B - Plant Physiology

with altered T6P content display various phenotypes suggestive for an involvement of T6P
signalling in a range of processes related to metabolic and developmental regulation (Goddijn
and Smeekens, 1998; Eastmond et al., 2002; Schluepmann et al., 2003; Avonce et al., 2004;
Pellny et al., 2004; Schluepmann et al., 2004; van Dijken et al., 2004; Gomez et al., 2010).
However, our understanding of the underlying signalling mechanisms is still in its infancy.
Here we presented a detailed molecular and physiological analysis of transgenic potato tubers
expressing E. coli enzymes to either reduce (B33-TPP) or to elevate (B33-TPS) the T6P
content in a tuber specific manner. Our results provide evidence that T6P acts as an integrator
of nutritional status and growth in potato tubers.
Manipulation of the T6P content affects carbon utilization and energy metabolism
Diminished T6P levels in B33-TPP tubers as well as elevated levels in B33-TPS eventually
led to a decrease in tuber yield in all transgenic lines, although the underlying cause is likely
to be different in both cases. Higher soluble sugar levels in B33-TPP tubers are consistent
with a model in which T6P is required to efficiently utilize carbohydrates for growth as was
previously shown for Arabidopsis plants overexpressing OtsB (Schluepmann et al., 2003).
Similar to what was observed in B33-TPP tubers, these transgenic Arabidopsis plants
accumulated hexose-phosphates. The increase in soluble sugars in B33-TPP tubers was not
accompanied by obvious changes in carbon partitioning between central metabolic pathways
further supporting the notion that increased sugar levels reflect reduced carbon consumption
for tuber growth rather than gross metabolic changes through T6P directly affecting
regulation of metabolic pathways. A range of studies implies that sucrose breakdown and
starch synthesis is limited by levels of adenine nucleotides as elevated ATP levels lead to an
increase in starch synthesis (Loef et al., 2001; Regierer et al., 2002; Oliver et al., 2008). ATP
levels are drastically increased in OtsB overexpressing lines indicating that ATP availability
is not the primary cause for the decrease in carbon utilization observed in these tubers. It has
recently been shown that the import into the amyloplast of G6P and ATP via the glucose-6phosphate/phosphate
translocator (GPT) and the adenylate translocator (NTT), respectively,
limits starch synthesis (Zhang et al., 2008). Thus, downregulation of these two transport
activities could also be the cause for accumulation of G6P and ATP in B33-TPP tubers.
However, the available transcript data provide no indication for a transcriptional regulation of
these two important transporters in TPP overexpressing tubers. Reduced T6P content seems to
uncouple ATP availability from sugar utilizing processes.
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