Ultrastructure of Tomato Fruit Ripening and the Role of Polygalacturonase Isoenzymes in Cell Wall Degradation

Ultrastructural changes in the pericarp of tomato (Lycopersicon esculentum Mill) fruit were followed during ripening. Ethylene production was monitored by gas chromatography and samples analyzed at successive stages of the ripening process.Changes in the cytoplasmic ultrastructure were not consistent with the suggestion that ripening is a ;senescence' phenomenon. A large degree of ultrastructural organization, especially of the mitochondria, chromoplasts, and rough endoplasmic reticulum, was retained by ripe fruit.Striking changes in the structure of the cell wall were noted, beginning with dissolution of the middle lamella and eventual disruption of the primary cell wall. These changes were correlated with appearance of polygalacturonase (EC 3.2.1.15) isoenzymes. Application of purified tomato polygalacturonase isoenzymes to mature green fruit tissue duplicated the changes in the cell wall noted during normal ripening. Possible roles of the polygalacturonase isoenzymes in cell wall disorganization are discussed.