Antisense repression of hexokinase 1 leads to an overaccumulation of starch in leaves of transgenic potato plants but not to significant changes in tuber carbohydrate metabolism

Potato (Solanum tuberosum L.) plants transformed with sense and antisense c onstructs of a cDNA encoding the potato hexokinase 1 (StHK1) exhibited alte red enzyme activities and expression of StHK1 mRNA. Measurements of the max imum catalytic activity of hexokinase revealed a 22-fold variation in leave s (from 22% of the wild-type activity in antisense transformants to 485% ac tivity in sense transformants) and a 7-fold variation in developing tubers (from 32% of the wild-type activity in antisense transformants to 222% acti vity in sense transformants). Despite the wide range of hexokinase activiti es, no change was found in the fresh weight yield, starch, sugar, or metabo lite levels of transgenic tubers. However, there was a 3-fold increase in t he starch content of leaves from the antisense transformants after the dark period. Starch accumulation at the end of the night period was correlated with a 2-fold increase of glucose and a decrease of sucrose content. These results provide strong support for the hypothesis that glucose is a primary product of transitory starch degradation and is the sugar that is exported to the cytosol at night to support sucrose biosynthesis.