Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2005
Publication Date: 11/18/2005
Citation: Mc Cue, K.F., Allen, P.V., Shepherd, L., Blake, A., Whitworth, J.L., Maccree, M., Rockhold, D.R., Stewart, D., Davies, H., Belknap, W.R. 2006. The primary in vitro steroidal alkaloid glucosyltransferase from potato (solanum tuberosum). Phytochemistry. 67 (15):1590-1597
Interpretive Summary: Steroidal glycoalkaloids (SGAs) are undesirable, bitter secondary metabolites found in Solanaceous plants including the potato and tomato. Potatoes produce two major SGAs species, alpha-chaconine and alpha-solanine. As part of a program to improve potato cultivars with molecular genetics we are using natural potato genes for a family of genes called Steroidal alkaloid Glycosyl Transferases (SGT) to develop potatoes with lower levels of SGAs. In this study we introduced an antisense SGT gene into potatoes believed responsible for one of two steps at the committed step leading into the SGA pathway. Inhibition of the SGT2 protein results in a large decrease in the accumulation of alpha-chaconine, the least desirable of the two major SGA species. The function of the SGT2 protein was confirmed using SGT2 protein isolated from yeast and tested with different sugar donor and alkaloid receptors to confirm its identity.
Technical Abstract: In an effort to control the flux through the steroidal glycoalkaloid pathway in potato we have been investigating the members of the steroidal alkaloid glycosyltransferase (Sgt) gene family. The committed step into the SGA pathway is the glycosylation of solanidine by either UDP-glucose or UDP-galactose leading to alpha-chaconine or alpha-solanine, respectively. In this study we identify the gene with primary responsibility for the glucosylation of solanidine in vivo, Sgt2. The Sgt2 gene was identified by deduced protein sequence homology to the previously identified Sgt1 gene, which has glucosylation activity in vitro, but in potatoes has primary responsibility for galactosylation of solanidine destined for accumulation as alpha-solanine. Using an antisense strategy, Sgt2 was shown to inhibit accumulation of alpha-chaconine in transgenic tubers potato (Solanum tuberosum) CVs. Lenape and Desirée. Studies with recombinant SGT2 protein expressed in yeast and purified by metal ion affinity show that SGT2 glycosylation activity is highly specific for UDP-glucose as a sugar donor. The Sgt2 gene in combination with Sgt1 is expected to block biosynthesis of both branches of the SGA pathway resulting in significant reductions of SGAs in tubers.