Simultaneous expression of NAD-dependent isocitrate dehydrogenase and other Krebs cycle genes after nitrate resupply to short-term nitrogen-starved tobacco

Mitochondrial NAD-dependent (IDH) and cytosolic NADP-dependent isocitrate d ehydrogenases have been considered as candidates for the production of 2-ox oglutarate required by the glutamine synthetase/glutamate synthase cycle. T he increase in IDH transcripts in leaf and root tissues, induced by nitrate or NH4+ resupply to short-term N-starved tobacco (Nicotiana tabacum) plant s, suggested that this enzyme could play such a role. The leaf and root ste ady-state mRNA levels of citrate synthase, acotinase, IDH, and glutamine sy nthetase were found to respond similarly to nitrate, whereas those for cyto solic NADP-dependent isocitrate dehydrogenase and fumarase responded differ ently. This apparent coordination occurred only at the mRNA level, since ac tivity and protein levels of certain corresponding enzymes were not altered . Roots and leaves were not affected to the same extent either by N starvat ion or nitrate addition, the roots showing smaller changes in N metabolite levels. After nitrate resupply, these organs showed different response kine tics with respect to mRNA and N metabolite levels, suggesting that under su ch conditions nitrate assimilation was preferentially carried out in the ro ots. The differential effects appeared to reflect the C/N status after N st arvation, the response kinetics being associated with the nitrate assimilat ory capacity of each organ, signaled either by nitrate status or by metabol ite(s) associated with its metabolism.