Fruit Ripening: Primary Metabolism - Fruit Ripening: Physiology, Signalling and Genomics - page 21

Solanum habrochaites identifi ed a key
enzyme in the biosynthesis of methyl-
ketones that serves this purpose (Fridman
et al. , 2005). In recent years, there have
been dramatic improvements in the
knowledge of volatiles, and it is clear that
synthesis of the fl avour volatiles is
associated with ripening. However, the
regulatory mechanisms have not been
established. It is likely that a large part of
this regulation is mediated by a subset of
the many ripening-associated transcription
factors.

cell-wall components whose solubility
increases and/or polymer size decreases in
parallel with decreasing fruit fi rmness
(Seymour et al. , 1987; Tucker and Grierson,
1987; Redgwell et al. , 1992); and (ii) the
characterization of proteins that are expres-
sed during ripening and whose bio-
chemical activities can be mechanistically
related to the observed cell-wall changes
(Goulao and Oliveira, 2007; Vicente et al. ,
2007). Among cell-wall hydrolases, pectin-
degrading enzymes are mostly implicated
in fruit softening. Increased solubilization
of the pectin substances, progressive loss of
tissue fi rmness and a rapid rise in the
polygalacturonase (PG) activity accompany
normal ripening in many fruits (Brady,
1987; Fisher and Bennett, 1991). A positive
correlation between PG activity and
initiation of softening is known in a
number of fruits like guava (El-Zoghbi,
1994), papaya (Paull and Chen, 1983),
mango (Roe and Bruemmer, 1981) and
strawberry (Garcia-Gago et al. , 2009;
Quesada et al. , 2009). However, experi-
ments with transgenic tomatoes have
shown that, even though PG is important
for the degradation of pectins, it is not the
sole determinant of tissue softening during
ripening (Gray et al. , 1992). Pectin methyl
esterase catalyses the de-esterifi cation of
pectin, and its activity together with that of
PG increase remarkably during ripening in
peach, tomato, pear and strawberry (Tucker
and Grierson, 1987; Osorio et al. , 2010).
The regulation of texture and shelf-life
is clearly far more complex than was
envisaged previously and so new
approaches are needed for a better under-
standing of the relationships among changes
in the texture properties of specifi c fruit
tissues, intact fruit 'fi rmness' and shelf-life.
Fruit softening is generally described as a
textural change that is associated with cell-
wall disassembly due to the activity of
degrading enzymes. However, an alternative
explanation is that polysaccharide de-
gradation is not the sole determinant of
fruit softening and that other structures
and ripening-related physiological pro-
cesses also play critical roles. The cuticle
has a number of biological functions that
could have an important impact on fruit

2.5 Cell-wall Metabolism

The primary cell wall is composed of
numerous polymers. These vary in structure
somewhat between species, but eight
polymeric components (cellulose, three
matrix glycans composed of neutral sugars,
three pectins rich in d-galacturonic acid and
structural proteins) are usually present.
During ripening, cell-wall architecture and
the polymers of which it is composed are
progressively modifi ed. The metabolic
changes during ripening include alteration
of cell structure involving changes in cell-
wall thickness, permeability of the plasma
membrane, hydration of the cell wall,
decreases in structural integrity and
increases in intracellular spaces (Redgwell
et al. , 1997). In fruit such as strawberry and
avocado, which develop a soft melting
texture during ripening, swelling and
softening of the cell wall is evident, but in
fruit such as apple, which ripen to a crisp,
fracturable texture, cell-wall swelling is not
observed (Redgwell et al. , 1997). Ripening is
also usually accompanied by a reduction in
cell turgor, due to an increasing con-
centration of solutes in the cell-wall space
and to wall loosening (Shackel et al. , 1991).
Cell-wall disassembly rate and extent are
crucial for the maintenance of fruit quality
and integrity (Matas et al. , 2009). For this
reason, maintenance of fi rmness has long
been the target for breeders in many crops
to minimize postharvest decay.
The conventional approach to elucidat-
ing fruit softening has typically been based
on two strategies: (i) the identifi cation of