| Summary: | Higher plants possess both high- and low-affinity nitrate transporters. Since the first
Arabidopsis thaliana low-affinity nitrate transporter gene, AtNRTl.l (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 NO3"
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. AtNRTl.l, 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 AtNRTl.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, AtNRTl.l, 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 AtNRTl.l showed expression in
leaves and flowers, while AtNRT2.6 was specifically expressed in pollen grains. ¹³NO₃⁻ -
influx from 100|iM 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.
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