Résumé
Although nitrate reductase (NR, EC 1.6.6.1) is thought to control the rate of nitrate assimilation, mutants with 40—45% of wildtype (WT) NR activity (NRA) grow as fast as the WT. We have investigated how tobacco (Nicotiana tabacum L. cv. Gatersleben) mutants with one or two instead of four functional nia genes compensate. (i) The nia transcript was higher in the leaves of the mutants. However, the diurnal rhythm was retained in the mutants, with a maximum at the end of the night and a strong decline during the photoperiod. (ii) Nitrate reductase protein and NRA rose to a maximum after 3—4 h light in WT leaves, and then decreased by 50—60% during the second part of the photoperiod and the first part of the night. Leaves of mutants contained 40—60% less NR protein and NRA after 3—4 h illumination, but NR did not decrease during the photoperiod. At the end of the photoperiod the WT and the mutants contained similar levels of NR protein and NRA. (iii) Darkening led to a rapid inactivation of NR in the WT and the mutants. However, in the mutants, this inactivation was reversed after 1—3 h darkness. Calyculin A prevented this reversal. When magnesium was included in the assay to distinguish between the active and inactive forms of NR, mutants contained 50% more activity than the WT during the night. Conversion of [15N]-nitrate to organic compounds in leaves in the first 6 h of the night was 60% faster in the mutants than in the WT. (iv) Growth of WT plants in enhanced carbon dioxide prevented the decline of NRA during the second part of the photoperiod, and led to reactivation of NR in the dark. (v) Increased stability of NR in the light and reversal of dark-inactivation correlated with decreased levels of glutamine in the leaves. When glutamine was supplied to detached leaves it accelerated the breakdown of NR, and led to inactivation of NR, even in the light. (vi) Diurnal changes were also investigated in roots. In the WT, the amount of nia transcript rose to a maximum after 4 h illumination and then gradually decreased. The amplitude of the changes in transcript amount was smaller in roots than in leaves, and there were no diurnal changes in NRA. In mutants, nia transcript levels were high through the photoperiod and the first part of the night. The NRA was 50% lower during the day but rose during the night to an activity almost as high as in the WT. The rate of [15N]-nitrate assimilation in the roots of the mutants resembled that in the WT during the first 6 h of the night. (vii) Diurnal changes were also compared in Nia30(145) transformants with very low NRA, and in nitrate-deficient WT plants. Both sets of plants had similar low growth rates. Nitrate reductase did not show a diurnal rhythm in leaves or roots of Nia30(145), the leaves contained very low glutamine, and NR did not inactivate in the dark. Nitrate-deficient WT plants were watered each day with 0.2 mM nitrate. After watering, there was a small peak of nia transcript, NR protein and NRA and, slightly later, a transient increase of glutamine and other amino acids in the leaves. During the night glutamine was low, and NR did not inactivate. In the roots, there was a very marked increase of nitrate, nia transcript and NRA 2—3 h after the daily watering with 0.2 mM nitrate. (viii) It is concluded that WT plants have excess capacity for nitrate assimilation. They only utilise this potential capacity for a short time each day, and then down-regulate nitrate assimilation in response, depending on the conditions, to accumulation of the products of nitrate assimilation or exhaustion of external nitrate. Genotypes with a lower capacity for nitrate assimilation compensate by increasing expression of NR and weakening the feedback regulation, to allow assimilation to
continue for a longer period each day.