| Higher plants possess both high- and low-affinity nitrate transporters. Since the first Arabidopsis thaliana low-affinity nitrate transporter gene, AtNRT1.1(CHL1), was isolated (Tsay et al., 1993), three low-affinity (NRT1), and seven high-affinity (NRT2) nitrate transporter gene homologues have been identified in this species. We investigated the transcript abundances of all eleven genes both in shoot and root tissues in response to NO 3− provision, by relative quantitative RT-PCR method. Based upon their patterns of expression following NO3− provision, the genes were classified into three groups: (1) nitrate-inducible, (2) nitrate-repressible, and (3) nitrate-constitutive. AtNRT1.1, 2.1, and 2.2, representatives of the first group, were strongly induced by 1 mM NO3−, peaking at 3 to 12 hours, and declining during 3 subsequent days. By contrast AtNRT2.4 showed only modest induction both in shoots and roots. Expression of AtNRT2.7, one of the nitrate-repressible genes from the second group, was strongly suppressed by nitrate provision in both roots and shoots. The last group, characterized by a constitutive expression pattern, contains the largest membership, including AtNRT1.2, 1.4, 2.3, 2.5 , and 2.6. Tissue-specific expression patterns of limited number of AtNRT genes were observed by using GUS reporter DNA fusion lines. In the root tip regions, AtNRT1.1, 2.1 and 2.4 were expressed, while in the epidermal cells of mature roots, AtNRT2.1, 2.2, 2.4, and 2.6 were expressed. Expression of AtNRT2.1 was also found in the cortex and endodermal cells. In shoots AtNRT1.1 showed expression in leaves and flowers, while AtNRT2.6 was specifically expressed in pollen grains. 13NO3 − influx from 100μM and 5mM [NO3− ], chosen to examine high-affinity and low-affinity transport, respectively, corresponded closely with the expression patterns of AtNRT2.1 and 1.1, respectively. These results indicate that despite the close homology among members of the NRT2 and NRT1 families, individual members appear to be differently regulated, and may therefore perform different functions with respect to nitrate transport. |
