Submitted to: American Journal of Potato Research
Publication Type: Abstract Only
Publication Acceptance Date: 3/23/2004
Publication Date: 4/23/2004
Citation: Mc Cue, K.F., Allen, P.V., Shepherd, L., Rockhold, D.R., Davies, H., Belknap, W.R. 2005. Identification and characterization of a udp-galactose:solanidine galactosly transferase, a branchpoint in triose formation of steroidal glycoalkaloids. American Journal of Potato Research. 82(1):80-81. 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, alpha-chaconine and alpha-solanine accumulate in potato tubers. The new sterol alkaloid glycosyl transferase (Sgt) gene has been identifed as a glycosterol rhamnosyl transferase (SOLtu:Sgt3). The Sgt3 nucleic acid sequence is 55.4% identical to the SOLtu:Sgt1 (Sgt1,) a glucosyl transferase, and the predicted SGT3 sequence shows only 42.9% amino acid conservation compared to the SGT1 protein. Sgt3 and Sgt1 had similar wound-induced expression of RNA in both leaves and tubers with transcripts accumulating to a maximum within the first 16 hours after wounding and then rapidly declining. Analysis of genomic DNA showed a simpler pattern for Sgt3 compared to Sgt1 indicative of a low copy number. The N-terminal encoding region of the Sgt3 cDNA was used to create an antisense transgene under control the Gbss6 promoter and the Ubi3 polyadenylation signal. Analysis of total SGAs in glass house tubers resulted in the expected variation seen in SGA levels by somaclonal variation. While the total levels of SGAs were essentially unchanged the profiles of component SGAs were dramatically affected in two of the ten lines analyzed showing a dramatic reduction of alpha-SGAs compensated by an increase in both beta-solanine and beta-chaconine with minor accumulation of alpha-SGAs