofucosterol. The results confirm the findings of Morikawa et al. (Plant Cell 18:1008-1022, 2006) regarding the function of CYP710A1 in stigmasterol synthesis, and show that CYP710A4 also has this capacity. Furthermore, our results suggest that an increased stigmasterol level alone is sufficient to stimulate esterification of other major sterols.
    Planta 02/2008; 227(2):309-17. · 3.35 Impact Factor
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    ABSTRACT: Transgenic potato (Solanum tuberosum cv Désirée) plants overexpressing a soybean (Glycine max) type 1 sterol methyltransferase (GmSMT1) cDNA were generated and used to study sterol biosynthesis in relation to the production of toxic glycoalkaloids. Transgenic plants displayed an increased total sterol level in both leaves and tubers, mainly due to increased levels of the 24-ethyl sterols isofucosterol and sitosterol. The higher total sterol level was due to increases in both free and esterified sterols. However, the level of free cholesterol, a nonalkylated sterol, was decreased. Associated with this was a decreased glycoalkaloid level in leaves and tubers, down to 41% and 63% of wild-type levels, respectively. The results show that glycoalkaloid biosynthesis can be down-regulated in transgenic potato plants by reducing the content of free nonalkylated sterols, and they support the view of cholesterol as a precursor in glycoalkaloid biosynthesis.
    Plant physiology 05/2003; 131(4):1792-9. · 6.56 Impact Factor
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    Lisa Arnqvist
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    ABSTRACT: Glycoalkaloids are secondary metabolites present within species of the family Solanaceae. They are toxic to humans and animals, and it is important to keep the levels in potato (Solanum tuberosum L.) tubers below the safe limit for consumption. Glycoalkaloids are derived from sterols, which are important components of cell membranes. The sterol composition in plants is complex, and varies between species. For instance, cholesterol is a minor sterol in most plants, but a major one in the Solanaceae. Cholesterol has been suggested as a metabolic precursor to the glycoalkaloids in potato, although the biosynthetic pathway is largely unknown. To investigate the role of sterols in glycoalkaloid biosynthesis, potato plants overexpressing a type 1 sterol methyltransferase (SMT1) were generated. SMT1 plants displayed an increased level of alkylated sterols, while the level of the non-alkylated sterol cholesterol was decreased. Along with this there was a reduction of glycoalkaloid levels. To gain more insight into the sterol biosynthesis in plants, Arabidopsis lines were generated overexpressing CYP710A1 and CYP710A4, encoding enzymes potentially involved in stigmasterol synthesis. Both transformants contained increased levels of stigmasterol and a decrease in the level of sitosterol. CYP710A1 transformants also displayed increased levels of esterified sterols, suggesting that an increased stigmasterol level alone is sufficient to stimulate esterification of other sterols. The possibility of downregulating cholesterol and glycoalkaloid levels by increasing cholesterol catabolism was investigated in plants by expression of four mouse cDNAs encoding enzymes hydroxylating cholesterol. In Arabidopsis, plant growth and sterol/steroid levels were altered, indicating that the introduced hydroxysterol synthesis affected regulatory steps in steroid homeostasis. However, an increased level of hydroxylated cholesterol in potato had no effect on glycoalkaloid levels. A sterol Δ24-reductase was downregulated in transgenic potato. Transformants displayed increased levels of Δ24-sterols, while the levels of 24-saturated sterols such as cholesterol were decreased, as was their glycoalkaloid level. This reveals a new role for this type of enzymes in plant steroid metabolism. Taken together, the results show that both cholesterol and glycoalkaloid biosynthesis can be downregulated in transgenic plants, and support the view of cholesterol as a metabolic precursor in glycoalkaloid biosynthesis.

Publication Stats

36 Citations
9.90 Total Impact Points

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Institutions

  • 2003–2008
    • Swedish University of Agricultural Sciences
      • Institutionen för växtbiologi och skogsgeneti
      Uppsala, Uppsala, Sweden