Molecular biology of grape berry ripening

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Abstract
Grapevines produce non-climacteric fruit that exhibit a double sigmoidal pattern of growth. Ripening occurs during the second growth phase when grapes change colour, start to soften, accumulate reducing sugars, metabolise organic acids and synthesise flavour compounds. Unlike many other fruit, grapes ripen while the berries are still expanding, and as with most non-climacteric fruit, ripening does not appear to be controlled by ethylene. Sugars and amino acids that accumulate in grapes during ripening are imported via the phloem, while many secondary metabolites are synthesised within the berry itself. Grapes import sucrose but accumulate hexoses. Conversion of sucrose to hexoses is most likely catalysed by invertase. cDNAs encoding vacuolar invertases have been isolated from grape berries. Expression of these genes and an increase in invertase activity occur before veraison, so it seems unlikely that synthesis of this enzyme is a controlling factor for sugar accumulation during ripening. Proteins that transport sugars into the berry vacuole may regulate sugar accumulation, and cDNAs encoding both sucrose and hexose transporters have been isolated from ripening grape berries. Determination of the role of these transporters may reveal the pathway of sugar accumulation in grapes. Anthocyanins are only synthesised in the skin of red grapes after veraison. Analysis of the patterns of expression of genes in the flavonoid pathway has shown that there is a dramatic increase in expression of many of these genes in skin cells at veraison. Expression of the gene encoding a glycosyl transferase involved in the lasts steps of anthocyanin synthesis was absolutely correlated with anthocyanin synthesis and may explain the lack of anthocyanin synthesis in white grapes and in the flesh of most red grapes. We infer that the synthesis of anthocyanins is regulated at the transcription level and is likely to be controlled by regulatory genes. Softening of fruit generally results from changes in the properties of cell walls. Analysis of the cell walls of grapes during ripening suggests that there are no dramatic changes in polysaccharide composition but modification of specific components may contribute to softening. A number of proteins are newly synthesised in grapes during ripening and several of these proteins have now been identified. The most abundant are pathogenesis-related (PR) proteins, including chitinases and thaumatin-like proteins. Expression of genes encoding a number of PR proteins increased dramatically in grapes during ripening. It is not clear what role the PR proteins play during ripening but they may provide resistance to pathogens. Differential screening of a post-veraison grape berry cDNA library has also identified ripening-related genes, some of which encode proline-rich cell wall proteins. Other grape ripening-related genes have homologues that are induced by stress in other plants. These studies indicate that a dramatic change in gene expression occurs in grape berries at veraison and suggest that ripening involves a coordinated increase in transcription of a number of different genes.