Journal of Biotechnology 96 (2002) 119 – 124 Potato tubers as bioreactors for palatinose production Frederik Bo ¨ rnke *, Mohammad Hajirezaei, Uwe Sonnewald Institut fu ¨r Pflanzengenetik und Kulturpflanzenforschung (IPK), Corrensstrasse 3, 06466 Gatersleben, Germany Received 5 July 2001; accepted 2 November 2001 Abstract Palatinose (isomaltulose, 6-O--D-glucopyranosyl-D-fructose) is a structural isomer of sucrose with very similar physico-chemical properties. Due to its non-cariogenicity and low calorific value it is an ideal sugar substitute for use in food production. Palatinose is produced on an industrial scale from sucrose by an enzymatic rearrangement using immobilized bacterial cells. To explore the potential of transgenic plants as alternative production facilities for palatinose, a chimeric sucrose isomerase gene from Erwinia rhapontici under control of a tuber-specific promoter was introduced into potato plants. The enzyme catalyses the conversion of sucrose into palatinose. Expression of the palI gene within the apoplast of transgenic tubers led to a nearly quantitative conversion of sucrose into palatinose. Despite the soluble carbohydrates having been altered within the tubers, growth of palI expressing transgenic potato plants was indistinguishable from wild type plants. Therefore, expression of a bacterial sucrose isomerase provides a valid tool for high level palatinose production in storage tissues of transgenic crop plants. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Agricultural biology; Metabolic engineering; Transgenic potato; Sucrose isomerase; Palatinose www.elsevier.com/locate/jbiotec 1. Introduction Sucrose is a unique substance of great versatil- ity, possessing not only in itself an exceptional combination of organoleptic and related proper- ties but also being able to contribute significantly to the properties of manufactured foods such as confectionery, jams, cakes and biscuits. Therefore, it is the main sweetener for use in food produc- tion. However, its high calorific value and caries- inducing properties have evoked a shift in consumer preference towards alternative sweeten- ers. A number of synthetic sugar substitutes have been introduced to the market and still novel compounds emerge. Sucrose substitutes vary greatly in their degree of sweetness, volume, tex- ture and stability under various conditions; and no sweetener is a perfect replacement for sucrose in all applications. The main limitations of these sweeteners are concerned with their taste profiles. They distinguish themselves from sucrose not only by having a bitter aftertaste, e.g. saccharin, more- over, they were suspected of provoking cancer when consumed in high amounts over long peri- ods (Price et al., 1970). Although, evidence for a carcinogenic effect is yet decidedly meager, e.g. * Corresponding author. Tel.: +49-3-9482-5490; fax: +49- 3-9482-5515. E-mail address: boernke@ipk-gatersleben.de (F. Bo ¨ rnke). 0168-1656/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0168-1656(02)00042-1