The tissue differentiation in the central carbon metabolism of tomato fruit

Respiration consumes O2 and produces CO2. As a consequence, the O2 and CO2 concentration in the centre of the fruit are below and above that of the storage atmosphere, respectively. This has been shown experimentally by several authors. As it is difficult to measure the O2 and CO2 concentration in vivo, models have been developed to describe gas exchange of fruit with their environment. Ho et al. developed a reaction-diffusion model to study gas exchange of intact fruit at the macroscale level.  Gas transport in fruit tissue is driven by gas concentration gradients due to O2 depletion and CO2 production by respiration and fermentation. So far, we have modelled these processes by Michaelis-Menten kinetics. However, understanding the effect of storage conditions on respiration and fermentation requires more detailed information on the various pathways that are involved in the central carbon metabolism, including glycolysis, Krebs cycle, oxidative phosphorylation, pentose phosphate shunt and fermentation. Indeed, hypoxic stress has been shown to change the metabolic composition of fruit dramatically. Metabolomics has been used to evaluate the effect of environmental stresses on plant physiology and to elucidate pathways.. Only few applications of metabolomics to postharvest biology have been described in literature. Metabolomic data can be used to construct mathematical models of metabolic networks in fruits and vegetables predicting the effect of postharvest operations on the physiology and quality of fruit and vegetables. To develop such dynamic models, experimental data on the uptake of isotope labelled substrates is generally adopted as they are able to reveal metabolite fluxes changing with time. Due to its complexity the mathematical modelling of dynamic isotope labelling in plants has received very little attention. However, quantitative knowledge at the metabolic level only, remains difficult to interpret without proper conceptual and mathematical models of the underlying biochemistry that can interpret these changes in terms of turnover rates and overall carbon fluxes. Recently we developed a workflow to measure fluxes in undifferentiated tomato cells, with the ultimate goal to construct kinetic pathway models of the respiration pathway in hypoxic and anoxic conditions. In addition the experimental approach has been extended to studying metabolic responses in thin slices of fruit tissue in developing apple fruit. So far this methodology has not been applied to describe metabolic changes in intact fruit. While for pome fruit the fleshy tissue is considered quite homogeneous the edible part of tomato fruit is known to consist of multiple tissue types each contributing in their own way to the overall fruit’s metabolism. This was explicitly shown with regard to the ethylene biosynthesis pathway in tomato. In addition, this tissue differentiation is clearly linked to the fruit’s developmental stage. So, next to the impact of oxygen gradients, metabolic tissue differentiation can be expected to impact on the overall fruit response as well.