Regulation of GmNRT2 expression and nitrate transport activity in roots of soybean (Glycine max)

A full-length cDNA, GmNRT2, encoding a putative high-affinity nitrate transporter was isolated from a Glycine max (L.) root cDNA library and sequenced. The deduced GmNRT2 protein is 530 amino acids in length and contains 12 putative membrane-spanning domains and a long, hydrophilic C-terminal domain. GmNRT2 is related to high-affinity nitrate transporters in the eukaryotes Chlamydomonas reinhardtii and Aspergillus nidulans, and to putative high-affinity nitrate transporters in barley and tobacco. Southern blot analysis indicated that GmNRT2 is part of a small, multigene family in soybean. Expression of GmNRT2 in roots was regulated by the type of nitrogen source provided to plants: GmNRT2 mRNA levels were barely detectable in ammonium-grown plants, higher in nitrogen-deprived plants, and highest in nitrate-grown plants. Induction of GmNRT2 mRNA levels in roots occurred within 1 h after exposure of plants to nitrate. Nitrate induction of GmNRT2 mRNA levels was accompanied by a fourfold increase in net nitrate uptake by soybean roots at 100 microM external nitrate. The molecular and physiological evidence indicates that GmNRT2 is probably a high-affinity nitrate transporter involved in nitrate uptake by soybean roots.