Agriculture Reference
In-Depth Information
15.4
Carbohydrate Metabolism
in Stored Potatoes
Starch synthesis and storage occur in amy-
loplasts. Glucose-
6-
phosphate (G-
6-
P) is trans-
ported from the cytosol into amyloplasts via a
transmembrane transport protein. Once inside,
the enzyme, plastidic phosphoglucomutase
(pPGM), converts G-
6-
P into glucose-
1-
phosphate
(G-
1-
P). ADP-glucose pyrophosphorylase (AG-
Pase) then converts G-
1-
P to the immediate sub-
strate for starch synthesis, ADP-glucose. The
reaction catalyzed by AGPase requires ATP
imported from the cytosol through the ATP/
ADP translocator, and both the enzyme and the
transporter exert strong control over the rate of
starch synthesis (Sowokinos, 1976; Tjaden
et al
.,
1998; Geigenberger, 2003; Geigenberger
et al
.,
2004). Starch, which is composed of branched
amylopectin and linear amylose domains, is syn-
thesized by several enzymes including granule
bound and soluble starch synthases (SS), starch
branching enzymes (SBE), and debranching en-
zymes (Takaha
et al
., 1993; Larsson
et al
., 1996;
Lloyd
et al
., 2004; Zeeman
et al
., 2007).
The details of starch breakdown in potato
tuber amyloplasts are poorly understood (Smith
et al
., 2005; Rathore
et al
., 2009; Zeeman
et al
.,
2010). There is evidence for two enzymatic
routes of breakdown: phosphorolytic, using starch
phosphorylase (SP), and hydrolytic, using amylase.
The major value of potatoes as a food stock is
due largely to the tuber's ability to accumulate
massive amounts of starch. The typical tuber is
15-
19% starch by weight. Starch is a dynamic
compound, and during the storage period, it is
broken down to sugars that are consumed by
respiration. Starch contributes to pools of sugar
and sugar phosphates in the cytosol and vacuole,
which are used for the synthesis of carbon-
containing compounds such as suberin and
additional starch. Carbohydrate metabolism as it
relates to the accumulation of reducing sugars is
a critical determinant of stored tuber quality for
chip and fry processing.
A simplified diagram illustrating carbohy-
drate metabolism in potato tubers is shown in
Fig. 15.1
. Indicated in the diagram are four sub-
cellular locations in which key metabolic activ-
ities take place: the amyloplast, cytosol, vacuole,
and mitochondria. Carbon intermediates are
shuttled between these locations, where enzyme
activities, specific to each, contribute to overall
changes in starch content, sugar content, and
respiration rate observed at the level of whole
tubers.
Fig. 15.1.
Simplified diagram of carbohydrate metabolism in potato tubers showing key enzymes and
metabolites in the amyloplast, cytosol, apoplast, mitochondria, and vacuole. See text for details and
explanation of abbreviations.