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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.

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 ...

SummaryNitrate, the major nitrogen source for most plants, is not only a nutrient but also a signaling molecule. For almost two decades, it has been known that nitrate can rapidly induce transcriptional expression of several nitrate-related genes, a process that is referred to as the primary nitrate response. However, little is known about how plants actually sense nitrate and how the signal is transmitted in this pathway. In this study, a calcineurin B-like (CBL) -interacting protein kinase (CIPK) gene, CIPK8, was found to be involved in early nitrate signaling. CIPK8 expression was rapidly induced by nitrate. Analysis of two independent knockout mutants and a complemented line showed that CIPK8 positively regulates the nitrate-induced expression of primary nitrate response genes, including nitrate transporter genes and genes required for assimilation. Kinetic analysis of nitrate induction levels of these genes in wild-type plants indicated that there are two response phases: a high-affinity phase with a K m of approximately 30 lM and a low-affinity phase with a K m of approximately 0.9 mM. As cipk8 mutants were defective mainly in the low-affinity response, the high-affinity and low-affinity nitrate signaling systems are proposed to be genetically distinct, with CIPK8 involved in the low-affinity system. In addition, CIPK8 was found to be involved in long-term nitrate-modulated primary root growth and nitrate-modulated expression of a vacuolar malate transporter. Taken together, our results indicate that CBL-CIPK networks are responsible not only for stress responses and potassium shortage, but also for nitrate sensing.

This study of the Arabidopsis thaliana nitrate transporter NRT1.9 reveals an important function for a NRT1 family member in phloem nitrate transport. Functional analysis in Xenopus laevis oocytes showed that NRT1.9 is a low-affinity nitrate transporter. Green fluorescent protein and β-glucuronidase reporter analyses indicated that NRT1.9 is a plasma membrane transporter expressed in the companion cells of root phloem. In nrt1.9 mutants, nitrate content in root phloem exudates was decreased, and downward nitrate transport was reduced, suggesting that NRT1.9 may facilitate loading of nitrate into the root phloem and enhance downward nitrate transport in roots. Under high nitrate conditions, the nrt1.9 mutant showed enhanced root-to-shoot nitrate transport and plant growth. We conclude that phloem nitrate transport is facilitated by expression of NRT1.9 in root companion cells. In addition, enhanced root-to-shoot xylem transport of nitrate in nrt1.9 mutants points to a negative correlation between xylem and phloem nitrate transport.