BLBS102-c32 BLBS102-Simpson March 21, 2012 14:2 Trim: 276mm X 219mm Printer Name: Yet to Come
622 Part 5: Fruits, Vegetables, and Cerealsproduce smaller starch grains than the wild type (Muller-R ̈ ober ̈
et al. 1992; see also later discussion and Table 32.2). To date, one
of the most successful approaches for elevating starch accumula-
tion in tubers was that of Stark et al. (1991), who overexpressed
an unregulated bacterial AGPase. This manipulation resulted in
up to a 30% increase in tuber starch content. However, it should
be noted that the expression of exactly the same enzyme within
a second potato cultivar did not significantly affect starch lev-
els (Sweetlove et al. 1996), indicating that these results might
be highly context dependent. Another more promising route to
increase starch yield would be to manipulate the regulatory net-
work leading to posttranslational redox activation of AGPase
(see Fig. 32.4). Based on future progress in this field, direct
strategies can be taken to modify the regulatory components
leading to redox regulation of starch synthesis in potato tubers.
More recent studies have shown that the adenylate supply to
the plastid is of fundamental importance to starch biosynthesis
in potato tubers (Loef et al. 2001, Tjaden et al. 1998). Over-
expression of the plastidial ATP/ADP translocator resulted in
increased tuber starch content, whereas antisense inhibition of
the same protein resulted in reduced starch yield, modified tuber
morphology, and altered starch structure (Tjaden et al. 1998).
Furthermore, incubation of tuber discs in adenine resulted in
a considerable increase in cellular adenylate pool sizes and a
consequent increase in the rate of starch synthesis (Loef et al.
2001). The enzyme adenylate kinase (EC 2.7.4.3) interconverts
ATP and AMP into 2 ADP. Because adenylate kinase is involved
in maintaining the levels of the various adenylates at equilibrium,
it represents an interesting target for modulating the adenylate
pools in plants. For this reason, a molecular approach was taken
to downregulate the plastidial isoform of this enzyme by the
antisense technique (Regierer et al. 2002). This manipulation
led to a substantial increase in the levels of all adenylate pools
(including ATP) and, most importantly, to a record increase in
tuber starch content up to 60% above wild type. These results
are particularly striking because this genetic manipulation also
resulted in a dramatic increase in tuber yield during several field
trials of approximately 40% higher than that of the wild type.
When taken in tandem, these results suggest a doubling of starch
yield per plant. In addition to the changes described above, more
moderate increases in starch yield were previously obtained by
targeting enzymes esoteric to the pathway of starch synthesis,
for example, plants impaired in their expression of the sucrose
synthetic enzyme, sucrose phosphate synthase (Geigenberger
et al. 1999b). This enzyme exerts negative control on starch syn-
thesis since it is involved in a futile cycle of sucrose synthesis
and degradation and leads to a decrease in the net rate of sucrose
degradation in potato tubers (Geigenberger et al. 1997).
Although these results are exciting from a biotechnological
perspective and they give clear hints as to how starch synthesis
is coordinated in vivo, they do not currently allow us to es-
tablish the mechanisms by which they operate. It is also clear
that these results, while promising, are unlikely to be the only
way to achieve increases in starch yield. Recent advances in
transgenic technologies now allow the manipulation of multiple
targets in tandem (Fernie et al. 2001), and given that several of
the successful manipulations described above were somewhatunexpected, the possibility that further such examples will be
uncovered in the future cannot be excluded. One obvious future
target would be to reduce the expression levels of the starch
degradative pathway since, as described above, starch content is
clearly a function of the relative activities of the synthetic and
degradative pathways. Despite the fact that a large number of
Arabidopsismutants has now been generated that are deficient
in the pathway of starch degradation, the consequence of such
deficiencies has not been investigated in a crop such as potato
tubers. Furthermore, there are no reports to date of increases in
starch yield in heterotrophic tissues displaying mutations in the
starch degradative pathway.
A further phenomenon in potatoes that relates to starch
metabolism is that of cold-induced sweetening, where the rate of
degradation of starch to reducing sugars is accelerated. As raw
potatoes are sliced and cooked in oil at high temperature, the
accumulated reducing sugars react with free amino acids in the
potato cell, forming unacceptably brown- to black-pigmented
chips or fries via a nonenzymatic, Maillard-type reaction. Pota-
toes yielding these unacceptably colored products are gener-
ally rejected for purchase by the processing plant. If a “cold-
processing potato” (i.e., one which has low sugar content even
in the cold) were available, energy savings would be realized in
potato-growing regions where outside storage temperatures are
cool. In regions where outside temperatures are moderately high,
increased refrigeration costs may occur. This expense would be
offset, however, by removal of the need to purchase dormancy-
prolonging chemicals, by a decreased need for disease control,
and by improvement of long-term tuber quality. Although such
a cold-processing potato is not yet on the market, several manip-
ulations potentially fulfill this criterion, perhaps most impres-
sively the antisense inhibition of GWD, which is involved in the
initiation of starch degradation (Lorberth et al. 1998). Further
examples on this subject are excellently reviewed in a recent
paper by Sowokinos (2001).
While only a limited number of successful manipulations of
starch yield have been reported to date, far more successful ma-
nipulations have been reported with respect to engineering starch
structure. These will be reviewed in Section “Manipulation of
Starch Structure.”MANIPULATION OF STARCH
STRUCTUREIn addition to attempting to increase starch yield, there have
been many, arguably more, successful attempts to manipulate
its structural properties. Considerable natural variation exists
between the starch structures of crop species, with potato starch
having larger granules, less amylose, a higher proportion of co-
valently bound phosphate, and less protein and lipid content than
cereal starches. The level of phosphorylation strongly influences
the physical properties of starch, and granule size is another im-
portant factor for many applications; for example, determining
starch noodle processing and quality (Jobling et al. 2004). In ad-
dition, the ratio between the different polymer types can affect
the functionality of different starches (Slattery et al. 1998). High
amylose starches are used in fried snack products to create crisp