The Metabolic Architecture of Plant Cells

Abstract

The changes in the intermediary metabolism of plant cells were quantified according to growth conditions at three different stages of the growth cycle of tomato cell suspension. Eighteen fluxes of central metabolism were calculated from 13 C enrichments after near steady-state labeling by a metabolic model similar to that described in Dieuaide-Noubhani et al. (Dieuaide-Noubhani, M., Raffard, G., Canioni, P., Pradet, A., and Raymond, P. (1995) J. Biol. Chem. 270, 13147-13159), and 10 net fluxes were obtained directly from end-product accumulation rates. The absolute flux values of central metabolic pathways gradually slowed down with the decrease of glucose influx into the cells. However, the relative fluxes of glycolysis, the pentose-P pathway, and the tricarboxylic acid cycle remained unchanged during the culture cycle at 70, 28, and 40% of glucose influx, respectively, and the futile cycle of sucrose remained high at about 6-fold the glucose influx, independently from carbon nutritional conditions. This natural resistance to flux alterations is referred to as metabolic stability. The numerous anabolic pathways, including starch synthesis, hexose accumulation, biosynthesis of wall polysaccharides, and amino and organic acid biosynthesis were comparatively low and variable. The phosphoenolpyruvate carboxylase flux decreased 5-fold in absolute terms and 2-fold in relation to the glucose influx rate during the culture cycle. We conclude that anabolic fluxes constitute the flexible part of plant cell metabolism that can fluctuate in relation to cell demands for growth.Plants are able to grow under a wide range of environmental conditions (extreme temperatures, insufficient or excessive light, and shortage of water or mineral nutrients) and show a robust physiological homeostasis. To ensure this homeostasis, plant metabolism has to be very flexible (1). In all cells, the central carbon metabolism provides energy, cofactor regeneration, and building blocks for biomass and secondary metabolism. The flexibility of plant metabolism, which is probably an evolutionary adaptation to the variable environmental conditions plants normally experience, has been explained by the buffering effect of carbon storage and allocation (2) and by the complexity of regulation or built-in redundancy owing to alternative enzymes and pathways for many processes (3). For example, parallel glycolytic pathways are present in the cytosol and plastid; in the cytosol, the classic key sites for regulation of glycolysis at phosphofructokinase and pyruvate kinase (PK) 1 can be bypassed by pyrophosphate:fructose-6-phosphate phosphotransferase and phosphoenolpyruvate carboxylase (PEPC) (3).The development of genetic engineering, which introduced a new dimension to pathway knowledge by allowing precise modifications of specific enzymatic reactions in metabolic pathways, also highlighted the flexibility of plant primary metabolism (4 -6). Pyrophosphate:fructose-6-phosphate phosphotransferase and cytosolic PK, which generally have been considered essenti...

Subcellular Flux Analysis of Central Metabolism in a Heterotrophic Arabidopsis Cell Suspension Using Steady-State Stable Isotope Labeling

“…These estimates of carbon use efficiency are within the range obtained for other heterotrophic plant systems, including a rosy periwinkle hairy root culture (24%; Sriram et al, 2007), maize root tips (42%-47%; Alonso et al, 2007b), developing sunflower embryos (50%; Alonso et al, 2007a), and a tomato cell culture (52%-68%; Rontein et al, 2002). Although considerably higher values have been reported for developing embryos of oilseed rape (85%-95%) and soybean (82%-83%), in these systems there is appreciable reassimilation of respired CO 2 through Rubisco, and light makes significant contributions to the provision of ATP and/or reductant required for biosynthesis (Goffman et al, 2005;Allen et al, 2009).…”

“…The bottom row contains an analysis of data combined from all three feeding strategies (1/2/U-13 C), in which the SD of all isotopomer measurements was reduced to 5%. a single plastidic pathway has been assumed in maize root tips (Dieuaide-Noubhani et al, 1995;Alonso et al, 2007b), tomato (Solanum lycopersicum) cell cultures (Rontein et al, 2002), and Arabidopsis cell cultures (Williams et al, 2008), whereas complete duplication of the pathway in the cytosol and plastid has been assumed in the analysis of cultured soybean embryos (Sriram et al, 2004;Iyer et al, 2008). In some tissues, such as oilseed rape embryos (Schwender et al, 2003) and soybean embryos (Allen et al, 2009), the modeling task is made easier because the observed labeling patterns imply that intermediates in glycolysis and the PPP are in fast exchange between the cytosol and the plastids, ensuring that duplicate pathways in different physical locations function indistinguishably from a single uncompartmented pathway.…”

Three Different and Tissue-specific NAD-Malic Enzymes Generated by Alternative Subunit Association in Arabidopsis thaliana

“…These characteristics represent a mechanism of allosteric regulation of enzymes of central metabolic pathways (40). The presence of three different NAD-MEs originating by alternative associations of NAD-ME1 and -2 may be a novel phenomenon unique to plant mitochondria.…”

Expression and evolutionary features of the hexokinase gene family in Arabidopsis

“…A number of different possible metabolic roles of HXKs have been recently described (Claeyssen and Rivoal 2007). Mitochondrial HXKs are thought to have preferred access to ATP produced in respiration for consumption by active metabolite fluxes through sucrose cycling, glycolysis, and sugar nucleotide syntheses (Rontein et al 2002;Graham et al 2007). …”