Contrasted responses to carbohydrate limitation in tomato fruit at two stages of development

The metabolic consequences of long-term carbohydrate depletion have been well documented in many sink organs but not extensively in fruit. Therefore, in the present study the response to sugar limitation in tomato fruit (Lycopersicon esculentum Mill.) was investigated at two developmental stages; during the cell division and cell expansion phases. First, the response in excised fruit cultured in vitro was characterized. Sugar depletion caused an arrest of growth and an exhaustion of carbon reserves. The proteins that were degraded and the nitrogen released was transiently stored as asparagine and glutamine in both developmental stages and also as gamma-aminobutyric acid (GABA) in expanding fruit. Fruit at the cell division stage appeared to be more sensitive to sugar limitation. The response to sugar depletion was then characterized in fruit from plants submitted to extended darkness. In planta, the effects of sugar-limitation were similar to those described in vitro but much more attenuated, especially in expanding fruit, which still accumulated dry matter. The expression of cell cycle genes, sugar- and nitrogen-related genes was reduced by darkness. Only asparagine synthetase gene expression was induced in both dark-treated fruit. Together the present data revealed that the effects of the carbon limitation are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit.