Agriculture Reference
In-Depth Information
4.3.4 Protein
calcium (Virk and Cleland, 1988). In this
regard, an early theory proposed that auxin
induced cell growth by acidifi cation of the
cell wall, known as the acid growth theory
(Rayle and Cleland, 1970; Rayle and
Cleland, 1980; Rayle and Cleland, 1992).
The acid growth theory was originally
explained as an effect on charge-related
interactions between polymers or changes
in enzyme activity (Rayle and Cleland,
1970). Subsequent to this early work, it
was demonstrated that expansins were
activated by cell-wall acidifi cation and
played a major role in loosening the cell
walls during auxin-induced growth
(McQueen-Mason and Cosgrove, 1994). As
discussed above, expansins probably also
play a role in fruit softening, but the role of
pH in this process has not been examined
(Brummell
et al.
, 1999b). Nevertheless,
although the optimum pH for tomato fruit
expansins was not reported, the activity of
fruit expansin was assayed at pH 4.5 (Rose
et al.
, 2000), presumably because the pH
optimum for the tomato fruit expansin is
low, as it is for other expansins examined
(McQueen-Mason and Cosgrove, 1994).
Also of interest in regard to pH is that the
activity maximum for the tomato fruit PG
is approximately pH 5 and remains
relatively high at pH 4.5 (Chun and Huber,
1998), which is the pH of the apoplast in
ripe fruit (Almeida and Huber, 1999). At
pH 6 and above, the pH of the apoplast in
mature green fruit, PG has very low activity
(Chun and Huber, 1998). Thus, the pH of
the cell wall may be important for the
activation of expansin, PG and other
enzymes.
A change in the concentration of
calcium can also affect cell-wall ex-
tensibility (Virk and Cleland, 1988), but the
concentration of calcium in the cell wall of
tomato does not change much during
ripening (Almeida and Huber, 1999). None
the less, the importance of calcium in fruit
fi rmness and storage is exemplifi ed by
postharvest studies with apple (Mason
et
al.
, 1975; Sams and Conway, 1984). More
than simply a research interest, fruits and
fruit slices are often commercially dipped
in a solution containing CaCl
2
to improve
The protein component of cell walls is
probably the least well understood part of
the cell wall. It seems logical that
something that accounts for as much as
20% of the mass of the cell wall (Carpita
and Gibeaut, 1993) might be important in
softening. Mutants of cell-wall proteins
and manipulation of the glycosylation of
these proteins clearly affects cell shape and
development (Knox, 1995), but how do
changes in this component of the cell wall
affect fruit softening? Not much is known
about this. An early study of tomato
demonstrated that total nitrogen content
from cell-wall protein changed very little
during fruit ripening, but the amount of
salt soluble protein that could be extracted
from the cell wall increased approximately
twofold from mature green to red-ripe fruit
(Hobson
et al.
, 1983). However, the
increase in extractable protein may have
been due to changes in the carbohydrate
fraction rather than the proteins them-
selves. In a recent paper, the change in
accumulation of several thousand tran-
scripts and a few hundred proteins was
examined during tomato fruit development
(Osorio
et al.
, 2011). The results suggested
that neither transcription nor the protein
content of structural cell-wall proteins
changed much during ripening. None the
less, it is possible that structural proteins
in the cell wall are simply modifi ed, which
might affect softening, but this remains to
be determined.
4.3.5 Other factors (pH, ionic composition,
synthesis and non-uniformity)
The pH of most cell walls is typically
between pH 6 and 7 (Almeida and Huber,
1999). When fruit ripen, the pH of the
tomato cell-wall fl uid (apoplast) drops
from 6.5 in mature green fruit to pH 4.5 in
ripe fruit (Almeida and Huber, 1999). A
drop in the apoplastic pH may be common
during fruit ripening. Changes in pH can
affect enzyme activity (Chun and Huber,
1998) and possibly ionic interactions with
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