REDUCTION OF TOMATO POLYGALACTURONASE BETA-SUBUNIT EXPRESSION AFFECTSPECTIN SOLUBILIZATION AND DEGRADATION DURING FRUIT RIPENING

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.