IN TOBACCO-LEAVES, THE GENES ENCODING THE NITRATE-REDUCTING OR THE AMMONIUM-ASSIMILATING ENZYMES ARE REGULATED DIFFERENTLY BY EXTERNAL NITROGEN-SOURCES

In nitrogen starvation experiments with whole plants, we have compared the effect of nitrate, ammonium, glutamine or glutamate on the tobacc o (Nicotiana tabacum L.) nitrate reductase (NR; EC 1.6.6.1), nitrite r eductase (NiR; EC 1.7.7.1), plastidic glutamine synthetase (GS-2; EC 6 .3.1.2) and ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4. 7.1) transcript levels, and on corresponding enzymatic activities. The NR and NiR mRNA levels decreased to the limit of detection during N-s tarvation and were rapidly and specifically inducible to their normal levels upon replenishment with nitrate. The GS-2 and the W-GOGAT mRNA pools were much less affected by N-starvation than the NR or NiR trans cript levels. The Fd-GOGAT mRNA level in leaves of N-stressed tobacco increased upon replenishment with nitrate: ammonium, glutamine or glut amate. The GS-2 mRNA level in N-starved tobacco leaves was unaffected by the addition of ammonium or glutamate, while replenishment with nit rate or glutamine caused an increase in the GS-2 transcript abundance to ca. 30% of the normal level. We conclude that when compared to the magnitude of the rapid and specific nitrate-inducibility of the NR and NiR transcripts, the GS-2 and the W-GOGAT mRNA level are not specific ally inducible by nitrate or ammonium. The limited changes observed in the GS-2 or the Fd-GOGAT transcript pools during nitrogen stress or u pon replenishment of N-starved tobacco with different N-sources may, t herefore, merely reflect the altered metabolic status of the leaf duri ng N-stress or upon recovery from N-starvation. The physiological sign ificance of the apparent nitrate- and ammonium-independent expression of the GS-2 and Fd-GOGAT genes of tobacco is discussed.