Root NO3uptake and expression of two root NO3transporter genes (Nrt2;1and Nrt1) were investigated in response to changes in the N- or C-status of hydroponically grown Arabidopsis thaliana plants. Expression of Nrt2;1 is up-regulated by NO3starvation in wild-type plants and by N-limitation in a nitrate reductase (NR) deficient mutant transferred to NO3as sole N source. These observations show that expression of Nrt2;1 is under feedback repression by N-metabolites resulting from NO3reduction. Expression of Nrt1 is not subject to such a repression. However, Nrt1 is over-expressed in the NR mutant even under N-sufficient conditions (growth on NH4NO3medium), suggesting that expression of this gene is affected by the presence of active NR, but not by N-status of the plant. Root15NO3influx is markedly increased in the NR mutant as compared to the wild-type. Nevertheless, both genotypes have similar net15NO3uptake rates due to a much larger14NO3efflux in the mutant than in the wild-type. Expressions of Nrt2;1 and Nrt1 are diurnally regulated in photosynthetically active A. thaliana plants. Both increase during the light period and decrease in the first hours of the dark period. Sucrose supply prevents the inhibition of Nrt2;1 and Nrt1 expressions in the dark. In all conditions investigated, Nrt2;1 expression is strongly correlated with root15NO3influx at 0.2 mmexternal concentration. In contrast, changes in the Nrt1 mRNA level are not always associated with similar changes in the activities of high- or low-affinity NO3transport systems.