Biochemical factors contributing to tomato fruit sugar content: a review

Introduction. Consumers and processors value tomatoes with high fruit sugar content; however, most breeding and cultural practices negatively impact this trait. Wild tomato species can accumulate two- to three-fold more fruit sugar than cultivars and are proving to be valuable both as a source of high-sugar loci to broaden the genetic base of currently produced cultivars, and as research material to understand this trait. Synthesis. While cutting-edge genomic approaches have taught us much about fruit phenotypes, it is still important to assess fruit enzyme activities and metabolic fluxes in lines with contrasting fruit sugar accumulation. These metabolic functions are closest to the ripe fruit sugar trait. In this review, we focus our attention on the biochemical pathways, especially starch biosynthesis, that may influence tomato fruit sugars. We try where possible to put this information into a physiological context because together they influence yield. We compare and contrast sugar metabolism in cultivars and wild tomato species and identify factors that may influence differences in their fruit size. Conclusion. Although difficult, we show that it is possible to develop fruit with high horticultural yield and use the breeding line 'Solara' as an example. In addition, we suggest avenues of further investigation to understand the regulation and control of fruit carbohydrate content.