RAPID COMMUNICATION Lee J. Sweetlove á Nicholas J. Kruger á Steven A. Hill Starch synthesis in transgenic potato tubers with increased 3-phosphoglyceric acid content as a consequence of increased 6-phosphofructokinase activity Received: 22 August 2000 /Accepted: 19 December 2000 / Published online: 31 March 2001 Ó Springer-Verlag 2001 Abstract The aim of this work was to test the hypothesis that changes in cytosolic 3-phosphoglyceric acid 3-PGA) content can regulate the rate of starch synthesis in potato Solanum tuberosum L.) tubers. The amount of 3-PGA was increased by expressing bacterial phospho- fructokinase PFK; EC 2.7.1.11) in transgenic potato tubers. The resultant 3-fold increase in PFK activity was accompanied by an increase in metabolites downstream of PFK, including a 3-fold increase in 3-PGA. There was also a decrease in metabolites upstream of PFK, most notably of glucose-6-phosphate. The increase in 3-PGA did not aect the amount of starch that accumulated in developing tubers, nor its rate of synthesis in tuber discs cut from developing tubers. This suggests that changes in cytosolic 3-PGA may not aect the rate of starch synthesis under all circumstances. We propose that in this case, a decrease in glucose-6-phosphate which is transported into the amyloplast as a substrate for starch synthesis) may be sucient to counteract the eect of increased 3-PGA. Keywords ADPglucose pyrophosphorylase á 3-Phosphoglyceric acid á 6-Phosphofructokinase á Solanum starch synthesis) á Starch synthesis á Transgenic potato Abbreviations AGPase: ADPglucose pyrophosphory- lase á PFK: 6-phosphofructokinase á 3-PGA: 3-phos- phoglyceric acid á WT: wild type Introduction The regulation and control of the rate of starch synthesis in non-photosynthetic tissues has been a subject of much debate, a key issue being the extent to which the enzyme ADPglucose pyrophosphorylase AGPase) dominates the control of this ¯ux. It is recognised that AGPase has the potential to be a site of regulation of ¯ux it is allosterically activated by 3-phosphoglyceric acid 3-PGA) and inhibited by inorganic phosphate) and indeedtheseregulatoryfeatureshavebeenshowntobeof physiological importance in regulating starch synthesis in photosynthetic tissues Preiss 1988). Furthermore, AGPase has a high ¯ux control coecient over photo- synthetic starch synthesis, indicating that small changes in the amount of the AGPase protein have a signi®cant eect on ¯ux Neuhaus and Stitt 1990). The case for AGPase having a similar key role in the regulation and control of starch synthesis in non-photosynthetic tissues has been largely based on the assumption that the reg- ulatory properties of the enzyme are also physiologically relevant in such tissues. Furthermore, it has often been erroneously) assumed that the regulatory properties of AGPase would automatically confer upon it a high ¯ux control coecient. The idea that AGPase exerts considerable control over the rate of starch synthesis in non-photosynthetic tissues has recently gained experimental support by the measurement of a high ¯ux control coecient of the enzyme in potato tuber discs Sweetlove et al. 1999). Additionally, studies of the eect of environmental perturbations on the metabolism of potato tuber discs have provided circumstantial evidence that AGPase may also be a site of regulation of the rate of starch synthesis i.e. maintaining a homeostatic balance be- tween starch synthesis and the rest of metabolism). For example, during moderate water stress there is an in- crease in sucrose synthesis to raise the osmotic poten- tial) and a balancing decrease in the rate of starch synthesis. The latter correlates with a decrease in the 3-PGA content Geigenberger et al. 1997). Similarly, the inhibition of starch synthesis that occurs at high tem- peratures is accompanied by a decrease in 3-PGA levels Geigenberger et al. 1998). It is argued that these changes in 3-PGA occur in the cytosol as a result of a Planta 2001) 213: 478±482 DOI 10.1007/s004250100544 L.J. Sweetlove &) á N.J. Kruger á S.A. Hill Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, UK E-mail: lee.sweetlove@plants.ox.ac.uk Fax: +44-1865-275074