Cf. Watson et al., REDUCTION OF TOMATO POLYGALACTURONASE BETA-SUBUNIT EXPRESSION AFFECTSPECTIN SOLUBILIZATION AND DEGRADATION DURING FRUIT RIPENING, The Plant cell, 6(11), 1994, pp. 1623-1634
The developmental changes that accompany tomato fruit ripening include
increased solubilization and depolymerization of pectins due to the a
ction of polygalacturonase (PG). Two Po isoenzymes can be extracted fr
om ripe fruit: PG2, which is a single catalytic PG polypeptide, and PG
1, which is composed of PG2 tightly associated with a second noncataly
tic protein, the beta subunit. Previous studies have correlated ripeni
ng-associated increases in pectin solubilization and depolymerization
with the presence of extractable PG1 activity, prior to the appearance
of PG2, suggesting a functional role for the beta subunit and PG1 in
pectin metabolism. To assess the function of the beta subunit, we prod
uced and characterized transgenic tomatoes constitutively expressing a
beta subunit antisense gene. Fruit from antisense lines had greatly r
educed levels of beta subunit mRNA and protein and accumulated <1% of
their total extractable Po activity in ripe fruit as PG1, as compared
with 25% for wild type. inhibition of beta subunit expression resulted
in significantly elevated levels of EDTA-soluble polyuronides at all
stages of fruit ripening and a significantly higher degree of depolyme
rization at later ripening stages. Decreased beta subunit protein and
extractable PG1 enzyme activity and increased pectin solubility and de
polymerization all cosegregated with the beta subunit antisense transg
ene in T-2 progeny. These results indicate (1) that PG2 is responsible
for pectin solubilization and depolymerization in vivo and (2) that t
he beta subunit protein is not required for PG2 activity in vivo but (
3) does play a significant role in regulating pectin metabolism in wil
d-type fruit by limiting the extent of pectin solubilization and depol
ymerization that can occur during ripening. Whether this occurs by dir
ect interaction of the beta subunit with PG2 or indirectly by interact
ion of the beta subunit with the pectic substrate remains to be determ
ined.