Potato glycosterol rhamnosyltransferase, the terminal step in triose side-chain biosynthesis.

Steroidal glycoalkaloids (SGAs) are potentially harmful specialty metabolites found in Solanaceous plants. Two tri-glycosylated alkaloids, alpha-chaconine and alpha-solanine accumulate in potato tubers. Expressed sequence tags (ESTs) were identified in the available database by searching for protein homology to the Sgt1 (SOLtu:Sgt1) steriodalalkaloid galactosyltransferase. The EST sequence data was used to isolate Sgt3 cDNA sequences by polymerase chain reaction (PCR) from a wounded potato tuber cDNA library. The resulting 1515bp 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. 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 selected transgenic tubers revealed a dramatic decrease in the accumulation of alpha-chaconine and alpha-solanine. This decrease was compensated by an increase in beta-solanine and beta-chaconine with minor accumulation of alpha-SGAs. These results allowed the identification of the function for SGT3 as the beta-solanine/beta-chaconine rhamnosyl transferase, the terminal step in formation of the potato glycoalkaloid triose side chains.