Sugar Metabolism and Fruit Development in the Tomato

Sugars are strongly related to fruit yield and quality, playing a critical role in fruit set, growth, ripening, and composition. The tomato (Solanum lycopersicum) is not only an important horticultural crop, but also a useful experimental model plant that can be used to further our understanding of fruit physiology. Therefore, in this review, we consider sugar metabolism and fruit development in the tomato. We begin by discussing how the sugar content of tomato fruit has been successfully increased in a tomato introgression line containing a chromosome segment from a wild relative, and how this has furthered our understanding of the mechanism controlling sugar content. We then outline current knowledge around how sugar sensing and signaling, proton pumps, and auxin affect sugar accumulation and fruit set. The prevention of fruit abscission by auxin, which is transported by PIN auxin efflux carriers and vacuolar proton phyrophosphatase (V-PPase), may retain sucrose transport to the fruit to inhibit programed cell death (PCD) and ensure successful fruit set. There is believed to be a trade-off between fruit sugar content and yield. However, fruit size and yield do not appear to decrease in the tomato introgression line IL8-3 and sucrose-induced repression of translation (SIRT)-engineered tomatoes, which contain higher fruit sugar contents. Future research needs to investigate the factors involved in sugar sensing and signaling, in addition to the sugar metabolic enzymes that have previously been studied for horticultural applications.