Isolation and Characterization of HvNRT2.3 and HvNRT2.4, cDNAs Encoding High-Affinity Nitrate Transporters from Roots of Barley

Two full-length cDNAs, HvNRT2.3 and HvNRT2.4, were isolated from roots of barley (Hordeum vulgare), using reverse transcriptase-PCR and RACE-PCR. The corresponding polypeptides, consisting of 507 amino acids (molecular masses of 54.6 kD), belong to the major facilitator superfamily (MFS), and are closely related (>87% identity) to those encoded by HvNRT2.1 and HvNRT2.2 (formerly BCH1 and BCH2, respectively) from roots of barley. The latter are considered to encode inducible high-affinity NO(3)(−) transporters (Trueman et al., 1996). HvNRT2 transcripts were undetectable in NO(3)(−)-deprived plants. Following exposure to either NO(3)(−) or NO(2)(−), transcript abundance and (13)NO(3)(−) influx increased to a maximum by 6 to 12 h, then declined in HvNRT2.1, HvNRT2.2, and HvNRT2.3. The pattern of HvNRT2.4 transcript abundance was different, remaining high after achieving peak abundance. When external NO(3)(−) concentrations were varied from 0 to 500 μm under steady-state conditions of NO(3)(−) supply, HvNRT2 transcript accumulation and (13)NO(3)(−) influx were highest in 50 μm NO(3)(−) -grown plants. When NH(4)(+) was provided together with NO(3)(−), transcript accumulation during the first 2 h was similar to that due to NO(3)(−) alone, but by 4 h the transcript level was significantly reduced. HvNRT2 transcript was undetectable in leaf tissues.