Planta

, Volume 219, Issue 1, pp 48–58

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

  • Zhi-Hui Chen
  • Robert P. Walker
  • László I. Técsi
  • Peter J. Lea
  • Richard C. Leegood
Original Article

Abstract

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.

Keywords

Cataplerosis Cucumis Nitrogen metabolism Phosphoenolpyruvate carboxykinase pH regulation Redox (NAD/NADH) regulation 

Abbreviations

Asp

Aspartate

Asn

Asparagine

Glu

Glutamate

Gln

Glutamine

NADP-ME

NADP-malic enzyme

OAA

Oxaloacetate

PEP

Phosphoenolpyruvate

PEPC

Phosphoenolpyruvate carboxylase

PEPCK

Phosphoenolpyruvate carboxykinase

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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  • Zhi-Hui Chen
    • 1
  • Robert P. Walker
    • 1
  • László I. Técsi
    • 1
  • Peter J. Lea
    • 2
  • Richard C. Leegood
    • 1
  1. 1.Robert Hill Institute and Department of Animal and Plant SciencesUniversity of SheffieldSheffieldUK
  2. 2.Department of Biological SciencesLancaster UniversityLancasterUK

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