|Novy, Richard - Rich|
|Belknap, William - Bill|
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., Blake, A., Maccree, M., Rockhold, D.R., Novy, R.G., Stewart, D., Davies, H., Belknap, W.R. 2007. Potato glycosterol rhamnosyltranferase, the terminal step in triose side-chain biosynthesis. Phytochemistry. 68(3): 327-334. 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 the last step in the production of alpha-chaconine and alpha-solanine. This resulted in a significant decrease in alpha-solanine and alpha-chaconine accumulation in selected transgenic lines, accompanied by increases in the accumulation of beta-solanine and beta-chaconine, the upstream compounds in the biosynthetic pathway. The net result was to leave total SGA levels essentially unchanged while altering the type of alkaloids accumulated. The results have confirmed the identity of a new gene in the pathway that is responsible for the addition of the rhamnose sugar to the final SGA products found in the potato. This gene and information from it will be used to isolate and identify other SGT genes that can also be used in future efforts to lower total SGA levels.
Technical Abstract: Steroidal glycoalkaloids (SGAs) are undesirable secondary metabolites in the Solanaceous plants potato and tomato. Two predominant tri-glycosylated alkaloids, a-chaconine and a-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 of Sgt3 is 58% homologous to that of Sgt1. The predicted SGT3 amino acid sequence is 18 residues longer than SGT1, and is 45% identical and 58% homologous to the SGT1 protein. Analysis of genomic DNA showed a simpler pattern for Sgt3 compared to Sgt1 suggestive of a lower copy number. The N-terminal encoded region of the Sgt3 cDNA was used to create an antisense transgene under control the granule bound starch synthase GBSS6 promoter and the Ubiquitin Ubi3 polyadenylation signal. Analysis of total SGAs in glass house tubers revealed a dramatic affect on the accumulation of a-chaconine and a-solanine compensated by an increase in both b-solanine and b-chaconine with minor accumulation of a-SGAs. These results allow the identification of the function for SGT3 as the b-solanine/b-chaconine rhamnosyl transferase, the terminal step in formation of the potato glycoalkaloid triose side chains.