Eukaryotic nitrate and nitrite transporters

Abstract

Nitrate transport is the key step controlling the amount of nitrate incorporated by the cells and subsequent of storage, reduction or export. Molecular, genetic and biochemical approaches to the study of eukaryotic nitrate/nitrite transporters allow an initial understanding of this step, which is much more complex and structured than previously suspected. At the plasma membrane level, two gene families, Nrt1 and Nrt2, account for high- and low-affinity nitrate transporters. Functionality of NRT1 from Arabidopsis and NRT2 proteins from Aspergillus and Chlamydomonas has been demonstrated. However, redundancy of these systems makes it difficult to assign particular physiological roles to each. Data on genes involved in the regulation of nitrate transport and reduction are still scarce. Information on nitrite transporters to the chloroplast is biased by the belief that in vivo nitrous acid diffuses freely to this organellum. The recent progress on these aspects is discussed in this review.

R2R3-type MYB transcription factor, CmMYB1, is a central nitrogen assimilation regulator in Cyanidioschyzon merolae

“…Nitrate is reduced by nitrate reductase and nitrite reductase, and the resulting ammonium is usually assimilated by glutamine synthetase (GS) and glutamate synthase (GOGAT), which is known as the GS-GOGAT cycle (1). In land plants and green algal cells, the nitrate anion is imported into the cell by specific transporters (2), and reduced to nitrite by NAD(P)H-dependent nitrate reductase (NAR) in the cytosol. Nitrite is subsequently reduced to ammonium by ferredoxin-dependent nitrite reductase (NiR) in plastids.…”

The putative high-affinity nitrate transporter NRT2.1 represses lateral root initiation in response to nutritional cues

“…NRT1 proteins have been demonstrated both biochemically and genetically to be lowaffinity nitrate transporters (although the Arabidopsis NRT1.1 appears to function in both low-affinity transport system and HATS) (3). Although they are unrelated to the NRT2 proteins at the nucleotide or amino acid sequence level, NRT1 proteins are predicted to be members of the MFS and share an overall structure and membrane topology with the NRT2 family (6)(7)(8). Significantly, inhibition of primary root growth in an Arabidopsis nrt1.1 mutant was observed even in the absence of nitrate, leading Guo et al (22) to suggest that NRT1.1 may contribute to growth ''in a way that goes beyond the simple uptake of nitrate,'' perhaps through developmental signaling.…”

“…NRT2 proteins are members of the major facilitator superfamily (MFS) of transporter and transporter-like proteins (5). Higher plant NRT2 proteins are predicted to be nitrate transporters based on their homology to the fungal nitrate transporter CRNA and on the changes in nitrate uptake kinetics observed in Arabidopsis plants with mutations in NRT2 genes (6)(7)(8). Direct studies of nitrate uptake kinetics in higher plants have revealed two distinct systems: the low-affinity transport system that is responsible for uptake when nitrate is plentiful (Ͼ1 mM) and the high-affinity transport system (HATS) that is able to scavenge nitrate from the soil at concentrations between 1 M and 1 mM.…”

“…Direct studies of nitrate uptake kinetics in higher plants have revealed two distinct systems: the low-affinity transport system that is responsible for uptake when nitrate is plentiful (Ͼ1 mM) and the high-affinity transport system (HATS) that is able to scavenge nitrate from the soil at concentrations between 1 M and 1 mM. Molecular and physiological studies in a number of plant species indicate that the NRT2 proteins are required for the HATS (6)(7)(8).…”

“…For example, HATS-dependent nitrate uptake is stimulated by low external nitrate and inhibited in response to downstream products of nitrate assimilation such as ammonium and certain amino acids. Similarly, low levels of external nitrate induce NRT2.1 expression, whereas expression is repressed by downstream nitrate metabolites (6)(7)(8)(9)(10)(11)(12)(13)(14).…”