Diurnal changes in the expression of 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/senescent leaves. In order to investigate diurnal regulation of gdh1 in leaves, the content in gdh1 mRNA was measured every 3 h over a 48 h period and compared to nia and gs2 mRNA levels, encoding, respectively, nitrate reductase (NR) and chloroplastic glutamine synthetase (GS2). In source leaves, gdh1 mRNA levels exhibit diurnal fluctuations. A 12 h shift was observed between the day-night rhythms of gdh1 and nia expression. Metabolite contents were also measured and a shift in the day-night fluctuations of both glutamate (GLU) and gamma-aminobutyric acid (GABA) was observed between sink and source leaves, whereas the diurnal rhythm of alpha-ketoglutarate showed no change. A possible role of GDH in the shift of GLU and GABA contents is discussed. Leaf disc experiments showed that gdh1 expression is enhanced in conditions of continuous darkness. This trend is inhibited 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 genes is suggested.