Cloning and functional characterization of a constitutively expressed nitrate transporter gene, OsNRT1, from rice

Elucidating how rice (Oryza sativa) takes up nitrate at the molecular level could help improve the low recovery rate (<50%) of nitrogen fertilizer in rice paddies. As a first step toward that goal, we have cloned a nitrate tr ansporter gene from rice called OsNRT1. OsNRT1 is a new member of a growing transporter family called PTR, which consists not only of nitrate transpor ters from higher plants that are homologs of the Arabidopsis CHL1 (AtNRT1) protein, hut also peptide transporters from a wide variety of genera includ ing animals, plants, fungi, and bacteria. However, despite the fact that Os NRT1 shares a higher degree of sequence identity with the two peptide trans porters from plants (approximately 50%) than with the nitrate transporters (approximately 40%) of the PTR family, no peptide transport activity was ob served when OsNRT1 was expressed in either Xenopus oocytes or yeast. furthe rmore, contrasting the dual-affinity nitrate transport activity of CHL1, Os NRT1 displayed only low-affinity nitrate transport activity in Xenopus oocy tes, with a K-m value of approximately 9 mM. Northern-blot and in situ hybr idization analysis indicated that OsNRT1 is constitutively expressed in the most external layer of the root, epidermis and root hair. These data stron gly indicate that OsNRT1 encodes a constitutive component of a low-affinity nitrate uptake system for rice.