Diurnal changes in the expressionof glutamate dehydrogenase and nitrate reductase are involved in the C/N balance of tobacco source leaves

A novel cDNA encoding glutamate dehydrogenase (GDH) from tobacco(Nicotiana tabacum), named gdh1, was characterized.The gdh1 mRNA was detected in roots, stems and source/senescentleaves. In order to investigate diurnal regulation of gdh1 inleaves, the content in gdh1 mRNA was measured every 3 h overa 48 h period and compared to nia and gs2 mRNAlevels, encoding, respectively, nitrate reductase (NR) and chloroplasticglutamine synthetase (GS2). In source leaves, gdh1 mRNA levelsexhibit diurnal fluctuations. A 12 h shift was observedbetween the day–night rhythms of gdh1 and nia expression.Metabolite contents were also measured and a shift in the day–nightfluctuations of both glutamate (GLU) and γ‐aminobutyricacid (GABA) was observed between sink and source leaves, whereasthe diurnal rhythm of α‐ketoglutarate showed no change.A possible role of GDH in the shift of GLU and GABA contents isdiscussed. Leaf disc experiments showed that gdh1 expressionis enhanced in conditions of continuous darkness. This trend isinhibited by sucrose feeding. The opposite was observed for nia expression.An important outcome of this work is the reverse regulation of gdh1 and nia genes.A possible role of sugars and amino acids in the co‐regulation of gdh1 and nia genesis suggested.