Carbon Fluxes in Potato (Solanum tuberosum) Remain Stable in Cell Cultures Exposed to Nutritional Phosphate Deficiency

SIMPLE SUMMARY: Phosphorus is an essential nutrient for plants. It is usually available in the form of inorganic phosphate in the plant environment. Yet, most environments contain extremely low amounts of phosphate, causing a major nutritional deficiency for plant life. During their evolution, plants have acquired a number of adaptations that help them to survive chronic phosphate deficit. Many of these adaptations are documented in the scientific literature and show changes in gene expression and modifications in the levels of enzymes and metabolites involved in plant carbon and respiratory metabolism. This research uses potato cells cultivated in vitro to measure major metabolic carbon fluxes (rates at which molecules are processed, consumed or degraded) in response to two phosphate regimes (normal and deficiency). Our results show a remarkable stability of several metabolic fluxes in cells regardless of their phosphate regime. This is the case for the rate at which the carbon source is taken up from the medium. Two important metabolic fluxes used to fuel cell respiration are also not affected by phosphate. In all the experiments, cell age is the main factor affecting carbon metabolic fluxes. These findings lead us to conclude that potato cells maintain stable carbon fluxes during phosphate deficiency. ABSTRACT: Nutritional phosphate deficiency is a major limitation to plant growth. Here, we monitored fluxes in pathways supporting respiratory metabolism in potato (Solanum tuberosum) cell cultures growing in control or limiting phosphate conditions. Sugar uptake was quantified using [U-(14)C]sucrose as precursor. Carbohydrate degradation through glycolysis and respiratory pathways was estimated using the catabolism of [U-(14)C]sucrose to (14)CO(2). Anaplerotic carbon flux was assessed by labeling with NaH(14)CO(3). The data showed that these metabolic fluxes displayed distinct patterns over culture time. However, phosphate depletion had relatively little impact on the various fluxes. Sucrose uptake was higher during the first six days of culture, followed by a decline, which was steeper in Pi-sufficient cells. Anaplerotic pathway flux was more important at day three and decreased thereafter. In contrast, the flux between sucrose and CO(2) was at a maximum in the mid-log phase of the culture, with a peak at Day 6. Metabolization of [U-(14)C]sucrose into neutral, basic and acidic fractions was also unaffected by phosphate nutrition. Hence, the well-documented changes in central metabolism enzymes activities in response to Pi deficiency do not drastically modify metabolic fluxes, but rather result in the maintenance of the carbon fluxes that support respiration.