POLYGALACTURONASE GENE-EXPRESSION IN RIPE MELON FRUIT SUPPORTS A ROLEFOR POLYGALACTURONASE IN RIPENING-ASSOCIATED PECTIN DISASSEMBLY

Ripening-associated pectin disassembly in melon is characterized by a decrease in molecular mass and an increase in the solubilization of po lyuronide, modifications that in other fruit have been attributed to t he activity of polygalacturonase (PC). Although it has been reported t hat PC activity is absent during melon fruit ripening, a mechanism for PC-independent pectin disassembly has not been positively identified. Here we provide evidence that pectin disassembly in melon (Cucumis me lo) may be PG mediated. Three melon cDNA clones with significant homol ogy to other cloned PGs were isolated from the rapidly ripening cultiv ar Charentais (C. melo cv Reticulatus F1 Alpha) and were expressed at high levels during fruit ripening. The expression pattern correlated t emporally with an increase in pectin-degrading activity and a decrease in the molecular mass of cell wall pectins, suggesting that these gen es encode functional PGs. MPG1 and MPG2 were closely related to peach fruit and tomato abscission zone PGs, and MPG3 was closely related to tomato fruit PG. MPG1, the most abundant melon PG mRNA, was expressed in Aspergillus oryzae. The culture filtrate exponentially decreased th e viscosity of a pectin solution and catalyzed the linear release of r educing groups, suggesting that MPC1 encodes an endo-PG with the poten tial to depolymerize melon fruit cell wall pectin. Because MPC1 belong s to a group of PGs divergent from the well-characterized tomato fruit PG, this supports the involvement of a second class of PGs in fruit r ipening-associated pectin disassembly.