Tomato fruit growth results in two distinct but overlapping phases, namely cell division in the very early days post anthesis, followed by a cell expansion phase. The vacuole, which occupies almost 80% of the volume of ...
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Tomato fruit growth results in two distinct but overlapping phases, namely cell division in the very early days post anthesis, followed by a cell expansion phase. The vacuole, which occupies almost 80% of the volume of cell fruit would play a major role in cell enlargement. Recently, our team developed a kinetic modeling approach, allowing a precise and dynamic description of tomato fruit metabolism throughout fruit growth (Beauvoit et al., 2014). This model emphasized the crucial role of sugars in generating the osmotic strength of the vacuole, allowing its enlargement and thus, fruit growth. Their transport across the tonoplast is mainly ensured by secondary active transporters. Thanks to a bioinformatic study, we identified the tomato proteins implicated in these transports: SUTs/SUCs, (Sucrose Transporters family), VGT (vacuolar Glucose Transporters), SFPs (Sugar Facilitator Protein), TMTs (Tonoplast monosaccharide Transporters) as well as two passive transporters, SWEET16 and SWEET17.In order to investigate the conclusions, put forward by the modeling approach, we generated transgenic RNAi lines for these transporters. After a phenotyping of the population of transgenic lines, we selected two candidates: TMT135S.2 and TMT235S.6 affected on TMT1 and TMT2 expression. A deeper phenotyping highlighted a drastic reduction of fruit size compared to the WT. At the metabolic level, TMTs under expression induced the reduction of hexoses (glucose and fructose) and hexoses phosphate contents to the profit of the accumulation of starch, amino acids and ions and to a lesser extent, organic acids and cell wall polysaccharides. However, despite this carbon reallocation, we showed that a shift from hexose accumulating metabolism to starch and amino acids accumulating metabolism, was inefficient to compensate the decrease in the total osmolarity, leading to the reduction of fruit size. Together with RNAseq published data (Tomato Expression Atlas - http://tea.solgenomics.net/) showing that TMT1 and TMT2 were expressed in specific area in the pericarp, our results suggest that TMT2 would be more implicated in generating the osmotic strength of the vacuole of the surrounding bundle cell, allowing phloem sucrose unloading in the early phase of fruit development, whereas TMT1 would be associated to pericarp cell enlargement.Finally, a flux balance modeling confirmed in the TMT135S.2 line the deep remodeling of the metabolic network, leading to an additional energy cost that could in part, explain the growth limitation.
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