Regulation of NRT1 and NRT2 gene families of Arabidopsis thaliana:: Responses to nitrate provision

Four low-affinity (NRT1), and seven high-affinity (NRT2) nitrate transporter gene homologues have been identified in Arabidopsis thaliana. We investigated the transcript abundances of all eleven genes in shoot and root tissues in response to the provision of 1 mM NO3-, using relative quantitative RT-PCR. Based upon this criterion, genes were classified as nitrate-inducible, nitrate-repressible, or nitrate-constitutive. AtNRT1.1, 2.1, and 2.2 were strongly induced by NO3-, peaking at 3-12 h and subsequently declining. By contrast AtNRT2.4 showed only modest induction both in shoots and roots. Expression of AtNRT2.5, one of the nitrate-repressible genes, was strongly suppressed by nitrate provision in both roots and shoots. The last group, characterized by a constitutive expression pattern, included AtNRT1.2, 1.4, 2.3, 2.6, and 2.7. Correlation coefficients between (NO3-)-N-13 influx from 100 muM and 5 mM [NO3-] suggest that high- and low-affinity transport systems are mediated primarily by AtNRT2.1 and AtNRT1.1, respectively. Functional roles for the other members of these families remain uncertain.