CO-REGULATED EXPRESSION OF NITRATE AND NITRITE REDUCTASES

The regulation of nitrate and nitrite reductases (NR and NiR, respectively) expression was compared in wild-type and NR-deficient (nia) Nicotiana plumbaginifolia plants. In wild-type plants, NiR and NR mRNA levels fluctuated according to a circadian rhythm, with similar timing of maximal and minimal transcript accumulation. NiR and NR mRNA levels were also nitrate- and light-inducible. It had been found previously that some of the nia mutants overexpressed NR mRNA. In mutant plants, irrespective of the mutation affecting the nia gene, NiR mRNA and activity were overexpressed. However, nitrate and light inducibility of NR and NiR mRNAs was still detectable. Normal NiR expression levels of mRNA were restored in nia leaky mutants obtained by intragenic complementation which contained low levels of NR activity. These data support the hypothesis of a common regulation of NR and NiR expression by a nitrate-derived metabolite. They also prove that the autogenous model of NR regulation described in fungi is not valid for higher plants.