A cDNA encoding solanidine glucosyltransferase (SGT) was isolated from potato. The cDNA was selected from a yeast expression library using a positive selection based on the higher toxicity of steroidal alkaloid aglycons relative to their associated glycosylated forms. The cDNA contained an open reading frame encoding a 56 kDa polypeptide with regions of similarity to previously characterized UDP-glucosyltransferases. The enzyme activity and reaction products of recombinant SGT in yeast were consistent with those observed for the endogenous enzyme from potato. SGT mRNA and protein accumulated in tubers in response to wounding. The time course for SGT mRNA accumulation paralleled that of 3-hydroxy-3-methylglutaryl-coenzymeA isoform 1 (hmg1) mRNA. Steady-state SGT mRNA levels also increased transiently upon wounding of leaves.

Steroidal saponins are widely distributed in many plant species. Their diverse structures have resulted in a wide range of applications, including drug and medicine production. It has been suggested that the nature of the non-saccharide and oligosaccharide portions of the saponin molecule both contribute to the properties of individual saponins. Despite numerous studies on the occurrence, chemical structure, and varying pharmaceutical activities of steroidal saponins, their biosynthesis pathway is poorly understood. Glycosylation is thought to be the final step in steroidal saponin biosynthesis and it is thought to be involved in regulating the biological activities of saponins. Isolation of the glycosyltransferases that catalyze the transfer of sugar molecules to steroidal compounds will help to clarify the mechanisms that produce diverse saponins and control their activities in plants. In this study, we obtained three cDNAs encoding putative glycosyltransferases from Solanum aculeatissimum. One of the three, SaGT4A showed UDP-glucosyltransferase activity. This is the first cloned glucosyltransferase involved in steroidal saponin biosynthesis. SaGT4A catalyzes the 3-O-glucosylation of steroidal sapogenins, such as diosgenin, nuatigenin, and tigogenin. This enzyme also glucosylates steroidal alkaloids, such as solanidine, solasodine, and tomatidine. Gene expression analysis revealed that the accumulation of SaGT4A transcripts showed a unique response to wounding stress indicating the involvement of SaGT4A in plant defense system.