Symposium V: From the soil to the seed

Abs # 50003: Insights into the Function and Regulation of Nitrate Transport Systems

Presenter:	Crawford, Nigel M., ncrawford@ucsd.edu
Authors	Crawford, Nigel M. (A)   Guo, Fang-Qing  (A)   Okamoto, Mamoru  (A)   Benz, Philipp  (A)   Wang, Rongchen  (A)
Affiliations:	(A): Division of Biology, Univ. of Calif. at San Diego, La Jolla, CA 92093

Nitrate uptake requires the action of several systems that are distinguished by their regulation and by their affinity for nitrate. Classically, three systems have been recognized: an inducible high affinity (iHATS), a constitutive high affinity (cHATS) and a constitutive low affinity system (cLATS). Two gene families are known to contribute to these systems: NRT1 and NRT2. The NRT2 family contains 11 genes. The NRT1 family contains 2 genes that are known to encode nitrate transporters (NRT1.1 and NRT1.2), but there are over 50 genes with significant sequence similarity to NRT1.1. Many of the NRT1 genes are likely to encode oligopeptide transporters (part of the POT/PTR families), but some may contribute to nitrate transport. Studies to date indicate that from these gene families, two transporters NRT1.1 and NRT2.1 appear to dominate in roots of Arabidopsis. NRT2.1 has the properties for a component of the inducible high affinity system, being induced by nitrate and repressed by ammonium and by signals from the shoot. NRT1.1, however, does not fit neatly into any particular uptake system; in fact, it can contribute to all three. NRT1.1 is a dual affinity transporter whose affinity for nitrate is regulated by phosphorylation. This post-translational regulation is responsive to the external nitrate concentration. Transcriptional regulation of NRT1.1 is also important, being induced by nitrate, acidic pH, sucrose, and auxin. The auxin regulation is significant in that NRT1.1 expression shows a marked bias to regions of active growth: root tips and nascent shoot organs. This pattern of expression is controlled, at least in part, to potential auxin-responsive elements. Why a nitrate transporter would be responsive to auxin is not yet clear, but a clue is provided by the fact that nrt1.1 mutants have impaired growth of nascent organs (young primary roots, leaves and flower buds). Surprisingly, this phenotype is independent of nitrate (occurs in the absence or presence of nitrate). These results suggest that NRT1.1 is involved in nascent organ growth in a way that is not predicted by its known biochemical properties. Possible explanations will be discussed to explain the function(s) of NRT1.1 especially in organ growth.