Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 55

1999; Cosgrove et al. , 2002). However,
based on the effect of expansins on plant
tissues and cellulose paper, it was
concluded that expansins act on hydrogen
bonding at the interface of cellulose micro-
fi brils (McQueen-Mason and Cosgrove,
1994). If the tethering of microfi brils to
each other is loosened during ripening, a
marked effect on both fi rmness and texture
might be expected. Expansins are a family
of genes in tomato and several are
expressed in fruit, but one in particular
(EXP1) increases specifi cally during ripen-
ing (Rose et al. , 1997, 2000; Brummell et
al. , 1999c). To examine a possible role for
EXP1 in fruit softening its expression was
both suppressed and enhanced in tomato
fruit (Brummell et al. , 1999b). Suppression
of EXP1 caused a signifi cant increase in
fruit fi rmness measured at all stages of
ripening fruit including over-ripe fruit.
Also of interest is that when EXP1 was
overexpressed throughout fruit develop-
ment, the fruit were less fi rm, even at the
mature green stage. Fruit where EXP1 was
overexpressed were also smaller and had
what was described as a 'rubbery texture'.
Thus, results with EXP1 are consistent
with expansins affecting the tethering of
cellulose microfi brils to each other.

glucan polymers can be broken and
rejoined, it is possible for cellulose micro-
fi brils to slide past one another during cell
expansion without evidence of xyloglucan
depolymerization. Nevertheless, it has
been demonstrated that xyloglucans are
fragmented (depolymerized) during ripen-
ing (Brummell, 2006). In tomato, 25 XTHs
have been identifi ed (Ohba et al. , 2011)
and at least ten are expressed in ripe fruit,
but only two appeared to increase with
ripening whilst most of the others
decreased during ripening (Miedes and
Lorences, 2009). Overexpression of XTH1
during fruit development actually caused
a slight decrease in depolymerization of
xyloglucans during ripening (Miedes et
al. , 2010). These researchers concluded
that the increased endotransglucosylase
activity over the hydrolase activity in the
XTHs was responsible for the lower
xyloglucan depolymerization in fruit and
suggested that the role of XTHs during
fruit growth and ripening might be to
maintain the structural integrity of the cell
wall. They suggested that a decrease in
XTH activity, rather than an increase,
during ripening might contribute to fruit
softening. A separate group both over-
expressed and inhibited expression of
XTH1 in tomato and found that the level
of expression correlated directly with fruit
size (Ohba et al. , 2011). These researchers
concluded that XTH1 clearly plays a role
in cell expansion in fruit development,
but they did not identify a clear link to
softening; nevertheless, they postulated
that a spike in XTH activity at the turning
stage early in the ripening process might
be linked to xyloglucan depolymerization
and a decrease in fi rmness. Of interest in
this regard is recent research using
carbohydrate-specifi c antibodies, which
showed that hemicellulose-like polymers
are also associated with the cell adhesion
layer (middle lamella) and that these
polymers tend to disappear in ripe tomato
fruit (Ordaz-Ortiz et al. , 2009). Currently, it
is not clear how important the loss of
hemicelluloses in the adhesion layer is to
fruit softening.

4.3.3 Xyloglucans

Expansins may affect the tethering of
cellulose microfi brils, but it is proposed that
xyloglucans are the polysaccharide sub-
strate that cross-links and ties the
microfi brils together (Carpita and Gibeaut,
1993). Modifi cation or depolymerization of
the cross-linking xyloglucan network might
also be expected to affect fruit softening.
Xyloglucanases, like cellulase and ex-
pansins, are another interesting family of
enzymes. In addition to hydrolysing xylo-
glucan polymers, most of these enzymes
also appear to be able to join together two
xyloglucan polymers. This family of
enzymes have been called xyloglucan
endotransglucosylase/hydrolases (XTHs)
(Rose et al. , 2002). Thus, because xylo-