Reduction of Tomato Polygalacturonase p Subunit Expression Affects Pectin Solubilization and Degradation during Fruit Ripening

The developmental changes that accompany tomato fruit ripening include increased solubilization and depolymerization of pectins due to the action of polygalacturonase (PG). Two PG isoenrymes can be extracted from ripe fruit: PG2, which is a single catalytic PG polypeptide, and PG1, which is composed of PG2 tightly associated with a second noncatalytic protein, the p subunit. Previous studies have correlated ripening-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 p subunit and PG1 in pectin metabolism. To assess the function of the p subunit, we produced and characterized transgenic tomatoes constitutively expressing a p subunit antisense gene. Fruit from antisense lines had greatly reduced levels of p subunit mRNA and protein and accumulated <1% of their total extractable PG activity in ripe fruit as PG1, as compared with 25% for wild type. lnhibition of p subunit expression resulted i n significantly elevated levels of EDTAsoluble polyuronides at all stages of fruit ripening and a significantly higher degree of depolymerizationat later ripening stages. Decreased p subunit protein and extractable PG1 enzyme activity and increased pectin solubility and depolymerization all cosegregated with the p subunit antisense transgene in T2 progeny. These results indicate (1) that PG2 is responsible for pectin solubilization and depolymerization in vivo and (2) that the p subunit protein is not required for PG2 activity in vivo but (3) does play a significant role in regulating pectin metabolism in wild-type fruit by limiting the extent of pectin solubilization and depolymerization that can occur during ripening. Whether this occurs by direct interaction of the p subunit with PG2 or indirectly by interaction of the p subunit with the pectic substrate remains to be determined.