Tuber Development - The Potato: Botany, Production and Uses - page 56

can be expected, as the early stages of tuber for-
mation requires the initiation of cell divisions
within the swelling stolon. Palmer and Smith
(1969) first reported an increase of tuberization
frequency on various CK-containing media.
Their observation was supported by further in-
vestigations of exogenous CKs (Kumar and Wa-
reing, 1974; Hussey and Stacey, 1984). The
addition of CK to the high-sucrose in vitro tuber-
ization media has been reported to increase the
speed of tuber induction (Xu et al., 1998a).
A role of CKs in tuber formation also comes from
transgenic potato plants expressing the Sho gene
from Petunia hybrida , a CK biosynthesis gene.
The most pronounced effects detectable in these
lines were enhanced shoot production, delayed
tuber formation, significant reduction in tuber
size, and inhibition of tuber dormancy (Zubko
et al., 2005). Strong overexpressers of the Agro-
bacterium ipt gene, catalyzing a rate-limiting step
in CK biosynthesis, also showed inhibition of tu-
berization; however, moderate overexpression of
the same gene resulted in a promoting effect on
tuberization (Gális et al., 1995).
Interestingly, overexpression of the ipt gene
in flowering tobacco plants induced tuber-like
organs from stem axillary buds accumulating
starch (Guivarc'h et al., 2002). Overexpression
of POTH1 produced dwarf plants with several
pleiotropic effects (Rosin et al., 2003a). In vitro
tuberization was enhanced in the POTH1
overexpressing lines and was associated with
decreased levels of GAs and a two- to fourfold
increase in bioactive CKs measured in the stolon
apical meristem.
A secondary role for CKs could be in sink
creation, by regulating the expression of genes
implicated in assimilate partitioning and source-
sink regulation (Roitsch and Ehneß, 2000). Sup-
pression of a potato MADS box gene, POTM1 ,
resulted in increased CK content associated with
increased cell division and increased starch ac-
cumulation in specific regions associated with
vegetative meristems (Rosin et al., 2003b). Ser-
geeva et al . (2000) hypothesized that the ratio
between auxin and CK was important, with a
slight increase in CK promoting tuberization and
a larger change resulting in inhibition of tuberi-
zation. One could thus imagine that CKs exert
their effect on tuber formation by redirecting
carbon fluxes towards the stolon cells and
promoting cell division in concert with auxin.

However, other hormones have been implicated
in this process as well. Phytohormone distribu-
tion in the pith and cortex was shown to be dif-
ferent in young tubers for IAA, abscisic acid
(ABA), and CKs ( Fig. 4.5) . Enhancement of
starch synthesis, accumulation in the pith, and
the retardation of these processes in the cortex
followed changes in ABA content (Borzenkova
and Borovkova, 2003).
The plant hormone, ABA, has also been
shown to have a promoting effect on tuber in-
duction. Tuber formation in medium containing
ABA and 8% sucrose started earlier than the
tuber formation in ABA-free medium with 8%
sucrose, in which sessile tubers or tubers on very
short stolons were formed (Xu et al., 1998a).
However, an ABA-deficient potato mutant forms
tubers normally, which indicates that ABA is not
essential for tuberization and its function is likely
due to antagonistic effects towards GAs (Xu
et al., 1998a). The importance of other hor-
mones such as jasmonic acid and ethylene in
regulating tuber formation is not clear. Jasmonic
acid has been linked with cell expansion in tuber
cells (Takahashi et al., 1994), and a clear role in
tuberization was demonstrated for an enzyme
(POTLX-1) involved in the generation of jas-
monic acid precursors (Kolomiets et al., 2001).
Plant lipoxygenases (LOXs) catalyze the oxygen-
ation of polyunsaturated fatty acids such as
linolenic and linoleic acids. POTLX- 1 expression
is upregulated during tuberization and is associ-
ated with cell enlargement in the region later
forming the perimedullary region. Downregula-
tion of POTLX- 1 reduced tuber yield and dis-
rupted the process of tuber formation.
Interpretation of the activities and func-
tions of the different endogenous hormones
with respect to tuber development remains diffi-
cult without precise information on their bio-
synthesis, sites of accumulation, and shifting
balances.

4.6 Gene Expression During

Tuber Development

Within the last decade, gene expression studies
based on microarrays have yielded large data
sets of differentially expressed genes under vari-
ous biotic and abiotic stresses, developmental
stages, or genetic backgrounds (Kloosterman