Biotechnology of Fruit Quality - Fruit Ripening: Physiology, Signalling and Genomics - page 284

fruit defi cient in ethylene production con-
verted the exogenously applied lycopene
into carotenoid-related volatiles. These
results suggest that carotenoid biosynthesis
is ethylene dependent, but degradation
into volatile compounds is ethylene inde-
pendent (Gao et al. , 2008). More in vivo
experiments are needed to separate out the
role of ethylene in carotenoid production
and their catabolism. Carotenoids are also
synthesized from chloroplast-derived iso-
prenoids, and their levels increase with
total chromoplast area per cell in ripe fruit
pericarp of the high-pigment-1 ( hp-1 )
tomato mutant (Cookson et al. , 2003; Wang
et al. , 2008). This led to testing of the
hypothesis that elevating biosynthesis of
structural chromoplast proteins would
increase the emission of carotenoid-
derived volatile compounds. Transgenic
tomato lines overexpressing pepper
fi brillin, a protein involved in the synthesis
of lipoprotein in the chromoplast,
exhibited increased lycopene (118%) and
E -carotene (64%) (Simkin et al. , 2007).
Elevations in the emission of E -ionone
(36%), E -cyclocitral (74%), citral (50%),
6-methyl-5-hepten-2-one (122%) and
geranylacetone (223%) were also recorded
in these transgenic fruit as a consequence
of increases in the availability of
carotenoids for cleavage activity (Simkin et
al. , 2007).
In addition to the production of volatile
compounds from phenylalanine, carot-
enoid or lipoxygenase-mediated pathways,
other sources of important volatile com-
pounds are now known and include
guaiacol, synthesized by methylation of
catechol, which contributes smoky aroma
to tomato fl avour. Tomato lines silenced for
or overexpressing catechol- O -methyl-
transferase ( CTOMT1 ) provided evidence
that this gene is responsible for the
production of guaiacol in tomato (Mageroy
et al. , 2012). Transgenic tomato lines con-
stitutively over- or underexpressing sali-
cylic acid methyl transferase ( SlSAMT ) due
to a sense or antisense chimeric gene
construct confi rmed the functional role of
SlSAMT in the production and emission of
methyl salicylate (Tieman et al. , 2010).

Transgenic approaches applied to apple
(Dandekar et al. , 2004; Defi lippi et al. ,
2004, 2005a,b; Schaffer et al. , 2007; Brown,
2009), cucumber (Zawirska-Wojtasiak et
al. , 2009), grape (Battilana et al. , 2011),
berries (Malowicki et al. , 2008), strawberry
and banana (Beekwilder et al. , 2004),
potato (Di, 2009), basil (Dudai and
Belanger, 2009), melon (Flores et al. , 2002)
and oranges (Rodríguez et al. , 2011b,c)
have identifi ed various enzymes involved
in the volatile biosynthesis pathway and
their interaction with genetic and
environmental factors such as ethylene and
pathogen responsiveness.

16.7 Future Perspectives

The fi rst edible transgenic crop, 'FlavrSavr'
tomato, was released for human con-
sumption in 1992, some 20 years ago
(USDA-APHIS, 1991, 1992; Kramer and
Redenbaugh, 1994). 'FlavrSavr' was pro-
duced by antisense RNA technology to
have reduced PG expression and a promise
to maintain texture of the ripened tomato
fruit after harvest and during long-distance
transportation (Kramer and Redenbaugh,
1994). This was a large leap but was not
suffi cient to meet market expectation
(Giovannoni et al. , 1989; Thakur et al. ,
1997). However, it provided the impetus
and a path to genetically modifi ed crops for
enhancing various desirable traits, some of
which have been discussed in this chapter.
Most of the fi rst-generation genetically
engineered agronomical crops were
developed based on manipulation of
simple monogenic traits such as herbicide
or insect resistance. Examples of successful
genetic engineering of fruit crops, dis-
cussed in this chapter, are a testament to an
approach that is robust and powerful.
Thus, rational strategies have resulted in
enhancing several desirable quality
attributes in fruit crops and have produced
novel phenotypes by using the gain or loss
of function of a candidate gene.
Many desirable crop traits are, however,
multigenic in nature, the fi nal outcome
being a function of a group of genes.