• Nicotiana tabacum;
  • Nicotiana plumbaginifolia;
  • nitrate reductase;
  • NO3 reduction;
  • NO3 uptake;
  • N efficiency;
  • transgenic tobacco;
  • whole-plant regulation

The physiological consequences for NO3 utilization by the plant of underexpression and overexpression of nitrate reductase (NR) were investigated in nine transformants of Nicotiana tabacum and Nicotiana plumbaginifolia. The in vitro NR activities (NRAs) in both roots and leaves of low- and high-NR tobacco transformants ranged from 5–10% to 150–200%, respectively, of those measured in wild-type plants. The level of NR expression markedly affected the NO3 reduction efficiency in detached leaves and intact plants. In both species, 15NO3 reduction ranged from 15–45% of 15NO3 uptake in the low-NR plants, to 40–80% in the wild-type, and up to 95% in high-NR plants. In the high-NR genotypes, however, total 15NO3 assimilation was not significantly increased when compared with that in wild-type plants, because the higher 15NO3 reduction efficiency was offset by lower 15NO3 uptake by the roots. The inhibition of NO3 uptake appeared to be the result of negative feedback regulation of NO3 influx, and is interpreted as an adjustment of NO3 uptake to prevent excessive amino acid synthesis. In genotypes underexpressing NR, the low 15NO3 reduction efficiency also was generally associated with a decrease in net 15NO3 uptake as compared with the wild type. Thus, underexpression of NR resulted in an inhibition of reduced 15N synthesis in the plant, although the effect was much less pronounced than that expected from the very low NRAs. The restricted NO3 uptake in low-NR plants emphasizes the point that the products of NO3 assimilation are not the only factors responsible for down-regulation of the NO3 uptake system.