Data from: Model-assisted analysis of sugar metabolism throughout tomato fruit development reveals enzyme and carrier properties in relation to vacuole expansion

A kinetic model combining enzyme activity measurements and subcellular compartmentation was parameterized to fit the sucrose, hexose, and glucose-6-P contents of pericarp throughout tomato (Solanum lycopersicum) fruit development. The model was further validated using independent data obtained from domesticated and wild tomato species and on transgenic lines. A hierarchical clustering analysis of the calculated fluxes and enzyme capacities together revealed stage-dependent features. Cell division was characterized by a high sucrolytic activity of the vacuole, whereas sucrose cleavage during expansion was sustained by both sucrose synthase and neutral invertase, associated with minimal futile cycling. Most importantly, a tight correlation between flux rate and enzyme capacity was found for fructokinase and PPi-dependent phosphofructokinase during cell division and for sucrose synthase, UDP-glucopyrophosphorylase, and phosphoglucomutase during expansion, thus suggesting an adaptation of enzyme abundance to metabolic needs. In contrast, for most enzymes, flux rates varied irrespectively of enzyme capacities, and most enzymes functioned at <5% of their maximal catalytic capacity. One of the major findings with the model was the high accumulation of soluble sugars within the vacuole together with organic acids, thus enabling the osmotic-driven vacuole expansion that was found during cell division.

本研究针对结合酶活性测定与亚细胞区室化（subcellular compartmentation）的动力学模型开展参数化工作，以拟合番茄（Solanum lycopersicum）果实发育全过程中果皮内的蔗糖、己糖以及葡萄糖-6-磷酸（glucose-6-P）含量。该模型进一步利用从栽培番茄、野生番茄物种及转基因株系中获取的独立数据完成验证。对计算得到的代谢通量与酶容量进行层次聚类分析后，共同揭示了其发育阶段依赖性特征。细胞分裂阶段以液泡具备较高蔗糖水解活性为典型特征；而细胞扩张阶段的蔗糖裂解过程则由蔗糖合酶（sucrose synthase）与中性转化酶（neutral invertase）共同维系，且伴随极低水平的无效循环。最为关键的是，在细胞分裂阶段，果糖激酶（fructokinase）与依赖焦磷酸的磷酸果糖激酶（PPi-dependent phosphofructokinase）的通量速率与酶容量间存在紧密相关性；而在细胞扩张阶段，蔗糖合酶、UDP-葡萄糖焦磷酸化酶（UDP-glucopyrophosphorylase）以及磷酸葡萄糖变位酶（phosphoglucomutase）也呈现出该关联特征，这提示酶的丰度会适配代谢需求。与之相反，对于多数酶而言，其通量速率与酶容量并无关联，且大多数酶仅以低于自身最大催化能力5%的水平发挥功能。本模型的一项核心发现为：可溶性糖与有机酸在液泡内大量积累，这为细胞分裂阶段观察到的渗透驱动型液泡扩张提供了必要条件。