The tomato nia gene complements a Nicotiana plumbaginifolia nitrate reductase-deficient mutant and is properly regulated.

A nitrate reductase (NR) deficient mutant of Nicotiana plumbaginifolia totally impaired in the production of functional nia transcript and protein was restored for NR activity by transformation with a cloned tomato nia gene. The transgenic plants expressed from undetectable to 17% of the control NR activity in their leaves. Restoration of growth rates comparable to the wild type was obtained for transgenic plants expressing as little as 10% of the wild-type activity showing that nitrate reduction is not a growth-limiting factor in the wild-type plant. The analysis of the transgene expression showed that the tomato nia gene transcription was regulated by light, nitrate and a circadian rhythm as in tomato plants. These results suggest that all the cis-acting sequences involved in these regulations are contained in the 3 kb upstream region of the tomato nia gene and are still functional in transgenic N. plumbaginifolia plants. The amount of NR transcript synthesized from the tomato nia gene was reduced when a functional N. plumbaginifolia nia locus was introduced by sexual crosses. These data support the hypothesis that nitrate reduction is regulated by nitrate-derived metabolites as demonstrated in fungi.