Simultaneous Expression of NAD-Dependent Isocitrate Dehydrogenase and Other Krebs Cycle Genes after Nitrate Resupply to Short-Term Nitrogen-Starved Tobacco

Abstract

Mitochondrial NAD-dependent (IDH) and cytosolic NADPdependent isocitrate dehydrogenases have been considered as candidates for the production of 2-oxoglutarate required by the glutamine synthetase/glutamate synthase cycle. The increase in IDH transcripts in leaf and root tissues, induced by nitrate or NH 4 ؉ resupply to short-term N-starved tobacco (Nicotiana tabacum) plants, suggested that this enzyme could play such a role. The leaf and root steady-state mRNA levels of citrate synthase, acotinase, IDH, and glutamine synthetase were found to respond similarly to nitrate, whereas those for cytosolic NADP-dependent isocitrate dehydrogenase and fumarase responded differently. This apparent coordination occurred only at the mRNA level, since activity and protein levels of certain corresponding enzymes were not altered. Roots and leaves were not affected to the same extent either by N starvation or nitrate addition, the roots showing smaller changes in N metabolite levels. After nitrate resupply, these organs showed different response kinetics with respect to mRNA and N metabolite levels, suggesting that under such conditions nitrate assimilation was preferentially carried out in the roots. The differential effects appeared to reflect the C/N status after N starvation, the response kinetics being associated with the nitrate assimilatory capacity of each organ, signaled either by nitrate status or by metabolite(s) associated with its metabolism.

Enzyme Redundancy and the Importance of 2-Oxoglutarate in Higher Plant Ammonium Assimilation

“…Recently, the coordinated expression of IDH with other genes of the Krebs cycle, nitrate reduction, and ammonium assimilation in N-starved tobacco plants after nitrate resupply was described (Lancien et al, 1999). Such an observation was taken as indirect evidence for such a role.…”

“…A putative role for NADH-GOGAT in Gln export to the phloem in senescing tissues and roots reflects its expression in vascular tissue cells of roots and developing leaves as well as in root epi-and exodermis cells of rice (Hayakawa et al, 1999). Since, in roots, NADH-GOGAT expression is stimulated by low levels of ammonium (Hirose et al, 1997) and by nitrate addition after N starvation (Lancien et al, 1999), a role in primary N assimilation in this organ is possible. Indisputably, an important role for NADH-GOGAT is in ammonium assimilation in legume nodules where its expression is developmentally regulated in response to functional N-fixing nodules (Temple et al, 1998).…”

Abiotic Stress Generates ROS That Signal Expression of Anionic Glutamate Dehydrogenases to Form Glutamate for Proline Synthesis in Tobacco and Grapevine

“…The Salt-Induced Increase in gdh-NAD;A1 Transcript Was Accompanied by Increase in Nt idh;A and Nt cyt icdh Transcripts and Immunoreactive mt ICDH and cyt ICDH Protein I(C)DHs supply the cell with 2OG to form Glu (Figure 1; Lancien et al, 1999). It was of interest to test whether or not the increase in the transcript of Nt gdh-NAD;A1 and immunoreactive GDH protein brought about by NaCl ( Figures 5A and 5B) was accompanied by similar trends in the transcript(s) and protein of I(C)DH.…”

Biochemical and molecular characterization of transgenic Lotus japonicus plants constitutively over-expressing a cytosolic glutamine synthetase gene

“…GS 1 is the main isoform involved in the assimilation of external ammonia (Sukanya et al 1994) and the ammonia derived from other sources of nitrogen, like the synthesis of secondary metabolites and the recycling of nitrogen from protein catabolism (Joy 1988). Root GS 1 may also have a role in the assimilation of the ammonia derived from nitrate reduction in the roots (Lancien et al 1999). GS 1 is also present in the shoots where it is localized in the vascular tissue Kamachi et al 1992;Pereira et al 1992;Dubois et al 1996;Sakurai et al 1996;Pérez-García et al 1998).…”