Fruit Ripening: Primary Metabolism - Fruit Ripening: Physiology, Signalling and Genomics - page 18

metabolites. Thus, the synthesis of these
compounds is necessarily supported by the
supply of the primary photoassimilate.
In general, there are three major classes
of chemicals responsible for fl avour (a
combination of taste and smell): sugars,
acids and volatiles. Some of these primary
metabolites can be essential components of
taste, as they may be, depending on the
species, main components of the harvested
fruit, being recognized by receptors for
sweet taste.

species accumulates hexoses (Klann et al. ,
1996), allowing genetic and biochemical
studies to be carried out that have provided
evidence that the kinetic properties of the
invertase from the domesticated cultivars
accounts for the hexose accumulation in
the fruit of these species (Fridman et al. ,
2004). By contrast, there is little evidence
of a role of sucrose synthase in fruit
metabolism (Carrari and Fernie 2006). In
tomato, utilizing a reverse genetics
approach, Zanor et al. (2009a) reported that
LIN5 (a gene encoding a cell-wall
invertase) antisense plants had decreased
glucose and fructose in their fruit,
indicating the importance of LIN5 in
planta in the control of total soluble solids
content. The transformants were char-
acterized by an altered fl ower and fruit
morphology, displaying increased numbers
of petals and sepals per fl ower, an
increased rate of fruit abortion and a
reduction in fruit size.
Apoplastic invertase has been studied in
the fruits of species other than tomato,
such as strawberry, not only in terms of its
critical role in determining the sucrose/
hexose ratio but also because this ratio
determines the sink strength of the fruit
and, indirectly, fruit size. Thus, in straw-
berry, the levels of sucrose and hexoses
(glucose and fructose) increased during
fruit ripening, whilst other sugars such as
xylose and galactose, and the polyol
inositol, decreased (Fait et al. , 2008). The
apoplastic invertase-defi cient miniature1
mutant of maize exhibits a dramatically
decreased seed size as well as altered
levels of phytohormones (Miller and
Chourey 1992; Sonnewald et al. , 1997;
LeClere et al. , 2008). This raises interesting
questions regarding the regulation of
carbon partitioning in fruit. Recently, a
metabolic and transcriptional study using
tomato introgression lines resulting from a
cross between S. lycopersicum and S.
chmielewskii revealed that the dramatic
increase in amino acid content in the fruit
is the result of upregulated transport of
amino acids via the phloem, although the
mechanism is still unknown (Thi Do et al. ,
2010).

2.3.1 Sugars

The sugar, or sugar alcohol, delivered to
the fruit, is converted to starch (e.g. mango,
banana, kiwifruit), stored as reducing sugar
(e.g. tomato, strawberry) or stored as
sucrose (e.g. wild tomato, watermelon,
grape), or may even be converted to lipids
(e.g. olive). Sucrose, glucose and fructose
are the most abundant carbohydrates and
are widely distributed food components
derived from plants. The sweetness of
fruits is the central characteristic deter-
mining fruit quality and is determined by
the total sugar content and by the ratios
among these sugars. Accumulation of
sucrose, glucose and fructose in fruits
such as melon, watermelon (Brown and
Summers 1985), strawberry (Fait et al. ,
2008) and peach (Lo Bianco and Rieger
2002) is evident during ripening. However,
in domesticated tomato ( Solanum lyco-
persicum ), only a high accumulation of
the two hexoses (glucose and fructose) is
observed, whereas some wild tomato
species (e.g. Solanum chmielewskii )
accumulate mostly sucrose (Yelle et al. ,
1991).
The variability in the content of sucrose
and hexoses is the result of activities of the
enzymes responsible for their degradation
and synthesis, with invertase and sucrose
synthase being the most studied. In tomato,
the involvement of apoplastic invertase in
the balance between sucrose and hexoses
has been thoroughly studied by taking
advantage of the fact that the wild species
accumulates sucrose but the cultivated