Transcriptome and Metabolome Analysis for Gene Discovery in the Steroidal Alkaloid Pathway

Steroidal alkaloids (SAs) are common constituents of numerous plants belonging to the Solanaceae family. Solanine, a major SA in potato was reported already 200 years ago and extensively studied as sprouting tubers may contain a relatively high content of SAs that is potentially dangerous for human health. Consisting of a C27 cholestane skeleton composed of several heterocyclic rings and containing nitrogen, SAs were suggested to be synthesized from cholesterol and to be further glycosylated to form steroidal glycoalkaloids (SGAs). The early stages of SA biosynthesis, predicted to start from cholesterol up to the alkamine, were not investigated at the molecular level although several possible pathways were suggested. Our main aim is to identify key genes in the biosynthesis of SGAs in the Solaneceae family. We recently reported that silencing of the tomato GLYCOALKALOID METABOLISM 1 gene (GAME1), putatively encoding a glycoalkaloid- glycosyltransferase, resulted in up to 50% reduction in the principal tomato SGA, α-tomatine. This study pointed to the importance of glycosylation in avoiding toxicity to the plant cell on one hand while increasing toxicity to pathogens on the other. Through a combination of metabolic and transcript profiling we have identified a different gene, GAME4, that appears to play a key role in the biosynthesis of the SGA aglycone from cholesterol in both potato and tomato as its silencing resulted in a dramatic reduction of SGAs levels in potato tubers, tomato fruit and leaves (of both species). Results of experiments in which GAME4 and Solaneaceae plants altered in its expression were characterized in detail will be presented.