Agriculture Reference
In-Depth Information
et al.,
2005, 2008, 2010; Rensink
et al.,
2005;
Stupar
et al.,
2007; Ducreux
et al.,
2008;
Vasquez-Robinet
et al.,
2008). With the comple-
tion of the potato genome sequence, additional
profiling techniques such as RNAseq and small
RNA sequencing have brought new tools to
study the process of tuber development and
growth. The Potato Genome Sequencing Con-
sortium collected RNAseq expression data from
a large number of potato tissues, including tu-
berous tissue of different developmental stages
(PGSC, 2011). When the RNAseq expression
data were queried for genes impacting tuber de-
velopment,
16,812
genes were found with con-
firmed expression in the tuberous tissues (stolon,
young tuber, mature tuber). From this set,
325
genes were upregulated strongly (at least five-
fold) in the early phase of tuber growth, and an-
other 1151 genes reached peak expression in
the more matured tuber tissues in comparison to
the non-tuberizing stolon tissue.
The differential expression of a gene during
the transition of a stolon into a tuber may indi-
cate a regulatory role in tuber initiation, growth,
or its involvement in one of the many metabolic
pathways upregulated as a result of the newly
developed storage organ. One could consider the
formation of a tuber as a new organ requiring a
unique set of genes; however, “true” tuber-specific
expression (i.e. only expressed in the tuber) is a
rare commodity. This is not surprising, as the
tuber can be considered, in its simplest form, a
modified underground stem. Although most
genes expressed in tubers are also expressed in
other parts of the potato plant, one can identify
tuber-related adaptations in gene transcrip-
tional regulation to accommodate tuber devel-
opment, such as very high expression levels of
starch biosynthesis genes, and especially storage
proteins. The capacity for differential expression
in tuber tissue could come from mutations in
promoter elements, increased substrate avail-
ability for specific enzymatic reactions, shifts in
hormonal balance, or as a result of the increased
sink strength of the growing tuber.
From the same RNAseq data, a comparison
between tissue types of a fully matured tuber
(pith, cortex, and peel) can be made. Unique sets
of expression profiles can be found for each
of the three tissue types, with genes having
elevated or reduced expression in one or two
of the studied tissues. For example, genes
providing the first line of defence against
insect attacks or wounding, such as polyphenol
oxidases and genes that are part of the
glycoalkaloid biosynthesis pathway, were ex-
pressed more highly in the peel than in the pith
or cortex tissues. As almost half of the pre-
dicted genes within the potato genome sequence
lack proper functional annotation cell type spe-
cific expression may prove a valuable resource
for the identification of trait-associated candi-
date genes.
A more detailed analysis of gene expression
changes during the early stages of tuber devel-
opment using
44
k
60-
mer oligo microarray
shows a number of common expression patterns
that can be linked with tuber developmental
stages (Kloosterman
et al.,
2008)
(Fig. 4.8
). With
respect to tuber initiation and early growth
stages, the group exhibiting a temporarily up-
regulated expression profile is of high interest.
This group of genes is highly enriched for genes
associated with cell cycle and division, includ-
ing several members of the cycling kinases and
a gene with homology to AINTEGUMENTA
(
StANT
-like), associated with the maintenance
of meristematic competence. Altered expression
of this potato
ANT
-like gene resulted in plants
with reduced leaf size and increased tuber ini-
tials, but reduced total tuber yield (Kloosterman,
unpublished data). A larger set of genes is down-
regulated on tuber formation. This group of
genes is associated with a wide variety of func-
tions likely to be required during stolon growth
but no longer necessary during tuber growth.
Further characterization of differentially ex-
pressed genes during tuber development will en-
hance our understanding of the regulatory
mechanisms impacting commercially interest-
ing traits such as the speed of growth, final tuber
size, size distribution, and shape.
4.7 The Potato Tuber
as a Storage Organ
Sucrose as a driver for tuber
initiation and growth
In the previous paragraphs, the focus has been on
physiological changes and gene transcriptional
signaling cascades facilitating tuber development.
