The two nitrogen mobilisation- and senescence-associated GS1 and GDH genes are controlled by C and N metabolites

In tobacco, the two enzymes of nitrogen metabolism, cytosolic glutamine synthetase (GS1; E. C. 6.3.1.2) and glutamate dehydrogenase (GDH; E. C. 1.4.1.2), are induced during leaf senescence, whereas the chloroplastic glutamine synthetase (GS2; E. C. 6.3.1.2) and nitrate reductase (NR; E. C.1.6.1.1) are repressed in the course of ageing. In this report, we showed in discs of fully expanded Nicotiana tabacum L. cv. Xanthi leaves that sucrose (Suc) and amino acids were involved in the regulation of the expression of GS1 and GDH genes. Suc induced the expression of GS1 and repressed that of GDH. Therefore, we concluded that in response to Suc, GS1 behaved as an early'' Senescence Associated Gene ( SAG), whereas GDH behaved as a late'' SAG. Moreover, amino acids induced the expression of both genes. Among the amino acids tested as signal molecules, proline ( Pro) and glutamate (Glu) were major inducers of GDH and GS1 expression, respectively. Interestingly, an opposite regulation of GS1 and GS2 by Pro and Glu was shown. The contrary effect of Suc on NIA ( NR encoding gene) and GDH mRNA accumulation was also emphasized.