DIFFERING PATTERNS OF PECTIN DISASSEMBLY AND PERICARP SOFTENING IN RIPENING AND CHILL-INJURED TOMATO FRUIT WITH DOWNREGULATED POLYGALACTURONASE ACTIVITY

ABSTRACT 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. PRACTICAL APPLICATIONS Chilling injury poses serious problems in the supply chain of sensitive commodities. Abnormal textural changes are one of the chilling-injury symptoms described in sensitive fruit, including tomato, but the mechanism for the accelerated softening rate and development of mealiness is not yet fully understood. Although ripening-related softening and pectin metabolism have been extensively characterized, the pectic metabolism in chill-injured fruit remains elusive. Transgenic plants with altered enzyme expression are excellent tools to test hypothesis regarding the physiological role of the enzymes. Because polygalacturonase (PG) expression is inhibited during low-temperature storage, it is possible that, at least in part, the differences in textural changes between chill-injured and normally ripening fruit are related to this enzyme. The data presented here indicate that PG plays an even smaller role in the softening of chilled fruit than in that of ripening fruit. Tomato can provide a model for the understanding of the relationships between cell wall metabolism and texture that applies to other chilling-sensitive fruit.