|Novy, Richard - Rich|
Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2006
Publication Date: 12/8/2006
Citation: Mc Cue, K.F., Allen, P.V., Shepherd, L.V., Blake, A., Maccree, M.M., Rockhold, D.R., Novy, R.G., Stewart, D., Davies, H.V., Belknap, W.R. 2007. Potato glycosterol rhamnosyltransferase, the terminal step in triose side-chain biosynthesis. Phytochemistry. 68 (3): 327-334. Interpretive Summary: Potatoes produce bitter compounds called steroidal glycoalkaloids (SGAs). SGAs are also found in relatives of the potato, tomato and eggplant. High levels of SGAs are undesirable. When potatoes are exposed to light they turn green and start to accumulate SGAs. High levels can also be caused by bruising, adverse environmental conditions during growth, and during breeding of potatoes for new varieties. As part of a program to improve potatoes with biotechnology we are using natural potato genes that attach sugar molecules to the SGAs. By introducing the genes backwards, we can stop the sugar addition and prevent the accumulation of SGAs. In this study we introduced the gene for the last sugar addition step. This resulted in a significant decrease in the accumulation of SGAs usually found. However, additional SGAs with fewer sugars still remained. The results have confirmed the identity of a new gene in the pathway that adds the final sugar (rhamnose) to SGAs. This gene and information from it will be used to isolate and identify other SGT genes that can be used in future efforts to reduce accumulation of all SGAs.
Technical Abstract: Steroidal glycoalkaloids (SGAs) are undesirable secondary metabolites in the Solanaceous plants potato and tomato. Two predominant tri-glycosylated alkaloids, '-chaconine and '-solanine, accumulate in potato tubers. Using protein homology to Sgt1 (SOLtu:Sgt1) sterol galactosyltransferase, Sgt3 expressed sequence tags (ESTs) were identified in The Institute for Genomic Research (TIGR) database with similar abundance and tissue expression as the Sgt1 ESTs. The EST sequence data was used to design primers to isolate full cDNA sequences by polymerase chain reaction (PCR) from a wounded potato tuber cDNA library. The resulting 1515 bp open reading frame of Sgt3 encodes a predicted SGT3 amino acid sequence that is 18 residues longer than, 45% identical to and 58% homologous to the SGT1 protein. Analysis of genomic DNA showed a simple pattern for Sgt3, suggesting a low copy number. The amino-terminal region of the Sgt3 cDNA was used to create an antisense transgene under control of the granule bound starch synthase GBSS6 promoter and the Ubiquitin Ubi3 polyadenylation signal. Analysis of SGA metabolites in tubers revealed a dramatic affect on the accumulation of '-chaconine and '-solanine compensated by an increase in both '-solanine and '-chaconine with minor accumulation of '-SGAs. These results allow the identification of the function for SGT3 as the '-solanine/'-chaconine rhamnosyl transferase, the terminal step in formation of the potato glycoalkaloid triose side chains.