Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 51

in a more random pattern, the cell may
expand in all directions, called isodia-
metric expansion. The cellulose micro-
fi brils give the cell wall its tensile strength
(Carpita and Gibeaut, 1993). However, to
provide structure to the cell, the micro-
fi brils must be tethered to one another. The
polysaccharides that provide this function
are often called hemicelluloses and consist
primarily of xyloglucans (Brummell, 2006).
The cellulose microfi brils and tethering
hemicelluloses are both embedded in a
pectin matrix. The pectin fraction consists
primarily of polygalacturonic acid (homo-
galacturonans) and rhamnogalacturans
(Carpita and Gibeaut, 1993; Brummell,
2006). Pectins add strength to the cell wall
and determine pore size in the cell wall,
which may limit the access of enzymes to
target sites on the xyloglucan matrix and
other polymers (Carpita and Gibeaut, 1993).
The unesterifi ed charged ends of the
individual subunits in polygalacturonic
acid are often cross-linked by calcium ions
to form a stable egg-box-like structure
(Carpita and Gibeaut, 1993). The avail-
ability of calcium and esterifi cation of
polygalacturonic acid with methyl or acetyl
groups limits cross-linking by calcium,
which can in turn affect the structural
properties of the cell wall. Proteins are
another major fraction of cell walls (Carpita
and Gibeaut, 1993). There are several types
of cell-wall proteins, but the bulk of the
protein is made up of extensins -
hydroxyproline-rich glycoproteins (Carpita
and Gibeaut, 1993). These proteins coalesce
into rod-shaped structures that add strength
to the cell wall and may play a role in cell-
wall assembly, cell shape and formation of
the intercellular spaces (Carpita and
Gibeaut, 1993; Carpita and McCann, 2000).
The rod-shaped protein structures may also
anchor the plasma membrane to the cell
wall (Knox, 1995).

research on fruit ripening in the early part
of the 20th century focused on respiration.
In 1925, Kidd and West reported that
apples underwent an increase in
respiration during ripening, which they
called the climacteric (Laties, 1995). At
approximately the same time, it was
discovered that a gaseous substance
emitted by a ripe apple could stimulate the
ripening of an unripe apple, and that
exposure to ethylene could induce a
similar response (Laties, 1995). In 1934,
Gane published a paper demonstrating that
some fruit produce ethylene. Assaying for
changes in enzyme activity during ripening
followed as these tools became available.
This included assays for enzymes known
to affect the cell wall. In 1979, several
articles were published describing an
increase in cellulase and polygalacturonase
(PG) activity in apples and pears (Ben-Arie
and Kislev, 1979), avocado (Awad and
Young, 1979) and tomato (Poovaiah and
Nukaya, 1979). In apples and pears, it was
also noted that, at advanced stages of
ripening, the middle lamella was clearly
degraded and the orderly arrangement of
microfi brils was lost (Ben-Arie and Kislev,
1979). Changes in cell-wall morphology
appeared to fi t with the rise in cellulase
and PG activity. With the advent of tools
for cloning mRNA, it was not too long
before cellulase and PG were cloned from
avocado and tomato and their expression
correlated with changes in enzyme activity
(Christoffersen et al. , 1984; Grierson et al. ,
1986; Kutsunai et al. , 1993; Lashbrook et al. ,
1994). Clones for PG, cellulases, pectin
esterases and xyloglucanases in many
different fruit soon followed (Marin-
Rodriguez et al. , 2002; Rose et al. , 2002,
2004; Brummell, 2006; Vicente et al. , 2007;
Bapat et al. , 2010). With improved tech-
niques (Bräutigam et al ., 2011; Osorio et al .,
2011), we now have the tools to sequence
and identify expression patterns for literally
hundreds of different cell-wall proteins in
practically any fruit we want to examine.
However, expression data alone is
simply correlative and does not really tell
us how the individual proteins function
during fruit softening. The fact that the

4.3 Cell-wall Metabolism during
Ripening

The tools available to a scientist determine
the experiments they can perform. Thus,