Differing patterns of pectin disassembly and pericarp softening in ripening and chill-injured tomato fruit with downregulated polygalacturonase activity

Pectin disassembly was examined in transgenic tomato fruit with reduced polygalacturonase (PG). Mature-green tomato fruit were stored at 5 and 12C for 21 days and subsequently transferred to 18C for an additional 5 days. Pericarp softened during storage at 12C and the softening rate increased after fruit were transferred to 18C. Firmness remained unchanged during storage at 5C but decreased after transfer of chilled fruit to 18C. Water-soluble pectins increased five- and ninefold during ripening of PG-antisense and wild-type fruit, respectively, while trans-1,2-cyclohexanediaminetetraacetic acid-soluble pectins increased only slightly. Solubilization of pectins was very restricted in fruit stored at 5C and upon subsequent transfer to 18C. Pectins exhibited a PG-mediated reduction in molecular mass during ripening at 12C, but no depolymerization was observed in chill-injured fruit. Chill-injured tomato fruit softened even though pectin solubilization and molecular mass distribution remained unchanged, suggesting that the softening mechanism differs in ripening and chill-injured tomato fruit.