Ultrastructure of Sweet Orange Ripening Fruit (Citrus sinensis L.Osbeck) and the Role of Hydrolases in Dietary Fiber Degradation

摘要

Sweet orange (Citrus sinensis L. Osbeck) fruit is an important source of dietary fiber, which consists of a variety of non starch polysaccharides. The change of dietary fiber and polysaccharides leading to fruit softening is complex, which includes the coordinated and interdependent activities of a range of cell wall modifying proteins. Polygalacturonase (PG) activity is largely responsible for pectin depolymerization and solubilization, but PG-mediated pectin depolymerization requires pectin to be de-methylesterified by Pectinesterase (PE). All these pectinmodifying proteins affect the integrity of the middle lamella, which controls cell-to-cell adhesion and thus influences fruit texture. In contrast, the primary cell wall changes caused by the activities of β-galactosidase (β-Gal) and cellulase (Cx) early in ripening may restrict or control the activities of other ripening-related enzymes necessary for the fruit softening process. Taken together, we could consider that the changes of dietary fibre concentration and composition were caused by the solubilisation and depolymerisation of pectins in middle lamella and the disintegration of the primary cell wall. The data described above show that the disassembly of sweet orange fruit cell wall components during fruit ripening was brought about by a range of enzyme activities, and the presence of these mRNAs, and enzymic activities were in accordance with enzyme action in the living plant cell.