Primary Cell Wall Metabolism during Fruit Ripening*

Fruit softening is an important part of the ripening process, and involves changes to cell turgor and primary cell wall structure. Most of the polysaccharide components of the cell wall are subjected to some degree of controlled degradation, resulting in a loosening and swelling of the wall structure, a weakening of cell wall strength, and reduced intercellular adhesion. Early ripening changes involve the degradation of the galactan/ arabinan side chains of rhamnogalacturonan-I, demethylesterification of homogalacturonan, and depolymerisation of matrix glycans (hemicelluloses). Solubilisation of pectins increases during ripening, but depolymerisation of pectins is usually most pronounced late in ripening. Considerable variation in the extent of pectin depolymerisation and galactan/arabinan loss exists between species. Transgenic studies have shown that expansin may control cell wall loosening, and β-galactosidase may be important in increasing cell wall porosity. Suppression of either of these enzymes resulted in a retention of fruit firmness. Suppression of endo-polygalacturonase and pectin methylesterase had little effect on fruit firmness during ripening, but influenced fruit shelf life due to alterations in the integrity of the middle lamella, which affected intercellular adhesion. The enzyme(s) responsible for depolymerisation of matrix glycans have not been defined, and the identity of the ripening-related xyloglucanase remains obscure.