The primary in vivo steroidal alkaloid glucosyltransferase from potato q Kent F. McCue a, * , Paul V. Allen a , Louise V.T. Shepherd b , Alison Blake b , Jonathan Whitworth c , M. Malendia Maccree a , David R. Rockhold a , Derek Stewart b , Howard V. Davies b , William R. Belknap a a USDA, Agricultural Research Service, Crop Improvement and Utilization Research Unit, Western Regional Research Center, 800 Buchanan St., Albany, CA 94710-1105, USA b Scottish Crop Research Institute, Quality, Health and Nutrition Programme, Dundee DD2 5DA, UK c USDA, Agricultural Research Service, Small Grains and Potato Germplasm Research Unit, 1693 S 2700 W, Aberdeen, ID 83210, USA Received 15 July 2005; received in revised form 23 September 2005 Available online 18 November 2005 Dedicated to Professor Rod Croteau on the occasion of his 60 th birthday. Abstract To provide tools for breeders to control the steroidal glycoalkaloid (SGA) pathway in potato, we have investigated the steroidal alka- loid glycosyltransferase (Sgt) gene family. The committed step in the SGA pathway is the glycosylation of solanidine by either UDP- glucose or UDP-galactose leading to a-chaconine or a-solanine, respectively. The Sgt2 gene was identified by deduced protein sequence homology to the previously identified Sgt1 gene. SGT1 has glucosyltransferase activity in vitro, but in vivo serves as the UDP-galac- tose:solanidine galactosyltransferase. Two alleles of the Sgt2 gene were isolated and its function was established with antisense transgenic lines and in vitro assays of recombinant protein. In tubers of transgenic potato (Solanum tuberosum) cvs. Lenape and Desire ´e expressing an antisense Sgt2 gene construct, accumulation of a-solanine was increased and a-chaconine was reduced. Studies with recombinant SGT2 protein purified from yeast show that SGT2 glycosylation activity is highly specific for UDP-glucose as a sugar donor. This data establishes the function of the gene product (SGT2), as the primary UDP-glucose:solanidine glucosyltransferase in vivo. Published by Elsevier Ltd. Keywords: Solanum tuberosum; Solanaceae; Potato; Molecular genetics; Steroidal glycoalkaloids; UDP-glucose:solanidine glucosyltransferase; Chaconine; Solanine; Sgt1; Sgt2 1. Introduction Crop species produce a variety of commercially undesir- able natural products. These are believed to have evolved as defense mechanisms against microbial and herbivorous pests. Steroidal glycoalkaloids (SGAs) are toxic metabolites found in potatoes (Solanum tuberosum) and other Solana- ceous plants including tomato (Lycopersicon esculentum) and eggplant (Solanum melongena). The two predominant SGAs, a-solanine and (6) a-chaconine (7) accumulate in tubers and leaves (Fig. 1). The accumulation of SGAs can influence the quality of tubers in fresh and processed pota- toes by introducing a bitter flavor. SGAs are a potential food safety hazard as the compounds are toxic and can inhi- bit acetylcholinesterase and disrupt cell membranes (Men- singa et al., 2005). Thus safety guidelines establish limits for SGA levels in commercial potato cultivars (Valkonen et al., 1996). As high levels of SGAs hamper breeding efforts to develop new and improved varieties of potatoes, we are exploring the use of transgenic approaches: (a) to under- stand the overall regulation of the biosynthetic pathway 0031-9422/$ - see front matter. Published by Elsevier Ltd. doi:10.1016/j.phytochem.2005.09.037 q Disclaimer: The mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable. * Corresponding author. Tel.: +1 510 559 5796; fax: +1 510 559 5775. E-mail address: kmccue@pw.usda.gov (K.F. McCue). www.elsevier.com/locate/phytochem Phytochemistry 67 (2006) 1590–1597 PHYTOCHEMISTRY