Source/Sink Relationships and Molecular Biology of Sugar Accumulation in Grape Berries

Phloem transport of assimilates provides the materials needed for the growth and development of reproductive structures, storage and developing organs, and has long been recognised as a major determinant in crop yield. Thus, the understanding of the mechanisms and regulations of sugar transport into sink tissues has an important basic and applied relevance. The Grapevine is a good example of a crop where sugar accumulation in the fruit has an important economic role. Massive sugar transport and compartmentation into the grape berry mesocarp cells (up to 1 M glucose and fructose) start at veraison and continues until the harvest. Sucrose transported in the phloem is cleaved into hexoses by invertases and stored in the vacuole. The Sugar content determines the sweetness of table grapes, wine alcohol content, and regulates gene expression, including, for example, several genes involved in the synthesis of secondary compounds which contribute to grape and wine quality. Many viticultural practices affect source/sink relationships, thus altering sugar concentration in the berry. For instance, the rootstock used, which is a potential sink, has a strong impact on source activity, by affecting the morphology and activity of the aerial part of the plant. Molecular approaches have also provided major advances in grapevine research. Monosaccharide and disaccharide transporter genes have been recently identified and their products studied in heterologous systems. The sequencing of the grapevine genome and the development of grape microarrays have made a valuable contribution to the study of the biochemistry of grape berry development and ripening, for example, low affinity glucose uniporters identified in the genome may also be involved in the sugar uptake. In the present chapter, the routes of sugar import and storage in the grape cells are updated and discussed and a model with the main transport steps and biochemical pathways is proposed.

Keywords: Assimilates, Disaccharide transporter, Fructose, Glucose, Invertase, Monosaccharide transporter, Phloem transport, Phloem unloading, Sucrose, Sugars, Veraison.