Potato hexokinase 2 complements transgenic Arabidopsis plants deficient in hexokinase 1 but does not play a key role in tuber carbohydrate metabolism

Potato plants (Solanum tuberosum L. cv. Desiree) transformed with sense and antisense constructs of a cDNA encoding the potato hexokinase 2 exhibited altered enzyme activities and expression of hexokinase 2 mRNA. Measurements of the maximum catalytic activity of hexokinase revealed an 11-fold variation in leaf (from 48% of the wild-type activity in antisense transformants to 446% activity in sense transformants) and an 8-fold variation in developing tubers (from 35% of the wild-type activity in antisense transformants to 212% activity in sense transformants). Despite the wide range of hexokinase activities, no substantial change was found in the fresh weight yield, starch, sugar and metabolite levels of transgenic tubers. However, both potato hexokinases 1 and 2 were able to complement the hyposensitivity of antisense hexokinase 1 Arabidopsis transgenic plants to glucose. In an in vitro bioassay of seed germination in a medium with high glucose levels, double transformants showed the same sensitivity to glucose as that of the wild-type ecotype, displaying a stunted phenotype in hypocotyls, cotyledons and roots.