Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

400 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
by showing phosphofructokinase from two potato cultivars differing in their temperature
responses and their difference in susceptibility to CIS. Consequently, at low temperatures,
glycolysis is restricted and thereby decreases the amount of glucose that enters the respiratory
pathway. Burrell et al. (1994) concluded that respiration in potato is not limited by
phosphofructokinase. This was based on results obtained from transgenic tubers expressing
Escherichia coli pfkA gene with a patatin promoter. The theory of cold-labile enzymatic
steps in glycolysis favors the reducing sugar accumulation, when tubers were kept under
4 ◦ C did not find enough evidence (Hill et al., 1996). Malone et al. (2006) concluded that
cold lability of glycolytic enzyme phosphofructokinase is not a major factor in sugar accumulation
at cold temperature. The authors hypothesized that repression of glycolysis
and carbohydrate oxidation has no role in the accumulation of hexose phosphates because
there is no preferential inhibition of recycling of triose and hexose phosphates at low
temperature.
19.4.3 Sucrose phosphate synthase
Another possible way of accumulating reducing sugar is by increased activity of sucrose
synthetic enzymes. SPS is involved in synthesis of sucrose-6-phosphate from UDP-glucose.
This reaction is followed immediately by dephosphorylation to form free sucrose. Reimholz
et al. (1997) showed a novel form of SPS enzyme involved in the CIS process. The time
course and temperature dependence of the appearance of this novel form of SPS correlates
with sugar accumulation (Deiting et al., 1998). Krause et al. (1998) studied the SPS role using
antisense and cosuppression techniques, and concluded that increased sucrose production
at low temperature is because of changes in the kinetic properties of SPS, rather than an
increase in the catalytic capacity. A 70–80% reduction in the SPS expression using the
above-mentioned methods resulted in only a 10–40% decrease of soluble sugars in tubers
stored at cold temperature.
By analyzing isozymes of targeted genes in greater detail, specific isoforms of SPS
and a novel isoform of β-amylase (debranching enzyme) have been identified (Hill et al.,
1996; Nielsen et al., 1997). This novel isoform β-amylase was isolated from the cultivar
Desiŕee induced in tubers when transferred to cold (Nielsen et al., 1997). The time course
and temperature dependence of induction of this β-amylase in potato tuber is similar to CIS
(Deiting et al., 1998).
19.4.4 ADP-glucose pyrophosphorylase
The synthesis of starch in plant cells begins with the enzyme ADP-glucose pyrophosphorylase
(AGPase), which catalyzes the reaction of glucose-1-phosphate with ATP to form
ADP-glucose (liberating pyrophosphate) (Fu et al., 1998). The ADP-glucose is a substrate
for starch synthase enzymes, which add glucose units to the end of a growing polymer
chain to build up a starch molecule (releasing the ADP). Using the antisense approach,
the importance of the AGPase role in starch biosynthesis is understood (Knutzon et al.,
1992). Overexpressing AGPase resulted in an increase in starch content in tubers (Stark
et al., 1992; Ballicora et al., 1995). An increase in the rate of starch biosynthesis resulted in
increased the capacity of the tubers to degrade starch as measured by [U-14C] sucrose. It