Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 27
isoform of PG. The increase in activity of PG1 is related to the rate of pectin solubilization
and tomato fruit softening during the ripening process.
Research into the understanding of the regulation of biosynthesis and activity of PG
using molecular biology tools has resulted in the development of strategies for enhancing
the shelf life and quality of tomatoes. PG mRNA is one of the first ripening-related mRNAs
isolated from tomato fruits. All the different isoforms of PGs are encoded by a single gene.
The PG cDNA which has an open reading frame of 1,371 bases encodes a polypeptide having
457 amino acids, that includes a 24-amino acid signal sequence (for targeting to the cell wall
space) and a 47-amino acid prosequence at the N-terminal end, which are proteolytically
removed during the formation of the active PG isoforms. A 13-amino acid long C-terminal
peptide is also removed resulting in a 373-amino acid long polypeptide, which undergoes
different degrees of glycosylation resulting in the PG2a and PG2b isozymes. Complex
formation between PG2a, PG2b, and the 38-kDa subunit in the apoplast results in the PG1
isozyme (Grierson et al., 1986; Bird et al., 1988). In response to ethylene treatment of mature
green tomato fruits, which stimulates ripening, the levels of PG mRNA and PG are found
to increase. These changes can be inhibited by treating tomatoes with silver ions, which
interfere with the binding of ethylene to its receptor and initiation of ethylene action (Davies
et al., 1988). Thus, there is a link between ethylene, PG synthesis, and fruit softening.
Genetic engineering of tomato with the objective of regulating PG activity has yielded
complex results. In the rin mutant of tomato, which lacks PG and does not soften, introduction
of a PG gene resulted in the synthesis of an active enzyme; however, this did not
cause fruit softening (Giovannoni et al., 1989). As a corollary to this, introduction of the
PG gene in the antisense orientation resulted in near total inhibition of PG activity (Smith
et al., 1988). In both these cases, there was very little effect on fruit softening, suggesting
that factors other than pectin depolymerization may play an integral role in fruit softening.
Further studies using tomato cultivar such as UC82B (Kramer et al., 1992) showed that
antisense inhibition of ethylene biosynthesis or PG did indeed result in lowered PG activity,
improved integrity of cell wall, and increased fruit firmness during fruit ripening. As
well, increased activity of pectin methylesterase, which removes the methyl groups from
esterified galacturonic acid moieties, may contribute to the fruit softening process.
The activities of pectin-degrading enzymes have been related to the incidence of physiological
disorders such as “mealiness” or “wooliness” in mature unripened peaches that
are stored at a low temperature. The fruits with such a disorder show a lack of juice and a
dry texture. Deesterification of pectin by the activity of pectin methyl esterase is thought
to be responsible for the development of this disorder. Pectin methyl esterase isozymes
with relative molecular masses in the range of 32 kDa have been observed in peaches, and
their activity increases after 2 weeks of low-temperature storage. Polygalacuronase activity
increases as the fruit ripens. The ripening fruits that possess both polygalacturonase and
pectin methyl esterase do not develop mealy symptoms when stored at low temperature,
implicating the potential role of pectin degradation in the development of mealiness in
peaches.
There are two forms of polygalacturonases in peaches: the exo- and endopolygalacturonases.
The endopolygalacturonases (endo-PG) are the predominant forms in the freestone
type of peaches, whereas the exopolygalacturonases (exo-PG) are observed in the mesocarp
of both freestone and clingstone varieties of peaches. As the name implies, exopolygalacturonases
remove galacturonic acid moieties of pectin from the terminal reducing end of the