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However, the transport of sucrose from the leaves
towards the stolon end is considered an essential
force promoting tuber initiation and driving sub-
sequent growth. The removal of a growing tuber
from the plant will enhance the tuberization of
other stolon tips, indicating tuber initiation is also
source driven. The
in vitro
potato tuberization sys-
tem, implemented in many tuber development
studies (Hendriks
et al.,
1991; Appeldoorn
et al.,
1997) is characterized by the highly synchronous
formation of microtubers from single nodal stem
cuttings on medium with high sucrose (8% w/v).
The frequency of tuberization is relative to the
amount of sucrose in the medium, suggesting an
inducing role for sucrose in tuber formation (Xu
et al.,
1998a). Although the
in vitro
tuberization
system has been a very useful research tool in
studying early tuber developmental processes, the
resulting microtubers remain small and do not ex-
ceed a certain diameter, usually ~0.8 cm
(Fig. 4.1
).
This lack of further cell division, and the growth
mainly of the perimedullary region, indicate
additional source-signaling molecules are absent
in vitro
, where sucrose availability is not a limiting
factor.
Lowering sucrose transport in potato plants
results in a reduction in tuber formation and
tuber yield (Riesmeier
et al.,
1994; Kuhn
et al.,
2003). Further evidence for a role of sucrose in
tuber initiation comes from the work performed
on sucrose transporters (Chincinska
et al.,
2008).
All three studied sucrose transporters (SUT) from
potato (
S. tuberosum
), SUT1, SUT2, and SUT4,
are co-localized and their RNA levels not only fol-
low a diurnal rhythm but also oscillate in con-
stant light, indicating strong circadian control.
The phenotype of StSUT4-RNAi plants includes
early flowering and higher tuber production. Su-
crose efflux from leaves of transgenic plants was
increased at the end of the light period, leading to
modified sucrose levels in sink organs. Of high
interest was the observation that the inhibition
of StSUT4 expression allowed tuber formation in
the strictly photoperiodic-dependent cultivars
under noninductive LD conditions.
photoassimilates, mainly in the form of sucrose,
from the leaves to the stolon tip is considered
one of the driving forces behind tuber growth
(Zrenner
et al.,
1995) (see also previous para-
graph). The switch from apoplastic towards
symplastic sucrose unloading at tuber organo-
genesis results in increased cellular sucrose
availability (Appeldoorn
et al.,
1997; Viola
et al.,
2001). As a result, carbohydrate metabolism
and sucrose-linked pathways have to adjust
their flux accordingly and are dedicated towards
the synthesis of starch. Sucrose synthase (SUSY)
gene expression is increased strongly and becomes
the primary sugar cleavage pathway increasing
the hexose pool entering the starch biosynthesis
pathway through the generation and subse-
quent import of glucose-
6-
P into the amyloplast
(Tauberger
et al.,
2000).
Within this organelle, plastidic phospho-
glucomutase (PGM), subunits of the ADP-glucose
pyrophosphorylase (AGPase) protein complex,
starch synthases, and several starch-modifying
enzymes (branching enzymes, amylases, phos-
phorylases) all follow an upregulated expres-
sion profile during tuber development in support
of starch accumulation (Geigenberger, 2003;
Kloosterman
et al.,
2005). The conversion of su-
crose to starch is ATP dependent. Tjaden
et al
.
(1998) showed that a relatively small decrease
in ATP/ADP transporter activity led to a reduced
level of total starch content and a lower amylose
to amylopectin ratio. The interplay between su-
crose mobility, breakdown, and starch biosyn-
thesis, together with increasing cell number and
cell size in the tuber, requires the coordinated
expression of many genes, involving many cellu-
lar processes.
A comparison of a relative modern-day
cultivar with a diploid landrace showed a higher
expression of starch breakdown genes in the
landrace, indicating a strong selection for high
starch-containing tubers in the breeding pro-
grams for modern European potato cultivars
(PGSC, 2011). The expression of starch-modifying
enzymes during tuber growth and maturation
can alter starch properties, impacting tuber
quality and providing different industrial uses.
Beside starch, potato storage proteins accumu-
late to high levels in the tuber. Gene expression
profiles for these proteins show strong upregula-
tion throughout all stages of tuber growth and
can reach extremely high levels, particularly
Starch and protein accumulation
The transition of a stolon into a tuber coincides
with a strong upregulation of genes associated
with starch biosynthesis. Transport of these
