Epsilon Open Archive

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.