Postharvest Storage and Physiology - The Potato: Botany, Production and Uses - page 260

Starch breakdown is promoted by the R1 enzyme,
an alpha-glucan water dikinase that phosphor-
ylates amylopectin at the C6 position (Ritte et al .,
2002). Transgenic plants with reduced amounts
of R1 mRNA are less prone to sugar accumulation
at low temperatures, and these data support the
involvement of R1 in starch breakdown during
CIS (Rommens et al ., 2006). Starch granules are
degraded into branched and linear polyglucans
by enzymes that have not been identified clearly
in potato tubers. Branched polymers are reduced
to linear glucans by debranching enzymes and
limit-dextrinase. The linear glucans produced
may be degraded into neutral sugars by L-type
starch phosphorylase (Smith et al ., 2005). This en-
zyme catalyzes the reversible conversion of alpha- 1,
4- glucan and inorganic phosphate into G- 1- P.
A second route for starch degradation likely
involves hydrolysis by amylases. Starch-associated
alpha-amylase activity has been identified in
potato tubers, and the isolated enzyme, alpha-
amylase, was able to hydrolyze potato starch
(Witt and Sauter, 1996). Since alpha-amylase
produces small oligomers of glucose, it is likely
that beta-amylase and alpha-glucosidase par-
ticipate in the complete breakdown of starch
to simple sugars (Cochrane et al ., 1991; Nielsen
et al ., 1997). The activity of beta-amylase in
tubers has been shown to increase as storage
temperature declines (Cochrane et al ., 1991;
Nielsen et al ., 1997).
The identity of the metabolites transported
out of amyloplasts during starch degradation is
unknown, but maltose and glucose are likely can-
didates based on research done primarily with
chloroplasts (Zeeman et al ., 2007; Rathore et al .,
2009). Once in the cytosol, maltose can be con-
verted to G- 6- P by a glucosyltransferase and hex-
okinase, or to G- 1- P by cytosolic H-type phos-
phorylase (Rathore et al ., 2009). Cytosolic
phosphoglucomutase (cPGM) readily converts G- 6- P
to G- 1- P, and phosphoglucoisomerase (PGI) con-
verts G- 6- P to fructose- 6- phosphate (F- 6- P). Both
reactions are freely reversible. Cytosolic G- 1- P, G- 6- P,
and F- 6- P constitute the hexose-phosphate pool and
are used to supply carbon for numerous biochemical
reactions and to provide substrates for respiration.

tubers, and the enzymes responsible for sucrose
synthesis have been identified (Sowokinos,
2001b). The most important is UDP-glucose py-
rophosphorylase (UGPase), which synthesizes
UDP-glucose and pyrophosphatase (PPi) from
UTP and G- 1- P. Isoenzymes of UGPase with differ-
ent enzymatic properties have been character-
ized (Sowokinos, 2001a; Gupta and Sowokinos,
2003). Those with reduced affinity (higher K m )
for G- 1- P form sucrose less readily in stored
tubers than those with greater affinity. In many
potato cultivars, favorable isoenzymes of UGPase
contribute to the maintenance of low sucrose
and reducing sugars during storage (Sowokinos,
2001a).
Sucrose is synthesized in the cytosol by the
combined activities of sucrose phosphate syn-
thase (SPS), which produces sucrose- 6- phosphate
(Suc- 6- P) from UDP-glucose and F- 6- P, and
sucrose phosphate phosphatase (SPP), which
dephosphorylates sucrose- 6- phosphate to sucrose.
Sucrose is partitioned between the cytosol
and vacuole. Transport through the tonoplast
is likely to be facilitated by a sucrose-H + anti-
porter. In two reports, the concentration of su-
crose in the cytosol of developing potato tubers
was found to be approximately equal to that in
the vacuole (Farre et al ., 2001, 2008). Compar-
able data are not available for mature tubers.
Vacuolar acid invertase (VINV) cleaves sucrose
into the reducing sugars, glucose and fructose,
in a non-reversible reaction. The expression of
the vacuolar acid invertase gene and activity of
the vacuolar acid invertase enzyme are strongly
upregulated by low-temperature storage of
tubers (Pressey and Shaw, 1966; Matsuura-
Endo et al ., 2004; Bhaskar et al ., 2010). Rates
of invertase activity vary widely between culti-
vars (Matsuura-Endo et al ., 2004; McKenzie
et al ., 2005).
When invertase activity has been strongly
reduced through biotechnology, dramatic re-
ductions in reducing sugar accumulation have
been observed in tubers stored at low temperat-
ures (Bhaskar et al ., 2010; Ye et al ., 2010; Liu
et al ., 2011; Wu et al ., 2011). Hence, vacuolar
acid invertase is a key determinant of reducing
sugar accumulation during low-temperature
storage. Interestingly, in lines of potato with very
low invertase activity, sucrose accumulation
depended on the UGPase isoforms present (Wu
et al ., 2011). In cultivars containing A-II isoforms,
such as Snowden, Dakota Pearl, and Atlantic,

Sucrose and invertase activity

Sucrose formation is the first committed step in
the accumulation of reducing sugars by potato