Phosphoenolpyruvate carboxykinase in cucumber plants is increased both by ammonium and by acidification, and is present in the phloem

• Published in: Planta Vol. 219; no. 1; pp. 48 - 58
• Main Authors: Chen, Z.H, Walker, R.P, Tecsi, L.I, Lea, P.J, Leegood, R.C
• Format: Journal Article
• Language: English
• Published: Berlin Springer-Verlag 01-05-2004; Springer; Springer Nature B.V
• Subjects: Acidification; Amino acids; Ammonium; ammonium chloride; Ammonium Chloride - metabolism; ammonium compounds; Biological and medical sciences; Cell Culture Techniques; Cotyledon - enzymology; Cotyledon - ultrastructure; Cotyledons; Cucumbers; Cucumis sativus; Cucumis sativus - enzymology; Cucumis sativus - genetics; cytosol; Cytosol - metabolism; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; Leaves; Metabolism; nutrient solutions; Petioles; Phloem; phosphoenolpyruvate carboxykinase (ATP); Photosynthesis, respiration. Anabolism, catabolism; Plant Leaves - enzymology; plant morphology; Plant physiology and development; plant proteins; Plant roots; Plant Roots - enzymology; Plant Stems - enzymology; plant veins; Plants; Protein-Serine-Threonine Kinases - genetics; Protein-Serine-Threonine Kinases - metabolism; Quaternary ammonium compounds; Quaternary Ammonium Compounds - pharmacology; Roots; Seedlings; Seeds - enzymology; Stems; tissue distribution; vegetable crops; Veins (plant anatomy); Cucumis; Cotyledon; Seeds; Carbon dioxide; Acidification; Lyases; Redox (NAD/NADH) regulation; pH regulation; Stem; Butyric acid; Nitrogen metabolism; Carboxy-lyases; Dicotyledones; Angiospermae; Gluconeogenesis; Flower; Fruit; Root; Enzyme; Phosphoenolpyruvate carboxykinase; Cucurbitaceae; Plant leaf; Cucumis sativus; Perturbation; Metabolism; Carbon-carbon lyases; Phloem; Aminoacid; Cataplerosis; Spermatophyta; Phosphoenolpyruvate carboxykinase (ATP); Petiole
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s00425-004-1220-y

In cucumber (Cucumis sativus L.), phosphoenolpyruvate carboxykinase (PEPCK) was shown by activity measurements and immunoblots to be present in leaves, stems, roots, flowers, fruit and seed. However, immunolocalisation showed that it was present only in certain cell types. PEPCK was present in the companion cells of the adaxial phloem of minor veins, the adaxial and abaxial phloem of larger veins, the internal and external phloem of vascular bundles in petioles and stems, the phloem in roots and the extra-fascicular phloem in leaves, cotyledons, petioles and stems. Immunohistochemical evidence suggests that both the extra-fascicular phloem and the adaxial phloem are involved in the transport of amino acids. In roots and stems, the abundance of PEPCK was greatly increased by watering plants with a solution of ammonium chloride at low, but not at high pH. PEPCK also increased in leaves, but not roots or stems, of seedlings grown in an atmosphere containing 5% CO2, and in roots and stems of seedlings watered with butyric acid. All these treatments are known to lower the pH of plant cells. Amino acid metabolism in the phloem may produce an excess of carbon skeletons, pH perturbations and an imbalance in the production/utilisation of NADH. This raises the possibility that PEPCK may function in the conversion of these carbon skeletons to PEP, which, depending on the energy requirements of the phloem, is subsequently utilised by either gluconeogenesis or the Krebs cycle, which both consume protons.