Aroma Volatiles - Fruit Ripening: Physiology, Signalling and Genomics - page 75

Apart from the functions involved in
aroma quality, recent reports have
indicated that changes in the content of
volatiles also play important roles in the
fruit response to biotic and abiotic stresses.
Downregulation of the citrus limonene
synthase gene ( CitMTSE1 ) in the antisense
orientation produced approximately 85
and 50 times less (+)-limonene and
E -myrcene, respectively, in the peels of
transgenic lines compared with the peels of
wild-type fruit (Rodríguez et al. , 2011).
Transgenic CitMTSE1 citrus fruit showed
increased resistance to economically
important fungal and bacterial citrus
pathogens and resulted in the repulsion of
a major insect pest (Rodríguez et al. , 2011),
suggesting a promising method for
developing broad-spectrum resistance or
tolerance in fruit and other crops.
Transgenic tomato fruit generated by over-
expressing the Z -3 fatty acid desaturases
FAD3 and FAD7 had a signifi cant increase
in linolenic acid, a considerable accumu-
lation of ( Z )-hex-3-enal and enhanced
resistance to CI (Domínguez et al. , 2010).

based mainly on yield, visual char-
acteristics, sugar content and postharvest
storability, whilst less attention is devoted
to enhancing or even maintaining aroma
quality. Recently, consumers have com-
plained about a loss of fl avour quality in
fruit and are thus willing to pay higher
prices for a product with a better fl avour
quality. The introduction of new long-
shelf-life cultivars using traditional
genetics or the inhibition of ethylene pro-
duction using biotechnology has generally
been accompanied by a loss of aroma
quality and low consumer acceptance.
The reintroduction of fruit aroma volatiles
can be obtained using classical crossing
with relatives that are rich in volatile
compounds, as performed in tomato fruit
by Kamal et al. (2001). It therefore would
be important and necessary to analyse and
compare volatile profi les among fruit
species or cultivars with different genetic
backgrounds, with the aim of fi nding 'the
lost aroma volatiles' caused by evolution
or breeding. However, although the
biosynthesis of aroma volatiles is clearly
ripening dependent and is associated with
ethylene, the regulatory mechanisms are
still not clear. A major limitation to such
an understanding is the identifi cation of
genes and enzymes responsible for the
biosynthesis of aroma volatiles. Once the
biosynthetic pathways for fruit aroma
volatiles have been established, the
isolation of genes and transcription factors
can be performed by exploiting the
extensive genome and expressed sequence
tag (EST) databases.

5.5 Conclusions

Volatiles are important determinants in
the overall aroma quality and taste of
fruit. In nature, volatiles serve as signals
involved in protecting fruit against
various stresses and contributing to seed
dispersion by increasing fruit attractive-
ness. However, in the past few decades,
breeders have selected new cultivars

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