Plant Molecular Biology 49: 491–501, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Potato hexokinase 2 complements transgenic Arabidopsis plants deﬁcient
in hexokinase 1 but does not play a key role in tuber carbohydrate
, Alisdair R. Fernie, Andrea Leisse, Lothar Willmitzer and Richard N.
Max-Planck-Institut für Molekulare Pﬂanzenphysiologie, Am Mühlenberg 1, 14476 Golm, Germany; present ad-
Instituto de Agrobiotecnolog´ıa, Universidad P´ublica de Navarra, Campus Arrosad´ıa, 31006 Pamplona,
author for correspondence; e-mail: firstname.lastname@example.org);
Metanomics GmbH & Co. KGaA, Tegeler Weg
33, 10589 Berlin, Germany
Received 10 May 2001; accepted in revised form 25 November 2001
Key words: Arabidopsis, hexokinase, potato, sugar sensing, tuber
Potato plants (Solanum tuberosum L. cv. Désirée) 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.
Hexokinase (HK, EC 18.104.22.168) catalyses the phos-
phorylation of hexoses to form hexose monophos-
phates. This reaction is especially important in plants
as the use of free hexoses is particularly complex
in higher plants (Kruger, 1997). There have been
many reports on the presence of glucokinase (glucose-
phosphorylating) and hexokinase (capable of phos-
phorylating a range of hexoses) enzymes in a variety
of species including tomato (Martinez-Barajas and
Randall, 1998), maize (Doehlert, 1989; Schnarren-
berger, 1990; Galina et al., 1995), Arabidopsis (Jang
et al., 1997), pea (Turner et al., 1977; Turner and
Copeland, 1981; Schnarrenberger, 1990), avocado
(Copeland and Tanner, 1988), castor bean (Miernyk
and Dennis, 1983), soybean (Copeland and Morrell,
1985) and tobacco (Sindelar et al., 1998). However,
HKs from potato tuber are the most thoroughlycharac-
terized at the biochemical level (Renz and Stitt, 1993;
Renz et al., 1993; Veramendi et al., 1999). Three iso-
forms of HK have been reported within the tuber, all
of which have a high substrate afﬁnity for glucose and
mannose but only a negligible afﬁnity for fructose.
In parallel to its metabolic role within glycolysis,
HK has been widely implicated as a sensor in the
sugar-dependent regulation of gene expression (Gra-
ham et al., 1994; Koch, 1996; Smeekens, 1998, 2000;
Sheen et al., 1999; Gibson, 2000). Furthermore it has
been convincingly demonstrated to act as a glucose
sensor within Saccharomyces cerevisiae,inwhichthe
expression of a large number of genes is regulated