Agriculture Reference
In-Depth Information
provides a synchronous tuber induction system
by transferring plants from noninductive long-day
(LD) to inductive SD conditions. Provided that
plants have obtained sufficient biomass, transfer
of plants from LD (
16
h) to SD (
8
h) conditions
generally results in the first visible swellings
around
8-
10 days after induction. The impact
of photoperiod on tuberization becomes evi-
dent when interrupting a long night (SD) with a
night break of red light, which inhibits tuberiza-
tion (Jackson, 1999).
Photoperiod perception takes place in the
leaves, mediated by photoreceptors including
several phytochromes (PHYA and PHYB), the
actions of which may be modulated by light
quality. Specifically, the action of phytochrome B
(PHYB) has been assigned a central role in con-
trolling tuber formation. Antisense
phyB
plants
lost their photoperiod-dependent tuberization
and tuberized equally well under both LD and SD
conditions (Jackson
et al.,
1996). Reduced PHYA
functionality in potato was shown to be involved
in photoperiod sensing by resetting the circa-
dian clock, impacting the timing of tuberization
(Yanovsky
et al.,
2000). Grafting experiments
showed that PHYB was involved in the transmis-
sible inhibition of tuberization in the aerial parts
of the plant, since wild-type plant stocks (SD)
were induced to tuberize in LDs when grafted
with a scion from an
anti
-
phyB
plant (Jackson
et al
., 1998).
stolon apex. Chailakhyan
et al
. (1981) already
showed in an early experiment that flowering to-
bacco shoots were able to induce tuberization,
indicating the presence of a mobile signal similar
to that of flowering.
In the case of flowering in
Arabidopsis
, a
flowering time locus T (FT) was shown to be the
mobile florigen protein transported from the
leaves to the shoot apex inducing floral transi-
tion (Corbesier
et al.,
2007). Navarro
et al
.
(2011) showed that in potato, floral and tuberi-
zation transitions were controlled by two differ-
ent
FT
-like genes (
StSP3D
and
StSP6A
), which
responded to independent environmental cues.
StSP6A
gene expression was shown to be in-
duced strongly on transfer to SD conditions, and
the protein was demonstrated to be transported
to the stolon and capable of inducing tuberiza-
tion. It is proposed that, during transport,
StSP6A is able to promote its own gene expres-
sion through an autoregulatory loop, thereby
maintaining a plant tuber induction status.
Silencing of
StSP6A
severely delays tuber forma-
tion in SD conditions, again confirming a crucial
role in promoting tuber formation.
StSP6A
expression analyses in commercial cultivars
with early (Jaerla), late (Baraka), and intermedi-
ate (Kennebec) tuberization periods show that
accumulation of this transcript in leaves correl-
ates with the tuberization time of these cultivars
(Navarro
et al.,
2011).
In a diploid potato population segregating
for timing of tuber initiation (Celis-Gamboa
et al.,
2003),
StSP6A
expression levels for early and
late genotypes reflect an inductive and nonin-
ductive state, and are correlated strongly with
tuber formation (
Fig. 4.2
). In early genotypes
of this population (
Fig. 4.2
),
StSP6A
gene ex-
pression is high, and these plants have started
setting tubers, while in the late genotype, ex-
pression is close to the detection limit and no
tuberization is observed (Kloosterman
et al.,
un-
published results). These findings further favor
the notion that StSP6A is the main developmen-
tal switch in the transition from stolon growth to
tuber initiation. StSP6A is, like most identified FT
proteins, a member of a family of proteins that
contain a phosphatidylethanolamine-binding
domain (PEBP) and is not itself a transcription
factor (Turck
et al.,
2008). Instead, StSP6A likely
needs one or more partners present in the stolon
tip to be able to induce tuberization, analogous to
4.3
The Tuberization Signal
Kumar and Wareing (1974) proposed the exist-
ence of a stimulus signal (named “tuberigen”,
analogous to the “florigen”) based on grafting
experiments in which scions grown under SD
conditions were able to induce tuberization on
potato stocks grown under LD conditions. The
precise nature of this mobile signal has long
been sought after, and only in recent years have
several key determinants been identified, as will
be discussed below.
There is a large similarity between photo-
periodic control of tuberization with that of
flowering. In both flowering and tuberization,
inductive conditions (day length) are perceived in
the leaves, and under favorable conditions a sig-
nal is produced and transported via the phloem
to either the shoot apex or the underground
