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 NO3- transporters (Trueman et al., 1996). HvNRT2 transcripts were undetectable in NO3--deprived plants. Following exposure to either NO3- or NO2-, transcript abundance and (NO3-)-N-13 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 NO3- concentrations were varied from 0 to 500 mu M under steady-state conditions of NO3- supply, HvNRT2 transcript accumulation and (NO3-)-N-13 influx were highest in 50 mu M NO3- -grown plants. When NH4+ was provided together with NO3-, transcript accumulation during the first 2 h was similar to that due to NO3- alone, but by 4 h the transcript level was significantly reduced. HvNRT2 transcript was undetectable in leaf tissues.