11 Starch Synthesis in the Potato Tuber 255lose, having a molecular weight of 10^7 –10^8 as
opposed to 5 105 –10^6. These molecules can thus
be fractionated by utilizing differences in their mo-
lecular size as well as in their binding behavior.
Moreover, amylose is able to complex lipids, while
amylopectin can contain covalently bound phos-
phate, adding further complexity to their structure.
In nature, amylose normally accounts between 20
and 30% of the total starch. However, the percentage
of amylose depends on the species and the organ
used for starch storage. The proportion of amylose
to amylopectin and the size and structure of the
starch grain give distinct properties to different
extracted starches (properties important in food and
industrial purposes; Dennis and Blakeley 2000).
Starch grain size is also dependent on species and
organ type. It is well established that starch grains
grow by adding layers, and growth rings within the
grain may represent areas of fast and slower growth
(Pilling and Smith 2003); however, very little is
known about how these highly ordered structures
are formed in vivo. The interested reader is referred
to articles by Buleon et al. (1998) and Kossmann
and Lloyd (2000).
ROUTES OF STARCH SYNTHESIS
AND DEGRADATION AND THEIR
REGULATION
With the possible exception of sucrose, starch is the
most important metabolite of plant carbohydrate me-
tabolism. It is by far the most dominant storage poly-
saccharide and is present in all major organs of most
plants, in some instances at very high levels. Due to
the high likelihood of starch turnover, its metabo-
lism is best considered as the balance between the
antagonistic operation of pathways of synthesis and
degradation.
To investigate the regulation of starch synthesis in
more detail, growing potato tubers have been used as
a model system. Unlike many other tissues the entry
of sucrose into metabolism is relatively simple, in
that it is unloaded symplasmically from the phloem,
degraded via sucrose synthase to fructose and UDP-
glucose, which are converted to hexose monophos-
phates by fructokinase and UDPglucose pyrophos-
phorylase, respectively (Geigenberger 2003a). In
contrast to sucrose degradation, which is localized
in the cytosol, starch is synthesized predominantly, if
not exclusively, in the plastid. The precise pathway
of starch synthesis depends on the form in which
carbon crosses the amyloplast membrane (Fig.
11.2). This varies between species and has been the
subject of considerable debate (Keeling et al. 1998,
Hatzfeld and Stitt 1990, Tauberger et al. 2000).
Categorical evidence that carbon enters potato tuber,
Chenopodium rubrumsuspension cell, maize endo-
sperm, and wheat endosperm amyloplasts in the
form of hexose monophosphates rather than triose
phosphates was provided by determination of the
degree of randomization of radiolabel in glucose
units isolated from starch following incubation of
the various tissues with glucose labeled at the C1 or
C6 positions (Keeling et al. 1988, Hatzfeld and Stitt
1990). The cloning of a hexose monophosphate
transporter from potato and the finding that the cau-
liflower homolog is highly specific for glucose-6-
phosphate provides strong support for this theory
(Kammerer et al. 1998). Further evidence in support
of glucose-6-phosphate import was provided by
studies of transgenic potato lines in which the activ-
ity of the plastidial isoform of phosphoglucomutase
was reduced by antisense inhibition, leading to a
large reduction in starch content of the tubers (Tau-
berger et al. 2000). These data are in agreement with
the observations that heterotrophic tissues lack plas-
tidial fructose-1,6-bisphosphatase expression and
activity (Entwistle and Rees 1990, Kossmann et al.
1992). The results of recent transgenic and immuno-
localization experiments have indicated that the sub-
strate for uptake is most probably species specific,
with clear evidence for the predominant route of
uptake in the developing potato tuber being in the
form of glucose-6-phosphate. By contrast, in barley,
wheat, oat, and possibly maize the predominant form
of uptake, at least during early stages of seed endo-
sperm development, is as ADP-glucose (Neuhaus
and Emes 2000).
Irrespective of the route of carbon import, ADP-
glucose pyrophosphorylase (AGPase, EC 2.7.7.27)
plays an important role in starch synthesis, catalyz-
ing the conversion of glucose-1-phosphate and ATP
to ADP-glucose and inorganic pyrophosphate. Inor-
ganic pyrophosphate is subsequently metabolized to
inorganic phosphate by a highly active inorganic
pyrophosphatase within the plastid. AGPase is gen-
erally considered as the first committed step of starch
biosynthesis since it produces ADP-glucose, the di-
rect precursor for the starch polymerizing reactions
catalyzed by starch synthase (EC 2.4.1.21) and
branching enzyme (EC 2.4.1.24; Fig. 11.3). These
three enzymes appear to be involved in starch