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Diurnal changes in the expression of glutamate dehydrogenase and nitrate reductase are involved in the C/N balance of tobacco source leaves
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International audience. A novel cDNA encoding glutamate dehydrogenase (GDH)from tobacco (Nicotiana tabacum), named gdh1, was char-acterized. The gdh1 mRNA was detected in roots, stemsand source/senescent leaves. In order to investigate diurnalregulation of gdh1 in leaves, the content in gdh1 mRNAwas measured every 3 h over a 48 h period and comparedto nia and gs2 mRNA levels, encoding, respectively, nitratereductase (NR) and chloroplastic glutamine synthetase(GS2). In source leaves, gdh1 mRNA levels exhibit diurnalfluctuations. A 12 h shift was observed between the day–night rhythms of gdh1 and nia expression. Metabolite con-tents were also measured and a shift in the day–night fluc-tuations of both glutamate (GLU) and gggg-aminobutyric acid(GABA) was observed between sink and source leaves,whereas the diurnal rhythm of alpha-ketoglutarate showed nochange. A possible role of GDH in the shift of GLU andGABA contents is discussed. Leaf disc experiments showedthat gdh1 expression is enhanced in conditions of continu-ous darkness. This trend is inhibited by sucrose feeding.The opposite was observed for nia expression. An impor-tant outcome of this work is the reverse regulation of gdh1and nia genes. A possible role of sugars and amino acids inthe co-regulation of gdh1 and nia genes is suggested.
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