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
Blackwell Publishing Ltd
Blackwell Publishing Journal Backfiles 1879-2005
The effect of N-source on the interaction between carbon and nitrogen metabolism was evaluated by measuring phosphoenolpyruvate carboxylase (PEPcase; EC 22.214.171.124) 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.
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