Résumé
Fruit softening during ripening is accompanied by changes in cell wall composition due to the action of cell wall modifying enzymes. Moreover, the cell walls of grape berries form a barrier to the diffusion of aromatic and polyphenolic compounds which are important for wine quality. Samples of grape berries (
Vitis vinifera L., cv Ugni blanc) were harvested in 1996 and 1997 at twelve different developmental stages. The development of berries was characterized by physical, chemical and biochemical analysis. Isolated cell walls were analysed for their neutral sugar contents. The main changes during grape berry development were a large decrease in galactose parallel with glucose accumulation, while other neutral sugars (arabinose, rhamnose, xylose, fucose and mannose) showed no significant variations. For individual berries, galactose loss seemed to be softening-related, while galactose removal per mg of cell wall material was involved in a more general ripening process. β-Galactosidase (EC 3.2.1.23) activity was temporally associated with the loss of cell wall-linked galactosyl residues. A 545-base long partial cDNA (
ϐ-gal10, accession No. AF159124, GenBank) was isolated from first strand cDNA, and shared significant similarities with several β-gals in data banks. The pattern of transcript expression showed that
β-gal10 was only detectable in the early stages of development, suggesting that
β-gal10 may encode for a β-galactosidase active on cell walls during the early development of grape berries. Relationships between galactose content of the cell wall, β-galactosidase activity and expression of the corresponding transcripts, and their possible involvement in grape berry softening and ripening are discussed.