Agriculture Reference
In-Depth Information
200000
plants, which overexpressed
StCO
, tuberized
later than wild-type plants under inductive SD
photoperiod.
StCO
silencing promoted tuberiza-
tion under both repressive and weakly inductive
photoperiods (Gonzalez-Schain
et al.,
2012).
Constitutive expression of the
Arabidopsis
CON-
STANS
(
AtCO
) gene in potato also impairs the
inductive effect of SDs on tuber initiation
(Martinez-Garcia
et al.,
2002). These results
demonstrate that StCO represses tuberization in
a photoperiod-dependent manner. The results
from González-Schain
et al
. (2012) and also
from the laboratory of S. Prat (2012, personal
communication), indicate that StCO protein
acts as an inducer of an FT homolog, called
StSP5G. This protein in turn seems to function
as a suppressor of the mobile tuberigen,
StSP6A
rhythm, and its expression in
Arabidopsis
is con-
trolled by members of the cycling DOF factor
(CDF) protein family. CDF1 has been shown to
be post-transcriptionally controlled by clock
gene GIGANTEA (GI) (Park
et al.,
1999) and a
number of blue light receptors such as FLA-
VIN-BINDING, KELCH REPEAT, F-BOX PRO-
TEIN
1
(FKF1), LOV-KELCH REPEAT PROTEIN
2
(LKP2) and ZEITLUPPE (ZTL) (reviewed in
Imaizumi, 2010). In turn, four of the five
Arabi-
dopsis
CDF proteins have been shown to suppress
CO. Moreover, it is thought that the FKF1 pro-
tein targets CDF proteins for degradation by the
proteasome (
Fig. 4.3
).
The latter finding is of interest as allelic vari-
ation was found in a member of the potato CDF
gene family (
StCDF1
), resulting in a truncated
protein that lost its ability to interact with FKF1,
thereby causing an early tuberization phenotype
(Kloosterman
et al.,
2013). In contrast to the
wild type, the StCDF1 truncated protein levels re-
mained high throughout the day, resulting in a
constitutive repression of two CO genes located
as a tandem repeat on chromosome
2
of potato
(
StCO1
and
StCO2
). Since it is thought that these
StCO
gene products indirectly release the inhib-
ition on the mobile StSP6A signal, genotypes
carrying one or more of the truncated variants
of the StCDF1 protein show an early tuberization
phenotype. This hypothesis was confirmed by
transforming late tuberizing genotypes with the
truncated StCDF allele under control of the
CaMV 35S promoter, resulting in extreme early
tuberization. Interestingly, overexpression of the
160000
120000
80000
40000
0
Late
1
Late
2
Early
1
Early
2
Fig. 4.2.
Relative gene expression level
comparison for
StSP6A
between two “late” and
“early” genotypes of the C × E population
segregating for timing of tuber formation. C × E is
a diploid potato F1 population descending from
Solanum tuberosum
and
Solanum phureja.
Expression of
StSP6A
in the late genotypes is
below reliable detection levels. Only the early
genotypes have formed tubers.
flowering locus D (FD) in the apex during flower
induction. The identity of the partners remains
the subject of further investigation, but several
potential downstream targets have been identi-
fied and will be discussed below.
4.4
Molecular Regulation
of Tuber Induction
If
StSP6A
expression and transport is con-
sidered crucial to the developmental switch to-
wards tuber formation, the question that then
arises is what sort of mechanism controls
StSP6A
expression in the leaf ? In
Arabidopsis
, a
major factor controlling flowering is the circa-
dian rhythm-regulated transcription factor,
CONSTANS (CO), which acts directly on FT ex-
pression in
Arabidopsis
(An
et al.,
2004). The ac-
tivity of the CO protein is post-transcriptionally
modified under the influence of light through
an interaction with the red and blue light recep-
tors, phytochrome A, B, and cryptochrome
2
(Yanovsky and Kay, 2002). Similarly, in potato
plants that are grown under LD conditions,
PHYB protein is conformationally inhibited
from inducing
StCO
gene expression, whereas
under SD conditions, PHYB induces
StCO
(Rodriguez-Falcon
et al.,
2006). Furthermore,
Gonzalez-Shain
et al
. (2012) showed that potato
