13003:

New models of nitrate uptake derived from the complex behaviors of the Arabidopsis nitrate transporter CHL1 and its homolog AtNRT1:2.

Authors:	Tsay, Yi-Fang (A)Liu, Kun-Hsiang(A)Huang, Nien-Chen(A)
Affiliations:	(A): Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan (B): Department of Botany, National Taiwan University, Taipei, Taiwan
Presenter:	Tsay, Yi-Fang , mbyftsay@ccvax.sinica.edu.tw

To take up nitrate, higher plants have both low- and high- affinity transport systems (LATS and HATS). In response to nitrate induction, they can be further divided into constitutive (c) and inducible (i) components, of which cLATS, cHATS and iHATS have been identified by uptake studies. These components are thought to be genetically distinct. Indeed, a number of nitrate transporter genes cloned belong to two distinct families, called NRT1 and NRT2, and initial characterizations suggest NRT1 encodes LATS while NRT2 encodes HATS. On further characterizing Arabidopsis CHL1 gene (AtNRT1) and its homolog AtNRT1:2 (previously named NTL1), we showed, however, that these prevailing descriptions of nitrate transporter systems need to be modified. CHL1 encodes a nitrate transporter and its mutant is defective in low-affinity nitrate uptake. However, CHL1 is a nitrate-inducible gene. This conflicts with the established notion that LATS is composed of only constitutive component. To reconcile this discrepancy, we had proposed a ¡§two-component¡¨ model for LATS of Arabidopsis. Namely, like HATS, LATS also contains inducible and constitutive components. Cloning and functional characterization of AtNRT1:2 indicated it encodes a cLATS, thereby confirming the ¡§two-component¡¨ model. Unlike CHL1, AtNRT1:2 was constitutively expressed. In un-induced transgenic plants of anti-AtNRT1:2, membrane depolarization of root cells elicited by nitrate was reduced, so was the LATS activity. Interestingly, whereas high-affinity nitrate uptake activity was normal in anti-AtNRT1:2 plants, it was defective in chl1 mutants. Consistently, nitrate uptake activity of CHL1-injected Xenopus oocytes is biphasic with Km of ~50 mM for the high-affinity phase and ~4 mM for the low-affinity phase; but, AtNRT1:2-injected oocytes showed only low-affinity nitrate uptake activity. Both inducible and constitutive phases of HATS were reduced in chl1 mutants ¡Xthe inducible phase severely. These data indicate that CHL1 is a dual-affinity transporter involving in multiple phases (iLATS, iHATS and cHATS) of nitrate uptake, while AtNRT1:2 is a pure low-affinity transporter. Together with other group¡¦s findings, a new model for nitrate uptake with the following new elements emerges: 1) both constitutive and inducible components are present in LATS, (2) a single protein can be involved in both low- and high- affinity uptake, and (3) more than one protein can participate in the same uptake phase.