Agriculture Reference
In-Depth Information
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 galac-
tose is a part of the side chains attached to rhamnose of the rhamnogalacturonan (RG-I)
backbone and is linked either in type I (1
→
β
4)
-
D
-galactan chains or in type II branched
(1
-
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-
→
3),(1
→
6)
β
-
D
-galactan chains (Carpita and Gibeaut, 1993). It is the (1
→
4)
β
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
-
D
-galactan-rich polymer prepared from tomato cell walls (Pressey,
1983). It also exhibits activity against a variety of galactoside substrates indicating a
β
→
4)
β
-
D
-galactosidase/exogalactanase
activity
(Smith
and
Gross,
2000).
Whereas
β
-
β
galactosidase I and III predominate in green fruit tissue,
-galactosidase II is a ripening-
regulated 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
-
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
TBG
1, 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.
TBG
6 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
-galactosidase II) activity
(Carey et al., 1995). These mutants show a marked reduction in TGB4 transcripts in fruits,
whereas the levels of
TBG
1, 3, and 5 are similar to wild type (Moctezuma et al., 2003).
Interestingly, the mRNA of
TBG
6 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
TBG
4 corresponds to the protein termed as
β
-galactanase (
β
-galactosidase II (Smith
et al., 1998). Taken together these results suggest that
TBG
4, 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).
β
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 facili-
tate entry of enzymes into the ripening fruit cell wall. These enzymes include xyloglucan
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