Abstract
AbstractA novel cDNA encoding glutamate dehydrogenase (GDH) from tobacco(Nicotiana tabacum), namedgdh1,was characterized.Thegdh1mRNA was detected in roots, stems and source/senescentleaves. In order to investigate diurnal regulation ofgdh1inleaves, the content ingdh1mRNA was measured every 3 h overa 48 h period and compared toniaandgs2mRNAlevels, encoding, respectively, nitrate reductase (NR) and chloroplasticglutamine synthetase (GS2). In source leaves,gdh1mRNA levelsexhibit diurnal fluctuations. A 12 h shift was observedbetween the day–night rhythms ofgdh1andniaexpression.Metabolite contents were also measured and a shift in the day–nightfluctuations of both glutamate (GLU) andγ‐aminobutyricacid (GABA) was observed between sink and source leaves, whereasthe diurnal rhythm ofα‐ketoglutarate showed no change.A possible role of GDH in the shift of GLU and GABA contents isdiscussed. Leaf disc experiments showed thatgdh1expressionis enhanced in conditions of continuous darkness. This trend isinhibited by sucrose feeding. The opposite was observed forniaexpression.An important outcome of this work is the reverse regulation ofgdh1andniagenes.A possible role of sugars and amino acids in the co‐regulation ofgdh1andniagenesis suggested.
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