Carbon conversion efficiency and central metabolic fluxes in developing sunflower (Helianthus annuus L.) embryos

• Published in: The Plant journal : for cell and molecular biology Vol. 52; no. 2; pp. 296 - 308
• Main Authors: Alonso, Ana P, Goffman, Fernando D, Ohlrogge, John B, Shachar-Hill, Yair
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01-10-2007; Blackwell Publishing Ltd; Blackwell Science
• Subjects: Agronomy. Soil science and plant productions; Biochemistry; Biological and medical sciences; Botany; Carbon; Carbon - metabolism; carbon conversion efficiency; Carbon Radioisotopes; Cellular biology; Economic plant physiology; Embryo development. Germination; fatty acid synthesis; Fatty Acids - biosynthesis; Fundamental and applied biological sciences. Psychology; Growth and development; Helianthus - embryology; Helianthus - metabolism; isotopic labeling; metabolic flux analysis; Metabolism; oilseed filling; Plant Oils - metabolism; Seeds - metabolism; Staining and Labeling; sunflower embryo; Tissue Culture Techniques; Seeds; metabolic flux analysis; Root; sunflower embryo; Starch; Carbon dioxide; Biomass; Compositae; Metabolism; Fatty acids; oilseed filling; Helianthus annuus; isotopic labeling; fatty acid synthesis; Cruciferae; carbon conversion efficiency; Dicotyledones; Aminoacid; Angiospermae; Brassica napus; Spermatophyta; Gas exchange; ATP; EC 4.1.1.39
• DOI: 10.1111/j.1365-313x.2007.03235.x
• EISSN: 1365-313X
• ISSN: 0960-7412

The efficiency with which developing sunflower embryos convert substrates into seed storage reserves was determined by labeling embryos with [U-¹⁴C₆]glucose or [U-¹⁴C₅]glutamine and measuring their conversion to CO₂, oil, protein and other biomass compounds. The average carbon conversion efficiency was 50%, which contrasts with a value of over 80% previously observed in Brassica napus embryos ( Goffman et al., 2005 ), in which light and the RuBisCO bypass pathway allow more efficient conversion of hexose to oil. Labeling levels after incubating sunflower embryos with [U-¹⁴C₄]malate indicated that some carbon from malate enters the plastidic compartment and contributes to oil synthesis. To test this and to map the underlying pattern of metabolic fluxes, separate experiments were carried out in which embryos were labeled to isotopic steady state using [1-¹³C₁]glucose, [2-¹³C₁]glucose, or [U-¹³C₅]glutamine. The resultant labeling in sugars, starch, fatty acids and amino acids was analyzed by NMR and GC-MS. The fluxes through intermediary metabolism were then quantified by computer-aided modeling. The resulting flux map accounted well for the labeling data, was in good agreement with the observed carbon efficiency, and was further validated by testing for agreement with gas exchange measurements. The map shows that the influx of malate into oil is low and that flux through futile cycles (wasting ATP) is low, which contrasts with the high rates previously determined for growing root tips and heterotrophic cell cultures.