264 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
above wild type. These results are particularly strik-
ing 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 addi-
tion 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 synthesis 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 bio-
technological perspective and they give clear hints
as to how starch synthesis is coordinated in vivo,
they do not currently allow us to establish the mech-
anisms 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 somewhat un-
expected, 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 gener-
ated that are deficient in the pathway of starch de-
gradation, the consequence of such deficiencies has
not been investigated in a crop such as potato tubers.
Furthermore, there are no reports to date of increas-
es 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 sweeten-
ing, where the rate of degradation of starch to reduc-
ing sugars is accelerated. As raw potatoes are sliced
and cooked in oil at high temperature, the accumu-
lated 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. Potatoes yielding these un-
acceptably colored products are generally 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 sav-
ings 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 mar-
ket, several manipulations potentially fulfill this cri-
terion, perhaps most impressively the antisense inhi-
bition 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 manip-
ulations of starch yield have been reported to date,
far more successful manipulations have been re-
ported with respect to engineering starch structure.
These will be reviewed in the following section.MANIPULATION OF STARCH
STRUCTUREIn addition to attempting to increase starch yield
there have been many, arguably more, successful at-
tempts to manipulate its structural properties. Con-
siderable natural variation exists between the starch
structures of crop species, with potato starch having
larger granules, less amylose, a higher proportion of
covalently bound phosphate, and less protein and
lipid content than cereal starches. The level of phos-
phorylation strongly influences the physical proper-
ties of starch, and granule size is another important
factor for many applications; for example, determin-
ing starch noodle processing and quality (Jobling et
al. 2004). In addition, the ratio between the different
polymer types can affect the functionality of differ-
ent starches (Slattery et al. 2000). High amylose
starches are used in fried snack products to create
crisp and evenly brown snacks, as gelling agents,
and in photographic films, whereas high amylo-
pectin starches are useful in the food industry (to
improve uniformity, stability, and texture) and in the
paper and adhesive industries. Both, conventional