BLBS102-c32 BLBS102-Simpson March 21, 2012 14:2 Trim: 276mm X 219mm Printer Name: Yet to Come
616 Part 5: Fruits, Vegetables, and CerealsFigure 32.3.Routes of amylose and amylopectin synthesis within
potato tuber amyloplasts. For simplicity, the possible roles of
starch-degrading enzymes in trimming amylopectin have been
neglected in this scheme.glucose-1-phosphate and ATP to ADP-glucose and inorganic
pyrophosphate. Inorganic pyrophosphate is subsequently me-
tabolized to inorganic phosphate by a highly active inorganic
pyrophosphatase within the plastid. AGPase is generally consid-
ered as the first committed step of starch biosynthesis since it pro-
duces ADP-glucose, the direct precursor for the starch polymer-
izing reactions catalyzed by starch synthase (SS) (EC 2.4.1.21)
and branching enzyme (EC 2.4.1.24; Fig. 32.3). These three en-
zymes appear to be involved in starch synthesis in all species.
With the exceptions of the cereal species described above, which
also have a cytosolic isoform of AGPase, these reactions are
confined to the plastid. The activities of AGPase and SS are
sufficient to account for the rates of starch synthesis in a wide
variety of photosynthetic and heterotrophic tissues (Smith et al.
1997). Furthermore, changes in the activities of these enzymes
correlate with changes in the accumulation of starch during de-
velopment of storage organs (Smith et al. 1995). AGPase from
a range of photosynthetic and heterotrophic tissues is well es-
tablished to be inhibited by phosphate, which induces sigmoidal
kinetics, and to be allosterically activated by 3-phosphoglycerate
(3PGA), which relieves phosphate inhibition (Preiss 1988;
Fig. 32.4). There is clear evidence that allosteric regulation of
AGPase is important in vivo to adjust the rate of starch synthesis
to changes in the rate of respiration that go along with changes in
the levels of 3PGA, and an impressive body of evidence has been
provided that there is a strong correlation between the 3PGA and
ADP-glucose levels and the rate of starch synthesis under a widevariety of environmental conditions (Geigenberger et al. 1998,
Geigenberger 2003a).
More recently, an important physiological role for posttransla-
tional redox regulation of AGPase has been established (Tiessen
et al. 2002). In this case, reduction of an intermolecular cys-
teine bridge between the two small subunits of the heterote-
trameric enzyme leads to a dramatic increase of activity, due to
increased substrate affinities and sensitivity to allosteric activa-
tion by 3PGA (Fig. 32.4). Redox activation of AGPasein planta
correlated closely with the potato tuber sucrose content across a
range of physiological and genetic manipulations (Tiessen et al.
2002), indicating that redox modulation is part of a novel regu-
latory loop that directs incoming sucrose towards storage starch
synthesis (Tiessen et al. 2003). Crucially, it allows the rate of
starch synthesis to be increased in response to sucrose supply and
independently of any increase in metabolite levels (Fig. 32.4),
and it is therefore an interesting target for approaches to improv-
ing starch yield (see later). There are at least two separate sugar
signaling pathways leading to posttranslational redox activation
of AGPase, one involving an SNF1-like protein kinase (SnRK1),
the other involving hexokinase (Tiessen et al. 2003). Both hex-
okinase and SnRK1 have previously been shown to be involved
in the transcriptional regulation of many plant genes. Obviously,
the transduction pathway that regulates the reductive activation
of AGPase in plastids and the regulatory network that controls
the expression of genes in the cytosol share some common com-
ponents. How the sugar signal is transferred into the plastid and
leads to redox changes of AGPase is still unknown, but may
involve interaction of specific thioredoxins with AGPase.
Subcellular analyses of metabolite levels in growing potato
tubers have shown that the reaction catalyzed by AGPase is far
from equilibrium in vivo (Tiessen et al. 2002), and consequently,
the flux through this enzyme is particularly sensitive to regula-
tion by the above-mentioned factors. It is interesting to note that
plants contain multiple forms of AGPase, which is a tetrameric
enzyme comprising two different polypeptides, a small subunit
and a large subunit, both of which are required for full activ-
ity (Preiss 1988). Both subunits are encoded by multiple genes,
which are differentially expressed in different tissues. Although
the precise function of this differential expression is currently
unknown, it seems likely that these isoforms will differ in their
capacity to bind allosteric regulators. If this is indeed the case,
then different combinations of small and large subunits should
show different sensitivity to allosteric regulation—such as those
observed in tissues from cereals.
While there is a wealth of information on the regulation of
the plastidial isoforms of AGPase—the only isoform present in
the potato tuber—very little is known about cytosolic isoforms
in other species. Several important studies provide evidence that
the ADP-glucose produced in the cytosol can be taken up by the
plastid (Sullivan et al. 1991, Shannon et al. 1998). From these
studies and from characterization of mutants unable to transport
ADP-glucose, it would appear that this is a predominant route
for starch synthesis within maize. Despite these findings, the
physiological significance of cytosolic ADP-glucose production
remains unclear for a range of species. However, it has been
calculated that the AGPase activity of the plastid is insufficient