Ions serve as essential nutrients in higher plants and can also act as signaling molecules. Little is known about how plants sense changes in soil nutrient concentrations. Previous studies showed that T101-phosphorylated CHL1 is a high-affinity nitrate transporter, whereas T101-dephosphorylated CHL1 is a low-affinity transporter. In this study, analysis of an uptake- and sensing-decoupled mutant showed that the nitrate transporter CHL1 functions as a nitrate sensor. Primary nitrate responses in CHL1T101D and CHLT101A transgenic plants showed that phosphorylated and dephosphorylated CHL1 lead to a low- and high-level response, respectively. In vitro and in vivo studies showed that, in response to low nitrate concentrations, protein kinase CIPK23 can phosphorylate T101 of CHL1 to maintain a low-level primary response. Thus, CHL1 uses dual-affinity binding and a phosphorylation switch to sense a wide range of nitrate concentrations in the soil, thereby functioning as an ion sensor in higher plants. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

This paper reports the identification and functional expression of a gene that is involved in nitrate uptake in plants, a process essential for the assimilation of nitrate and the biological removal of nitrate from the soil solution. The CHL1 gene of Arabidopsis, which when mutated confers resistance to the herbicide chlorate and a decrease in nitrate uptake, was isolated and found to encode a protein with 12 putative membrane-spanning segments. Injection of CHL1 mRNA into Xenopus oocytes produces a nitrate- and pH-dependent membrane depolarization, inward current, and nitrate uptake. These data show that the CHL1 gene encodes an electrogenic nitrate transporter. CHL1 mRNA is found predominantly in roots and displays nitrate- and pH-dependent regulation.

No abstract

In higher plants, two types of nitrate transporters, NRT1 and NRT2, have been identified. In Arabidopsis, there are 53 NRT1 genes and 7 NRT2 genes. NRT2 are high-affinity nitrate transporters, while most members of the NRT1 family are low-affinity nitrate transporters. The exception is CHL1 (AtNRT1.1), which is a dual-affinity nitrate transporter, its mode of action being switched by phosphorylation and dephosphorylation of threonine 101. Two of the NRT1 genes, CHL1 and AtNRT1.2, and two of the NRT2 genes, AtNRT2.1 and AtNRT2.2, are known to be involved in nitrate uptake. In addition, AtNRT1.4 is required for petiole nitrate storage. On the other hand, some members of the NRT1 family are dipeptide transporters, called PTRs, which transport a broad spectrum of di/tripeptides. In barley, HvPTR1, expressed in the plasma membrane of scutellar epithelial cells, is involved in mobilizing peptides, produced by hydrolysis of endosperm storage protein, to the developing embryo. In higher plants, there is another family of peptide transporters, called oligopeptide transporters (OPTs), which transport tetra/pentapeptides. In addition, some OPTs transport GSH, GSSH, GSH conjugates, phytochelatins, and metals.

To counteract¯uctuating nutrient environments, plants have evolved high-and low-af®nity uptake systems. These two systems were traditionally thought to be genetically distinct, but, recently, two Arabidopsis transporters, AtKUP1 and CHL1, were shown to have dual af®nities. However, little is known about how a dual-af®nity transporter works and the advantages of having a dual-af®nity transporter. This study demonstrates that, in the case of CHL1, switching between the two modes of action is regulated by phosphorylation at threonine residue 101; when phosphorylated, CHL1 functions as a high-af®nity nitrate transporter, whereas, when dephosphorylated, it functions as a low-af®nity nitrate transporter. This regulatory mechanism allows plants to change rapidly between high-and low-af®nity nitrate uptake, which may be critical when competing for limited nitrogen. These results demonstrate yet another regulatory role of phosphorylation in plant physiology.

No abstract

Little is known about the molecular and regulatory mechanisms of long-distance nitrate transport in higher plants. NRT1.5 is one of the 53 Arabidopsis thaliana nitrate transporter NRT1 (Peptide Transporter PTR) genes, of which two members, NRT1.1 (CHL1 for Chlorate resistant 1) and NRT1.2, have been shown to be involved in nitrate uptake. Functional analysis of cRNA-injected Xenopus laevis oocytes showed that NRT1.5 is a low-affinity, pH-dependent bidirectional nitrate transporter. Subcellular localization in plant protoplasts and in planta promoter-b-glucuronidase analysis, as well as in situ hybridization, showed that NRT1.5 is located in the plasma membrane and is expressed in root pericycle cells close to the xylem. Knockdown or knockout mutations of NRT1.5 reduced the amount of nitrate transported from the root to the shoot, suggesting that NRT1.5 participates in root xylem loading of nitrate. However, root-to-shoot nitrate transport was not completely eliminated in the NRT1.5 knockout mutant, and reduction of NRT1.5 in the nrt1.1 background did not affect rootto-shoot nitrate transport. These data suggest that, in addition to that involving NRT1.5, another mechanism is responsible for xylem loading of nitrate. Further analyses of the nrt1.5 mutants revealed a regulatory loop between nitrate and potassium at the xylem transport step. INTRODUCTIONNitrate and ammonium ions are the two major nitrogen sources for higher plants. Due to its toxicity, ammonium is preferentially assimilated in the root and then transported in an organic form to the aerial parts. By contrast, nitrate can be assimilated into ammonium and then amino acids in the root or shoot. Partitioning of nitrate assimilation between the root and shoot depends on the plant species, external nitrate concentration, temperature, and light intensity (reviewed in Smirnoff and Stewart, 1985). If there is sufficient light, nitrate assimilation in the leaf has a lower energy cost than in the root. However, some disadvantages of leaf nitrate assimilation include (1) if light is limited, nitrate assimilation and carbon dioxide fixation will compete directly for the reductants and ATP generated by photosynthetic electron transport (Canvin and Atkins, 1974), and (2) hydroxyl ions generated in the leaf need to be neutralized by the synthesis of organic acids (in the root, the pH balance may possibly be maintained by reducing proton excretion or increasing bicarbonate excretion). Due to these factors, the partition of nitrate assimilation between the root and shoot shows both seasonal and diurnal fluctuations, allowing the plant to sustain maximal growth. In turn, the partition of nitrate assimilation depends on the partition of nitrate between the root and shoot.To transport nitrate to the aerial parts of the plant, nitrate has to be loaded into the xylem vessels of the root vascular stele. In Arabidopsis thaliana roots, four layers of cells are found surrounding the xylem, these being the epidermis, cortex, endodermis, and pericycle (in the order external to ...

Nitrogen accounts for approximately 60% of the fertilizer consumed each year; thus, it represents one of the major input costs for most nonlegume crops. Nitrate is one of the two major forms of nitrogen that plants acquire from the soil. Mechanistic insights into nitrate transport and signaling have enabled new strategies for enhancing nitrogen utilization efficiency, for lowering input costs for farming, and, more importantly, for alleviating environmental impacts (e.g., eutrophication and production of the greenhouse gas NO). Over the past decade, significant progress has been made in understanding how nitrate is acquired from the surroundings, how it is efficiently distributed into different plant tissues in response to environmental changes, how nitrate signaling is perceived and transmitted, and how shoot and root nitrogen status is communicated. Several key components of these processes have proven to be novel tools for enhancing nitrate- and nitrogen-use efficiency. In this review, we focus on the roles of NRT1 and NRT2 in nitrate uptake and nitrate allocation among different tissues; we describe the functions of the transceptor NRT1.1, transcription factors, and small signaling peptides in nitrate signaling and tissue communication; and we compile the new strategies for improving nitrogen-use efficiency.