Planta (2006) 224:1495–1502 DOI 10.1007/s00425-006-0387-9 123 RAPID COMMUNICATION Evidence for intracellular spatial separation of hexokinases and fructokinases in tomato plants Hila Damari-Weissler · Michal Kandel-KWr · David Gidoni · Anahit Mett · Eddy Belausov · David Granot Received: 23 July 2006 / Accepted: 17 August 2006 / Published online: 15 September 2006  Springer-Verlag 2006 Abstract Four hexokinase (LeHXK1–4) and four fructokinase (LeFRK1–4) genes were identiWed in tomato plants. Previous GFP fusion studies indicate that the gene product of LeHXK3 is associated with the mitochondria while that of LeHXK4 is located within plastids. In this study we found that the enzyme encoded by the fructokinase gene LeFRK3 is also located within plastids. The presence of LeFrk3 enzyme in plastids raises the question of the origin of fructose in these organelles. The other three FRKs enzymes, LeFrk1&2&4, are located in the cytosol. Unlike LeFrk1&2&4, the two additional HXKs, LeHxk1&2, share a common membrane anchor domain and are associated with the mitochondria simi- lar to LeHxk3. The diVerence in the locations of the cytoplasmic FRK and HXK isozymes suggests that glu- cose phosphorylation is conWned to deWned special intracellular localizations while fructose phosphoryla- tion is less conWned. Keywords Fructokinase · GFP · Hexokinase · Hexose phosphorylation · Intracellular localization · Tomato (Lycopersicon esculentum) Abbreviations FRK Fructokinase GFP Green Xuorescent protein HXK Hexokinase Introduction Sugars regulate major metabolic and developmental pathways in plants. In recent years, it has been sug- gested that sugar phosphorylation by hexose phosphor- ylating enzymes mediate sugar-sensing processes in plants, and it has been shown that Arabidopsis AtH- XK1 mediates sugar sensing in Arabidopsis and tomato (Jang et al. 1997; Dai et al. 1999). Plants have several hexose phosphorylating enzymes encoded by indepen- dent genes whose speciWc roles in sugar sensing remain to be elucidated. Hexose phosphorylating enzymes are characterized by their aYnities to various sugars. Hexokinases (HXKs) phosphorylate glucose and fruc- tose, but their aYnity for glucose is usually two orders of magnitude higher than for fructose (Menu et al. 2001; Dai et al. 2002b; Kandel-KWr et al. 2006). Fructo- kinases (FRKs) have high aYnities for fructose, and so phosphorylate only fructose (Renz and Stitt 1993; Kanayama et al. 1997, 1998; German et al. 2004). HXKs and FRKs are the only glucose and fructose phosphorylating enzymes that have been identiWed in plants (Dai et al. 2002b). A few HXK genes have been identiWed in various plant species (Veramendi et al. 1999, 2002; Giese et al. 2005; Cho et al. 2006). In tomato, four HXK genes, LeHXK1–4, were identiWed (Menu et al. 2001; Dai et al. 2002b; Kandel-KWr et al. 2006). Two of these, LeHXK1 and LeHXK4, are the major HXK genes expressed in most tissues, including Contribution from the Agriculture Research Organization, The Volcani Center, Bet Dagan, Israel, No. 126/2006 series. H. Damari-Weissler · M. Kandel-KWr · D. Gidoni · A. Mett · E. Belausov · D. Granot (&) Institute of Plant Sciences, Agricultural Research Organization, The Volcani Center, Bet Dagan, 50250, Israel e-mail: granot@agri.gov.il