Cloning and functional characterization of an Arabidopsis nitrate transporter gene that encodes a constitutive component of low-affinity uptake

The Arabidopsis CHL1 (AtNRT1) gene encodes an inducible component of low-af finity nitrate uptake, which necessitates a "two-component" model to accoun t for the constitutive low-affinity uptake observed in physiological studie s. Here, we report the cloning and characterization of a CHL1 homolog, AtNR T1:2 (originally named NTL1), with data to indicate that this gene encodes a constitutive component of low-affinity nitrate uptake. Transgenic plants expressing antisense AtNRT1:2 exhibited reduced nitrate-induced membrane de polarization and nitrate uptake activities in assays with 10 mM nitrate. Fu rthermore, transgenic plants expressing antisense AtNRT1:2 in the ch/1-5 ba ckground exhibited an enhanced resistance to chlorate (7 mM as opposed to 2 mM for the ch/1-5 mutant). Kinetic analysis of AtNRT1:2-injected Xenopus o ocytes yielded a K-m for nitrate of similar to 5.9 mM. In contrast to CHL1, AtNRT1:2 was constitutively expressed before and after nitrate exposure (i t was repressed transiently only when the level of CHL1 mRNA started to inc rease significantly), and its mRNA was found primarily in root hairs and th e epidermis in both young (root tips) and mature regions of roots. We concl ude that low-affinity systems of nitrate uptake, like high-affinity systems , are composed of inducible and constitutive components and that with their distinct functions, they are part of an elaborate nitrate uptake network i n Arabidopsis.