The aim of this work was to use tubers from transgenic lines of potato (Solanum tuberosum) containing increased amounts of ADPglucose pyrophosphorylase to study the role of this enzyme in the control of starch synthesis. A 4–5-fold increase in activity of the enzyme, achieved by transformation with the Escherichia coli ADPglucose pyrophosphorylase gene glgC-16, had no detectable effect on the starch content of developing or mature tubers. No significant effects were found on the contents of ADPglucose, UDPglucose, glucose 1-phosphate, glucose 6-phosphate, PPi, ATP and ADP. Flux from [U-14C]sucrose, supplied to tubers still attached to the plant, to starch increased roughly in proportion to the increase in ADPglucose pyrophosphorylase activity. These measurements of flux gave a response coefficient close to 1 for the activity of the pyrophosphorylase in respect of starch synthesis. Pulse–chase experiments with [U-14C]sucrose showed that the increased flux into starch in the transformed tubers was accompanied by an increased rate of starch turnover. Further experiments suggested that the increased turnover was associated with an increase in the capacity of the tubers to degrade starch.

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