In vitro characterization of tomato fruit softening : the use of enzymically active cell walls

Cell wall isolated from pericarp of normal tomato (Lycopersicon esculentum Mill. cv `Rutgers') fruit released pectic polymers in a reaction apparently mediated by wall-bound polygalacturonase that appears with the onset of ripening. Release was negligible in wall preparations from normal green and the ripening mutant rin fruit. Pectin solubilization was most extensive at pH 2.5 with a less significant peak at 5.5. Brief exposure to low (1.5) or high (7.5) pH resulted in reduction of autolytic activity, which was also inhibited by high temperature, Ca²⁺, and treatments employed to dissociate protein from cell wall. Uronic acid solubilization was significantly enhanced by 150 millimolar NaCl and by increasing temperature within the physiological range. These data indicate that the release of polyuronide from isolated cell walls is enzymic and may provide a convenient and reliable system for the study of softening metabolism.