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 an d contains 12 putative membrane-spanning domains and a long, hydrophil ic C-terminal domain. GmNRT2 is related to high-affinity nitrate trans porters in the eukaryotes Chlamydomonas reinhardtii and Aspergillus ni dulans, and to putative highaffinity nitrate transporters in barley an d tobacco. Southern blot analysis indicated that GmNRT2 is part of a s mall, multigene family in soybean. Expression of GmNRT2 in roots was r egulated by the type of nitrogen source provided to plants: GmNRT2 mRN A levels were barely detectable in ammonium-grown plants, higher in ni trogen-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 accompa nied by a fourfold increase in net nitrate uptake by soybean roots at 100 mu M external nitrate. The molecular and physiological evidence in dicates that GmNRT2 is probably a high-affinity nitrate transporter in volved in nitrate uptake by soybean roots.