Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE BREAKDOWN OF CELL WALL COMPONENTS 173
8.8 Enzymatic regulation of galactose loss during ripening
During ripening, fruits exhibit a large decrease in galactose from their cell wall polymers
(Gross and Sams, 1984). In tomato, the decline in galactose starts early and increases
with ripening, and occurs primarily from pectic fraction with matrix glycan and cellulose
fractions showing only a slight loss (Gross, 1984; Seymour et al., 1990). Most of the galactose
is a part of the side chains attached to rhamnose of the rhamnogalacturonan (RG-I)
backbone and is linked either in type I (1→4)β-D-galactan chains or in type II branched
(1→3),(1→6)β-D-galactan chains (Carpita and Gibeaut, 1993). It is the (1→4)β-D-galactan
that is depolymerized during ripening of tomato fruit (Seymour et al., 1990). Due to the
lack of endogalactanases in higher plants, the exo-β-D-galactosidase (EC 3.2.1.23) enzyme
activity has been implicated in depolymerization of β-galactan present in cell wall. Exo-β-
D-galactosidase catalyzes hydrolysis of the terminal nonreducing β-D-galactosyl residues
from β-D-galactosides. Multiple forms of β-D-galactosidase are present in fruits. Among
the three forms designated as β-galactosidase I, II, and III, only the β-galactosidase II is
active against a (1→4)β-D-galactan-rich polymer prepared from tomato cell walls (Pressey,
1983). It also exhibits activity against a variety of galactoside substrates indicating a
β-D-galactosidase/exogalactanase activity (Smith and Gross, 2000). Whereas β-
galactosidase I and III predominate in green fruit tissue, β-galactosidase II is a ripeningregulated
activity that increases over sevenfold during tomato fruit ripening (Pressey, 1983;
Carey et al., 2001).
The tomato genome contains at least seven genes that encode β-D-galactosidase that
are designated as TBG1 to TBG7 (Smith et al., 1998; Smith and Gross, 2000; Carey et al.,
2001). Transcripts of all members of family are detected in tomato fruit and other plant
tissues but expression of TBG1, 3, 4, and 5 increases during ripening. Among them the
transcript abundance of TBG1 and TBG3 remained low and unchanged during ripening
whereas the levels of TGB4 and 5 mRNAs continue to increase until the turning stage of
ripening before declining. TBG6 mRNA exhibits highest abundance in fruit tissue but is
undetectable in the ripening fruit. The nonsoftening tomato mutants rin and nor, that exhibit
much reduced loss of “ripening-related” cell wall Gal compared to wild-type fruit (Gross,
1984), show absence of ripening-related rise in β-galactanase (β-galactosidase II) activity
(Carey et al., 1995). These mutants show a marked reduction in TGB4 transcripts in fruits,
whereas the levels of TBG1, 3, and 5 are similar to wild type (Moctezuma et al., 2003).
Interestingly, the mRNA of TBG6 continues to be present in rin and nor fruits of the same
chronological age as wild-type ripening fruit (Smith and Gross, 2000). The deduced amino
acid sequence of TBG4 corresponds to the protein termed as β-galactosidase II (Smith
et al., 1998). Taken together these results suggest that TBG4, which is ca. 100 amino acids
shorter at the carboxyl terminal end than other TBGs and encodes a predicted polypeptide of
78 kDa, is responsible for the Gal loss from cell wall during fruit ripening (Smith and Gross,
2000).
8.9 Enzymatic solubilization of other polysaccharide components
Multiple enzymes are required to disassemble higher-order structural components of the
fruit cell wall. Structural modifications in hemicellulose–cellulose domains may facilitate
entry of enzymes into the ripening fruit cell wall. These enzymes include xyloglucan