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formation may be even higher. The identification
of several RNA-binding proteins differentially
present in potato petioles on transfer from LD to
SD conditions can provide important evidence
in understanding transport of StBEL5 , POTH1
and other genes in relation to photoperiodic-
dependent tuberization (Banerjee et  al., 2009;
Shah et  al., 2011). Within the identified tuber
signaling pathways, a challenging task now
awaits to establish the individual importance
of the associated genes and their relative position
within the signaling network. Generation of
complete knockouts for all of these genes should
provide more clear evidence as to whether they
are crucial for inducing tuber formation, have
redundant roles, or merely have a supportive
function. The clear link with circadian clock
control for several of these signaling genes
warrants further analysis of circadian rhythm
genes and output of signalling, or so-called
“gating”. Modern potato cultivars have lost the
strict photoperiodic control with respect to tuber
formation; however, the signaling components
(StCDF1, StSP6A, StBEL5, POTH1 miR172 ) re-
main under strict control of the circadian clock
to regulate seasonal timing of phase transitions
such as tuber formation.
dramatically prior to first visible swelling. Shiba-
oka showed that GA 3 might be involved in the
orientation of the microtubules and microfibrils
in plant cells (Shibaoka, 1993). It was hypothe-
sized that a reduction of GA in inducing condi-
tions, as found in stolon tips, might cause the
reorientation of the plane of cell expansion and
cell division, resulting in subapical swelling of
the stolon (Fujino et al., 1995; Xu et al., 1998a).
StGA2ox1 catabolizes bioactive GAs and/or GA
precursors, and its expression is strongly in-
duced prior to first visible swelling. Tubers of
StGA2ox1 suppression clones displayed occa-
sionally elongated tuber shapes indicative of
incomplete reorientation of the plane of cell
divisions (Kloosterman et  al., 2007) ( Fig. 4.4 ).
35S overexpression plants exhibit a dwarf
phenotype, reduced stolon growth, and earlier in
vitro tuberization.
The expression of StGA3ox2 , involved in
the last biosynthesis step towards bioactive GAs
(GA 1 , GA 4 ), is downregulated in the stolon on
tuber induction (Bou-Torrent et al., 2011). Both
actions, the upregulation of GA breakdown
( StGA2ox1 ) and decrease of GA biosynthesis
( StGA3ox2 ), allow for a rapid reduction of GA
in  the stolon tip. Overexpression of StGA3ox2
under control of constitutive or leaf-specific pro-
moters results in taller plants, which, surprisingly,
tuberize earlier under SD conditions than the con-
trols; whereas StGA3ox2 tuber-specific overex-
pression results in non-elongated plants with
slightly delayed tuber induction (Bou-Torrent
et al ., 2011). From these studies, it is interesting
to speculate that the differential mobility of GA
precursors and bioactive GAs may be important
in the control of tuberization. GA 20 can be trans-
ported readily throughout the plant, whereas
GA 1 preferentially remains in the vicinity of the
cells where it is produced, so that it exerts its ac-
tion mainly in these and directly adjacent cells
(Prat, 2010) ( Fig. 4.5 ). Independent evidence for
differential mobility of these two GA molecules
has been shown through grafting experiments
in pea (Proebsting et  al., 1992). Increase of po-
tato GA 3- oxidase activity through StGA3ox2
overexpression would lead to an increase in
the rate of GA 20 to GA 1 conversion in the aerial
tissues, resulting in taller plants but leaving
less GA 20 available for transport towards the sto-
lons. Consequently, GA levels in the stolon are
reduced in the overexpressing plants, thereby
4.5
Hormonal Regulation of Potato
Tuberization
Gibberellic acid (GA)
The plant hormone, gibberellic acid (GA), has
long been implicated to play a key role in tuber
formation inhibition (Booth, 1963; Kumar and
Wareing, 1974; Ewing, 1987). GAs are cyclic di-
terpenoid hormones that regulate many plant
growth and developmental processes including
germination, stem growth, flowering, and fruit
and tuber development (Lange, 1998). Applica-
tions of biologically active GAs or inhibitors of
GA biosynthesis have shown either to delay or to
promote tuber formation under tuber-inducing
conditions (Vreugdenhil and Struik, 1989; Jack-
son and Prat, 1996). Xu et al . (1998a) quanti-
fied endogenous GA levels during various stages
of stolon elongation and tuber formation. They
found that GA 1 levels were high during the lon-
gitudinal elongation of stolons and decreased
 
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