Biotechnology of Fruit Quality - Fruit Ripening: Physiology, Signalling and Genomics - page 270

16.3 Molecular Engineering of Fruit
Texture

(Kramer and Redenbaugh, 1994). The
impaired SlPG2 expression resulted in
enhanced juice viscosity, but fruit soften-
ing was not signifi cantly affected, and thus
it failed to meet the market expectation of
an extended-shelf-life fruit (Giovannoni et
al. , 1989; Thakur et al. , 1997). The ectopic
expression of SlPG2 also failed to enhance
softening of the ripening mutant, rin ,
suggesting a limited role of SlPG2 in
tomato fruit softening (Giovannoni et al. ,
1989). In contrast, antisense inhibition of
FaPG1 expression in strawberry ( Fragaria ×
ananassa ) resulted in reduced fruit
softening (Quesada et al. , 2009). Reduced
softening of FaPG1 -antisense fruit occurred
in spite of only a slight reduction in total
PG activity, as most of the PG activity was
contributed by another isozyme, FaPG2 ,
whose expression was not impaired by the
FaPG1 antisense gene (Quesada et al. ,
2009).
Multiple isozymes of pectin methyl-
esterase (PME), an enzyme that demethoxy-
lates pectin, are expressed during fruit
development, but their roles in fruit texture
are not as yet understood (Harriman et al. ,
1991; Gaffe et al. , 1994; Tieman and
Handa, 1994; Phan et al. , 2007). Over a
95% reduction in PME transcripts, protein
and enzymatic activity by antisense
expression of SlPME3 under the CaMV 35S
promoter did not affect fruit softening but
greatly enhanced juice viscosity and
increased the total soluble solids (Tieman
et al. , 1992; Tieman and Handa, 1994;
Thakur et al. , 1996a,b). The fruit integrity,
however, was compromised if low-PME
fruits were stored for extended periods
(Tieman and Handa, 1994). In another
study, silencing of SlPMU1 , a ubiquitously
expressed PME isozyme, enhanced
softening of transgenic fruit even when the
reduction in PME activity was only 25%
compared with wild-type fruit (Gaffe et al. ,
1997; Phan et al. , 2007). These studies on
PG and PME further emphasize that only
specifi c isozymes among cell-wall-
modifying isozymes contribute to fruit
textural changes. Interestingly, tomato fruit
with reduced PME expression also
exhibited a reduction in fruit blossom end

Changes in fruit texture are essential for
fruit softening and for making a fruit edible
and desirable for human consumption.
Fruit softening is associated with several
attributes including crispness, mealiness,
grittiness, chewiness, succulence and juici-
ness, fi brousness, toughness and oiliness.
Furthermore, fruit textural changes are
connected with the development of
organoleptic characteristics, such as sweet-
ness, sourness, astringency, bitterness and
the production of volatile compounds that
provide the aroma. However, excessive
fruit softening can cause some undesirable
attributes including the development of off
fl avours and susceptibility to phyto-
pathogens. The fact that excessive fruit
softening makes most fruit unacceptable,
leading to large economic losses, has
generated considerable interest among
plant biologists to understand the mol-
ecular basis of fruit softening and modify
this process using recombinant technology
(Negi and Handa, 2008). As softening and
cell-wall metabolism are intertwined
during fruit ripening, this subject is the
focus of another chapter (see Tucker,
Chapter 4, this volume). We limit this
chapter to biotechnological approaches for
enhancing the textural qualities of fruits.
Based on observed modifi cations of the
polysaccharides in the primary cell wall
and dissolution of the middle lamella
during fruit softening, it had been
hypothesized that cell-wall depolymerizing
enzymes play important roles in fruit
textural changes (Brady, 1987). This
hypothesis gained further credence when it
was shown that expression of several cell-
wall-degrading enzymes was severely
reduced in tomato mutants impaired in
fruit ripening (Tigchelaar et al. , 1978; Biggs
and Handa, 1989; DellaPenna et al. , 1989).
A test of this hypothesis led to the
development of the fi rst genetically
engineered tomato cultivar designated
'FlavrSavr'. In 'FlavrSavr' fruit, the poly-
galacturonase (PG) gene ( SlPG2 ) was
silenced by antisense RNA technology