The effect of Glc6P uptake and its subsequent oxidation within pea root plastids on nitrite reduction and glutamate synthesis

Caroline G. Bowsher, Anne E. Lacey, Guy T. Hanke, David T. Clarkson, Les R. Saker, Ineke Stulen, Michael J. Emes

    Research output: Contribution to journalArticlepeer-review

    Abstract

    In roots, nitrate assimilation is dependent upon a supply of reductant that is initially generated by oxidative metabolism including the pentose phosphate pathway (OPPP). The uptake of nitrite into the plastids and its subsequent reduction by nitrite reductase (NiR) and glutamate synthase (GOGAT) are potentially important control points that may affect nitrate assimilation. To support the operation of the OPPP there is a need for glucose 6-phosphate (Glc6P) to be imported into the plastids by the glucose phosphate translocator (GPT). Competitive inhibitors of Glc6P uptake had little impact on the rate of Glc6P-dependent nitrite reduction. Nitrite uptake into plastids, using 13N labelled nitrite, was shown to be by passive diffusion. Flux through the OPPP during nitrite reduction and glutamate synthesis in purified plastids was followed by monitoring the release of 14CO2 from [1-14C]-Glc6P. The results suggest that the flux through the OPPP is maximal when NiR operates at maximal capacity and could not respond further to the increased demand for reductant caused by the concurrent operation of NiR and GOGAT. Simultaneous nitrite reduction and glutamate synthesis resulted in decreased rates of both enzymatic reactions. The enzyme activity of glucose 6-phosphate dehydrogenase (G6PDH), the enzyme supporting the first step of the OPPP, was induced by external nitrate supply. The maximum catalytic activity of G6PDH was determined to be more than sufficient to support the reductant requirements of both NiR and GOGAT. These data are discussed in terms of competition between NiR and GOGAT for the provision of reductant generated by the OPPP. © 2007 The Author(s).
    Original languageEnglish
    Pages (from-to)1109-1118
    Number of pages9
    JournalJournal of Experimental Botany
    Volume58
    Issue number5
    DOIs
    Publication statusPublished - Mar 2007

    Keywords

    • Glucose-6-phosphate dehydrogenase
    • Glucose-6-phosphate uptake
    • Glutamate synthase
    • Nitrite reductase maximum catalytic activity
    • Nitrite uptake
    • Oxidative pentose phosphate pathway competition

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