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Background Sugar content is an important indicator of fruit quality. Except for a few wild tomato species that accumulate sucrose in the fruits, most cultivated tomato species accumulate hexose. Although several studies have focused on wild sucrose-accumulating tomato, the sucrose accumulation mechanism is still unclear. Results Here, two homozygous inbred cherry tomato lines (‘TB0023’ and ‘TB0278’, which accumulated sucrose and hexose, respectively) were selected to analyze the sugar accumulation mechanism. Carbohydrate analysis, cytological observation, gene expression and enzyme activity analysis and proteomics methods were used in this study. The results indicated that glucose and fructose were absolutely dominant in the soluble sugar content of hexose-accumulating cherry tomato fruit, while sucrose and a certain proportion of hexose were the main forms of soluble sugar in sucrose-accumulating cherry tomato fruit. The phloem unloading pathway of the hexose-accumulating cherry tomato fruit switched from symplastic to apoplastic during fruit development, and the sucrose-accumulating cherry tomato probably had a mixed unloading pathway involving the symplastic and apoplastic. High activity of acid invertase (AI), sucrose phosphate synthase (SPS), sucrose synthase (SS) and sugar transporters LeSUT1, SlSWEET2a and SlSWEET12c were important factors for hexose accumulation in the hexose-accumulating cherry tomato fruit, while LeSUT2, SPS, SS, SlSWEET1b, SlSWEET5b, SlSWEET11b, SlSWEET7a, SlSWEET14 were responsible for solute sugar accumulation in the sucrose-accumulating cherry tomato. Conclusions This study provides detailed evidence for elucidation of the tomato sugar accumulation mechanism from the perspective of cell structure, physiology and molecular biology, providing a theoretical basis for the improvement of tomato quality and aiding the utilization of tomato genetic resources.

Sugar is the key factor in the formation of fruit quality. Hexose is the main form of sugar accumulated in ordinary cultivated tomatoes, while a small number of wild-type tomato varieties can also accumulate sucrose. Although several studies have focused on wild sucrose-accumulating tomatoes, the sugar accumulation mechanism in tomato is still unclear. Here, two cherry tomatoes lines that accumulated sucrose and hexose respectively were selected to analyze the assimilates unloading pathway and sugar accumulation mechanism using CF tracing, cytological observation, proteomics methods, etc. The results indicated that the later stages of fruit development were key stages for sugar accumulation, and sucrose-accumulating (S) cherry tomatoes had higher sucrose contents in the fruits, while hexose-accumulating (H) cherry tomatoes accumulated more glucose, fructose and starch. The unloading pathway of assimilates in the S cherry tomato was switched from apoplastic to symplastic during fruit development, and the opposite was true in the H type. Plasmodesmata transport may be the main means of sucrose accumulation and high activity or expression levels of acid invertase (AI) and SUT1 may be important factors in hexose accumulation in H and S cherry tomatoes, respectively. In addition to sugar metabolism, photosynthesis, fatty acid metabolism and other secondary metabolism pathways also play important roles in sugar accumulation. This study provides detailed evidence for the tomato sugar accumulation mechanism from the perspective of cell structure, physiology and molecular biology, providing a theoretical basis for the improvement of tomato quality and aiding the utilization of tomato genetic resources.