Effects of the source of inorganic nitrogen on C and N interaction in maize callus tissue: phosphoenolpyruvate carboxylase activity, cytosolic pH and 15N amino acids
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Medientyp:
E-Artikel
Titel:
Effects of the source of inorganic nitrogen on C and N interaction in maize callus tissue: phosphoenolpyruvate carboxylase activity, cytosolic pH and 15N amino acids
Beteiligte:
Amâncio, Sara;
Diogo, Eugénio;
Santos, Helena
Erschienen:
Wiley, 1993
Erschienen in:
Physiologia Plantarum, 89 (1993) 3, Seite 618-625
Beschreibung:
The effect of N‐source on the interaction between carbon and nitrogen metabolism was evaluated by measuring phosphoenolpyruvate carboxylase (PEPcase; EC 4.1.1.31) activity in callus tissue of maize (Zea mays L. cv. Prisma) sub‐cultured under different N‐nutrition conditions: nitrate, ammonium or combinations of both. By comparison with the condition where both salts were supplied (control), nitrate as the sole N‐source led to an increase in PEPcase activity. Ammonium alone gave a drastic decrease of tissue growth. Extracts from calli grown on equivalent media supplied with 15N‐nitrate or 15N‐ammonium were analysed by 15N‐NMR. The labelling of amino acids in the NMR spectra showed that when 15NO−3 was the unique N‐source, 15N mainly accumulated in NδGln, Glu and Ala. With 15NH+4 only the NδGln and γ‐aminobutyric acid were labelled. The addition of both gave rise to labelled Gln, Asn, Glu, Asp, Ala, Val and γ‐aminobutyric acid independently of the origin of the label. In vivo 31P‐NMR allowed the cytoplasmic and vacuolar pH to be measured. The cytoplasmic pH showed an increase of approximately 0.3 units when nitrate was the sole source of nitrogen and a corresponding decrease when ammonium was added alone. Vacuolar pH decreased in both treatments. These results are discussed on the basis of the effect of the N‐source on carbon metabolism. A hypothesis of PEPcase activation as due to the increase of cytoplasmic pH upon nitrate uptake is proposed.