Fruit Ripening: Primary Metabolism - Fruit Ripening: Physiology, Signalling and Genomics - page 20

measured by the taste and olfactory
systems. These metabolites comprise a
diverse set of chemicals derived from
essential amino acids (phenylalanine,
leucine and isoleucine), essential fatty acids
(principally linolenic acid) and carotenoids
such as E -carotene, the precursor of retinol
(vitamin A), which is the immediate
precursor of one of the most important
fl avour volatiles, E -ionone (Goff and Klee,
2006). More than 400 volatiles compounds
have been identifi ed in tomato (Petro-Turza,
1987; Buttery, 1993; Buttery and Ling, 1993;
Fulton et al. , 2002), although a smaller set
of only 15-20 are made in suffi cient
quantities to have an impact on human
perception (Baldwin et al. , 2000).
Any study of the metabolic pathways
leading to the synthesis of its various
metabolites must be considered in the
context of this developmental process.
During the rapid growth phase, fruits act as
strong sinks, importing massive amounts of
photoassimilates from photosynthesizing
organs. The content of most, although not
all, of the fl avour volatiles in tomato
increases at the onset of ripening and peaks
either at or shortly before full ripening.
This timing suggests that synthesis of
fl avour volatiles is highly regulated.
Recently, a metabolomic approach was
used to describe the phenotypic variation
of a broad range of primary and volatile
metabolites across a series of tomato lines
resulting from crosses between a cherry
tomato and three independent large fruit
cultivars ('Levovil', 'VilB' and 'VilD')
(Zanor et al. , 2009b). The results of the
most highly abundant primary metabolite
analysis of cherry and large-fruited tomato
lines were largely in accordance with those
obtained from previous studies (Causse et
al. , 2002). The low sugar and high malate
content of the 'Levovil' parental and the
corresponding very low sugar:acid ratio
could explain the lower acceptance of the
fruit by food panel tasters, especially as
malate is perceived as tasting more sour
than citrate (Marsh et al. , 2003). In
addition to the changes observed in sugars
and acids in cherry tomatoes, the glutamate
level, known to be sensed as the fi fth basic
taste (umami), which evokes a savoury

feeling, was found to be considerably
higher in the cherry variety than in the
large-fruited varieties. This fi nding is also
in accordance with the fact that cherry
tomatoes were found to be tastier than the
other parental lines used in this study.
Additionally, considerable correlations
within the levels of primary metabolites or
volatile compounds were also observed.
However, there was relatively little
association between the levels of primary
metabolites and volatile compounds,
implying that they are not tightly linked to
one another, with the exception of sucrose,
which showed a strong association with a
number of volatile compounds (Zanor et
al. , 2009b).
A broad profi ling of tomato volatiles in a
tomato introgression line population har-
bouring introgression of the wild species
Solanum pennellii yielded over 100
quantitative trait loci (QTLs) that were
reproducibly altered in one or more
volatiles contributing to fl avour (Tieman et
al. , 2006b). These QTLs have been used as
tools to identify the genes responsible for
controlling the synthesis of many volatile
compounds. Very few genes involved in the
biosynthetic pathways of tomato fl avour
volatiles have been identifi ed, although the
detection of malodorous , a wild species
allele that affects tomato aroma, allowed
the identifi cation of a QTL that is linked to a
markedly undesirable fl avour within the
S. pennellii introgression line IL8-2 (Tadmor
et al. , 2002). A complementary approach
utilizing broad genetic crosses has been
used to identify QTLs for organoleptic
properties of tomatoes (Causse et al. , 2002).
The lines identifi ed as preferable by the
consumer could now be comprehensively
characterized with respect to volatile and
non-volatile compounds alike. By using a
combination of metabolic and fl ux profi ling
alongside reverse genetic studies on IL8-2,
it was possible to confi rm the biological
pathway of a set of phenylalanine-
derived volatiles, 2-phenylacetaldehyde
and 2-phenylethanol, important aromatic
compounds in tomato (Tieman et al. ,
2006a). A combined metabolic, genomic
and biochemical analysis of glandular
trichomes from the wild tomato species