Isolation and Characterization of HvNRT2.3 and HvNRT2.4, cDNAs Encoding High-Affinity Nitrate Transporters from Roots of Barley1

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American Society of Plant Physiologists

RESUMO

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 13NO3− 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 μm under steady-state conditions of NO3− supply, HvNRT2 transcript accumulation and 13NO3− influx were highest in 50 μm 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.

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