EFFECTS OF GENETIC-MODIFICATION OF NITRATE REDUCTASE EXPRESSION ON (NO3-)-N-15 UPTAKE AND REDUCTION IN NICOTIANA PLANTS

The physiological consequences for NO3- utilization by the plant of un derexpression and overexpression of nitrate reductase (NR) were invest igated in nine transformants of Nicotiana tabacum and Nicotiana plumba ginifolia. 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 N R expression markedly affected the NO3- reduction efficiency in detach ed leaves and intact plants, In both species, O-15(3)- reduction range d 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 genoty pes, however, total (NO3-)-N-15, assimilation was not significantly in creased when compared with that in mild-type plants, because the highe r (NO3-)-N-15 reduction efficiency was offset by lower (NO3-)-N-15 upt ake by the roots, The inhibition of NO3- uptake appeared to be the res ult of negative feedback regulation of NO3- influx, and is interpreted as an adjustment of NO3- uptake to prevent excessive amino acid synth esis, In genotypes underexpressing NR, the low (NO3-)-N-15 reduction e fficiency also was generally associated with a decrease in net (NO3-)- N-15 uptake as compared with the wild type. Thus, underexpression of N R resulted in an inhibition of reduced N-15 synthesis in the plant, al though 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 fac tors responsible for down-regulation of the NO3- uptake system.