Decreased sink/source ratio enhances hexose transport in the fruits of greenhouse tomatoes: integration of gene expression and biochemical analyses.

To examine the physiological role of hexose transporters in determining the sink strength of individual fruits, the regulation of hexose transporters gene expression was studied when the sink/source ratio was artificially altered under the greenhouse condition; this was done in two cultivars of tomato, i.e. Grandella and Isabella. The sink/source ratio treatments included: saving one fruit per truss (1F), two fruits per truss (2F), three fruits per truss (3F) and no fruit pruning (control). The results showed that fruit thinning could increase starch, sucrose, and hexose contents in the fruits; it could also modulate the activity of the key enzymes and the expression of tomato hexose transporter genes (LeHTs). Based on the relative transcript levels, all examined LeHTs were unregulated at the end of cell division and the cell expansion stage of fruit development, but the strongest expression level observed at the onset of ripening was related to LeHT1 and LeHT2. Given the concomitancy of cell wall invertase (EC 3.2.1.26) activity and the LeHTs relative expression cell wall, invertase activity seemed to be involved in the expression level of LeHTs. The increased trends of the LeHTs expression with the decrease of the sink/source ratio confirmed the role of hexose transporters in determining the sink strength of the tomato fruits.