Functional Genomics in Stevia reabudiana

[seetharam annadana]

Functional genomics uncovers three glucosyltransferases
involved in the synthesis of the major sweet glucosides
of Stevia rebaudiana
Alex Richman
†
, Andrew Swanson
†,‡
, Tania Humphrey, Ralph Chapman, Brian McGarvey, Robert Pocs and Jim Brandle
*
Agriculture and Agri-Food Canada, Southern Crop Protection and Food Research Centre, 1391 Sandford St, London, Ontario,
Canada N5V 4T3
Received 20 July 2004; revised 15 September 2004; accepted 24 September 2004.
*
For correspondence (fax þ 1 519 457 3997; e-mail brandleje@agr.gc.ca).
†
These authors contributed equally to this work.
‡
Present address: Department of Biology, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106 7080, USA.
Summary
Stevia rebaudiana leaves accumulate a mixture of at least eight different steviol glycosides. The pattern of glycosylation heavily influences the taste perception of these intensely sweet compounds. The majority of the
glycosides are formed by four glucosylation reactions that start with steviol and end with rebaudioside A. The steps involve the addition of glucose to the C-13 hydroxyl of steviol, the transfer of glucose to the C-2捥湴and C-3 of the 13-O-glucose and the addition of glucose to the hydroxyl of the C-4 carboxyl group. We used our collection of ESTs, an UDP-glucosyltransferase (UGT)-specific electronic probe and key word searches to identify candidate genes resident in our collection. Fifty-four expressed sequence tags (ESTs) belonging to 17 clusters were found using this procedure. We isolated full length cDNAs for 12 of the UGTs, cloned them into an expression vector, and produced recombinant enzymes in Escherichia coli. An in vitro glucosyltransferase activity enzyme assay was conducted using quercetin, kaempferol, steviol, steviolmonoside, steviolbioside, and stevioside as sugar acceptors, and
14 C-UDP-glucose as the donor. Thin layer chromatography was used to
separate the products and three of the recombinant enzymes produced labelled products that co-migrated with known standards. HPLC and LC-ES/MS were then used to further define those reaction products. We
determined that steviol UGTs behave in a regioselective manner and propose a modified pathway for the synthesis of rebaudioside A from steviol.