Effects of genetic modification of nitrate reductase expression on 15NO3- uptake and reduction in Nicotiana plants

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 irt vitro NR activities (NRA) 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, O-15(3)- reduction ranged from 15-45% of (NO3-)-N-15 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 (NO3-)-N-15, assimilation was not significantly increased when compared with that in mild-type plants, because the higher (NO3-)-N-15 reduction efficiency was offset by lower (NO3-)-N-15 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 (NO3-)-N-15 reduction efficiency also was generally associated with a decrease in net (NO3-)-N-15 uptake as compared with the wild type. Thus, underexpression of NR resulted in an inhibition of reduced N-15 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.