Cell-wall Metabolism and Softening during Ripening - Fruit Ripening: Physiology, Signalling and Genomics - page 50

2009). In recent years, considerable
research has gone into developing modifi ed
atmosphere packaging that reduces
mechanical damage and water loss, and
creates a gaseous environment that limits
oxygen exchange but not so much as to
create anaerobic conditions within the fruit
(Rojas-Graü et al. , 2009; Sandhya, 2010;
Singh, 2011).
Of particular interest in regard to water
relationships is recent research on a tomato
variety that has a long shelf-life (remains
fi rm and palatable for an extended period
of time) (Saladie et al. , 2007). The authors
of this study discovered that the long shelf-
life was due to a mutation that altered the
cuticle that surrounds and protects the
fruit. This is of interest because it suggests
an avenue for engineering or breeding that
can have profound effects on water loss
and maintenance of fi rm fruit.
None the less, the cell wall must ripen
along with the rest of the fruit. What is
cell-wall ripening? It is changes in the
cell wall that make the fruit desirable -
the perfect fi rmness and texture. But
desirability depends on which fruit you are
eating. An apple or pear is not the same as
a grape, blackberry, banana or tomato.
However, there are some general concepts
about cell structure and cell-wall metab-
olism that can be applied to many fruits.

to the cell (Carpita and Gibeaut, 1993).
However, as mentioned above, there must
be a contiguous gas space between cells to
sustain adequate exchange of gases. The
gas space can, however, vary widely
depending on the fruit; for example, in
pear it is ~5% (Verboven et al. , 2008), in
apple ~23% (Drazeta et al. , 2004; Verboven
et al. , 2008) and in tomato ~6% (Calbo and
Nery, 1995). It is at this early step in cell
division and primary cell-wall synthesis
that the air spaces form. The air space
forms at the corners where three or four
cells make contact with each other. When
the middle lamella joins with the middle
lamella from an existing cell, this region
(corner) is modifi ed. Carbohydrate-specifi c
antibodies have been used to demonstrate
that this region of the middle lamella
contains more unesterifi ed homogalact-
uronans (pectin polymers) than the middle
lamella that binds adjacent cells together
(Ordaz-Ortiz et al. , 2009). Although the
airspace at these corners may change in
volume during fruit development, a con-
tiguous gas space must be maintained
during cell expansion and ripening (Ho et
al. , 2011).
A typical primary cell wall of a dicot
cell consists of 50% cellulose/xyloglucan,
30% pectic polysaccharides and 20%
structural proteins (Carpita and Gibeaut,
1993). Polymers of cellulose (1,4- E -glucans)
are twisted together to form a cellulose
microfi bril (Carpita and Gibeaut, 1993). A
single microfi bril can include several dozen
glucan polymers twisted together to form a
very strong thread that is not easily
stretched or broken, yet is fl exible (Carpita
and Gibeaut, 1993). The direction in which
the microfi brils are synthesized and laid
down in the cell wall largely determines
the direction in which the cell can expand.
For example, if the microfi brils are laid
down in a helical fashion around the cell,
the cell expands primarily in the
longitudinal direction, much like stretching
a spring (Carpita and Gibeaut, 1993). A
helical orientation of microfi brils would be
the structure found in elongating cells
immediately behind the stem or root
meristem. If the microfi brils are laid down

4.2 Cell-wall Structure

Before discussing how the cell wall
changes during ripening, we must know
something about how it formed. When
cells divide to increase cell numbers, a cell
plate forms between the dividing cells
during a process called cytokinesis (Allen,
1901; Carpita and McCann, 2000). The cell
plate will become the middle lamella - the
glue that binds cells together. The middle
lamella consists primarily of pectins. Very
little or no cellulose is found in the middle
lamella (Allen, 1901; Carpita and McCann,
2000). Soon after the formation of the cell
plate, the primary cell wall is synthesized
on either side of the plate. The primary cell
wall provides structure and tensile strength