Regulation of sterol and glycoalkaloid biosynthesis in potato (Solanum tuberosum L.) – Identification of key genes and enzymatic steps

Steroidal glycoalkaloids (SGA) are toxic secondary metabolites present in some members of the Solanaceae family, including potato and tomato. The SGA level in tubers of potato (Solanum tuberosum L.) depends on genetic factors, but can also increase in response to e.g. wounding and light exposure. An upper limit of 200 mg SGA/kg f.w. is recommended in tubers used for human consumption. The SGA biosynthesis and its regulation are not fully understood, although cholesterol is often suggested as a likely SGA precursor. To gain more knowledge about the genetic regulation of SGA biosynthesis, a microarray study was performed during mechanical wounding or light exposure treatment of tubers from two potato cultivars. The results revealed six genes related to sterol and SGA biosynthesis as up-regulated during both treatments, and to be associated with increased SGA content. One of the genes, StDWF1, encoding a sterol Δ24-reductase similar to Arabidopsis DWF1, was further investigated in transgenic potato plants. Down-regulation of StDWF1 lowered the level of cholesterol as well as of SGA, demonstrating an important role of this gene in SGA synthesis. Homeostatic regulation of cholesterol metabolism in plants was investigated by over-expression of mouse cholesterol hydroxylases. In Arabidopsis, increased levels of hydroxylated sterols altered sterol/steroid metabolism as well as reduced plant growth. Similar effects were not observed in corresponding potato transformants, indicating species differences in sterol metabolism. To evaluate cholesterol as a SGA precursor, deuterium-labeled cholesterol was applied to potato shoots. Using LC-MS, label was shown to be incorporated into SGA. The work shows that increased SGA synthesis in potato tubers is mediated by the concerted action of at least six key genes, acting at different positions in steroid biosynthesis. Results also establish cholesterol as a SGA precursor in potato plants.