266 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
Given that some of the changes observed on alter-
ation of a single isoform of starch synthase were so
dramatic, several groups have looked into the effects
of simultaneously modifying the activities of more
than one isoform by the generation of chimeric anti-
sense constructs. When potato tubers were produced
in which the activities of SSII and SSIII were simul-
taneously repressed (Edwards et al. 1999, Lloyd et
al. 1999b), the effects were not additive. The amy-
lopectin from these lines was somewhat different
from that resulting from inhibition of the single iso-
forms, with the double antisense plants exhibiting
grossly modified amylopectin consisting of more
short and extra long chains but fewer medium length
chains, leading to a gross alteration in the structure
of the starch granules. In another study, the parallel
reduction of GBSSI, SSII, and SSIII resulted in
starch with less amylose and shorter amylopectin
chains, which conferred additional freeze-thaw sta-
bility of starch with respect to the wild type (Jobling
et al. 2002). This is beneficial since it replaces the
necessity for expensive chemical substitution reac-
tions, and in addition, its production may require
less energy, as this starch cooks at much lower tem-
peratures than normal potato starches.
Simultaneously inhibiting two isoforms of starch-
branching enzyme to below 1% of the wild-type
activities resulted in highly modified starch in potato
tubers. In this starch, normal, high molecular weight
amylopectin was absent, whereas the amylose con-
tent was increased to levels above 70%, comparable
to that in the highest commercially available maize
starches (Schwall et al. 2000; Table 11.2). There was
also a major effect on starch granule morphology. In
addition, the phosphorus content of the starch was
increased more than five-fold. This unique starch,
with its high amylose, low amylopectin, and high
phosphorus levels, offers novel properties for food
and industrial applications. A further example of
altered phosphate starch is provided by research into
potato plants deficient in GWD (previously known
as R1, Lorberth et al. 1998, Ritte et al. 2002). The
importance of starch phosphorylation becomes clear
when it is considered that starch phosphate mon-
oesters increase the clarity and viscosity of starch
pastes and decrease the gelatinization and retrogra-
dation rate. Interestingly, crops that produce high-
amylose starches are characterized by considerably
lower yield (Jobling 2004). Additional transgenic or
breeding approaches, as discussed in the previous
section, will be required to minimize the yield pen-
alty that goes along with the manipulation of crops
for altered starch structure and functionality.
As discussed above for starch yield, the impor-
tance of starch degradative processes in determina-
tion of starch structural properties is currently poor-
ly understood. However, it represents an interesting
avenue for further research.CONCLUSIONS AND FUTURE
PERSPECTIVESIn this chapter we have reviewed the pathway organ-
ization of starch synthesis within the potato tuber
and detailed how it can be modulated through trans-
genesis to result in higher starch yield or the produc-
tion of starches of modified structure. While several
successful examples exist for both types of manipu-
lation, these are yet to reach the field. It is likely that
such crops, once commercially produced, will yield
both industrial and nutritional benefits to society. In
addition to this, other genetic and biochemical fac-
tors may well also influence starch yield and struc-
ture, and further research is required in this area in
order to fully optimize these parameters in crop spe-
cies. In this context, transgenic approaches comple-
mentary to conventional breeding will allow more
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