Agriculture Reference
In-Depth Information
Starch is another carbohydrate that
undergoes modifi cations during ripening
and is metabolized to glucose and fructose,
the two main sugars in ripe fruit (Ho et al. ,
1983). Tomato introgression lines con-
taining the exotic allele of LIN5 (IL 9-2-5)
accumulated signifi cantly more starch
in both pericarp and columella tissues
(Baxter et al. , 2005). This is in agreement
with the fi nding that starch accumulation
plays an important role in determining the
soluble solids content, or Brix index, of
mature fruit (Schaffer and Petreikov, 1997).
Recently, in tomato fruit, it has been shown
that malic acid infl uences starch content
via redox control of the enzyme
responsible for starch synthesis, ADP-
glucose pyrophosphorylase (Centeno et al. ,
2011). Given that starch synthesis is
positively correlated with reducing sugar
content in ripe fruit, the malic acid content
of the immature fruit is predicted to be
inversely correlated to the sugar content of
the ripe fruit.
The structure of the TCA cycle is well
known in plants. However, until recently,
its regulation was poorly characterized. In
our laboratory, several studies have been
done to determine the role of the mito-
chondrial TCA cycle in plants. Bio-
chemical analysis of the ACO1 mutant
revealed a decreased fl ux through the TCA
cycle, decreased levels of TCA cycle
intermediates, enhanced carbon assimi-
lation and dramatically increased fruit
weight (Carrari et al. , 2003). Tomato plants
with reduced mitochondrial malate de-
hydrogenase (mMDH) showed an increase
in fruit weight that was probably due to
enhanced photosynthetic activity and
carbon assimilation in the leaves, which
also led to increased accumulation of
starch and sugars, as well as some organic
acids (succinate, ascorbate and de-
hydroascorbate) (Nunes-Nesi et al. , 2005).
In fruits, malic acid also infl uences starch
content via redox control of the ADP-
glucose pyrophosphorylase (Centeno et al. ,
2011). Those plants also showed a small
effect on the total fruit yield, as well as
unanticipated changes in postharvest shelf-
life and susceptibility to bacterial infection.
Tomato plants with reduced fumarase
activity showed the opposite effect to those
with reduced mMDH (Nunes-Nesi et al. ,
2007). Additionally, biochemical analyses of
antisense tomato mitochondrial NAD-
dependent isocitrate dehydrogenase plants
showed a reduction in fl ux through the TCA
cycle, decreased levels of TCA cycle
intermediates and relatively few changes in
photosynthetic parameters; however, fruit
size and yield were reduced (Sienkiewicz-
Porzucek et al. , 2010). Despite the fact that
much research work is needed to under-
stand the exact mechanism for the increase
in the fruit dry weight, manipulation of
central organic acids is clearly a promising
approach to enhance fruit yield (Nunes-Nesi
et al. , 2011).
2.3.2 Organic acids
Organic acids are the other intermediate
metabolites important as fl avour com-
ponents, either by themselves, because the
organic acid:sugar ratio defi nes quality
parameters at harvest time in fruits, or as
precursors of other secondary metabolites.
Therefore, organic acid manipulation is
highly valuable in terms of metabolic
engineering. The organic acid content is
dependent on the activity of the main
metabolic pathways such as glycolysis, the
tricarboxylic acid (TCA) cycle and respir-
ation. The main organic acids are the TCA
intermediates citrate and malate, together
with quinate, with other TCA inter-
mediates like oxalate, succinate, isocitrate,
fumarate and aconitate being present at
lower levels. The pH of a ripe fruit is
typically around pH 4. Interestingly, in an
independent study that focused on early
fruit development, the levels of both citrate
and malate were also highly correlated to
many important regulators of ripening
(Mounet et al. , 2009).
2.4 Volatiles
Flavour is the sum of a large set of primary
and secondary metabolites that are
 
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