Tuber Development - The Potato: Botany, Production and Uses - page 50

LKP2
ZTL
StFKF1

StGI

?

StSP5G ? StSP6A

StCDFs StCOs

miR172

StBEL5 StPHYs

StRAP1

PotH1

Leaf

POTH1 StBEL5 StSP6A Stolon

miR172

Key:

Repression

StGA20ox1

Induction
Interaction
Tr ansport

Tuberization

StGA3ox2 StGA2ox1

Fig. 4.3. Schematic overview of identified genes associated with timing of tuberization and promoting
tuber formation. LKP1, ZTL and FKF1 are blue light receptors that are regulated by the clock genes. FKF1
and GI interact and bind to the transcription regulator CDF. The interaction of GI and FKF1 with CDF
targets this protein for degradation by the proteasome in the afternoon. During the early morning of long
days, CDF is not yet degraded and inhibits the expression of CO genes. CO protein is also a transcription
factor thought to be a specific inducer of the FT homolog, StSP5G . Evidence suggests that StSP5G
inhibits the expression of StSP6A , which in turn is considered the primary mobile tuberigen transported
to the stolon tip. Within the stolon tip, StSP6A activity is expected to trigger expression of tuber identity
genes through interaction with a yet unidentified partner. StBEL5 , POTH1 have also been shown to be
mobile and associated with transport to the stolon tip promoting tuber formation. PHYB plays a central
role in the regulation of a number of proteins inducing both StCO and StBEL5 under the correct light
conditions (LD), while suppressing miR172 in stolons under LDs, possibly through StRAP1 (Banerjee
et al., 2006; Jung et al., 2007; Martin et al., 2009). At the stolon tip, hormones such as GA play a key role
in tuber initiation. It is known that the BEL-KNOX interaction is capable of suppressing GA production by
inhibiting StGA20ox1 . The downregulation of StGA3ox2 and the upregulation of StGA2ox1 allows for
further rapid reduction of bioactive GA in the stolon associated with tuber formation. Question marks
indicate uncertain interactions/mobility or yet-to-be-identified interactors. Note : no differentiation has been
made between protein and gene in this figure.

same allele also phenocopied a number of other
characteristics of early potatoes such as drastic-
ally reduced life cycle and more bushy growth
habit (Kloosterman et al., 2013). The StCDF1
gene was identified after fine mapping of the
earliness QTL on chromosome 5. The wide variation
in plant earliness in modern potato cultivars
appears thus to be a result of allelic variation at a
single locus. This is likely to be due in part to the
selection for StCDF1 genes that evade the circadian

rhythm control by StFKF1, as demonstrated in a
genome-wide association study in 83 diverse
tetraploid genotypes where a unique and very
high association was found between earliness
and the CDF1 locus on chromosome 5 (Klooster-
man et al., 2013).
The similarity between the flowering and
tuberization regulation pathways makes it
likely that signaling cascade between the clock
output proteins and FT in potato has been re- or