Mutation of the Arabidopsis NRT1.5 nitrate transporter causes defective root-to-shoot nitrate transport

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-β-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 root-to-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.     

The Arabidopsis nitrate transporter NRT2.4 plays a double role in roots and shoots of nitrogen-starved plants

Plants have evolved a variety of mechanisms to adapt to N starvation. NITRATE TRANSPORTER2.4 (NRT2.4) is one of seven NRT2 family genes in Arabidopsis thaliana, and NRT2.4 expression is induced under N starvation. Green fluorescent protein and β-glucuronidase reporter analyses revealed that NRT2.4 is a plasma membrane transporter expressed in the epidermis of lateral roots and in or close to the shoot phloem. The spatiotemporal expression pattern of NRT2.4 in roots is complementary with that of the major high-affinity nitrate transporter NTR2.1. Functional analysis in Xenopus laevis oocytes and in planta showed that NRT2.4 is a nitrate transporter functioning in the high-affinity range. In N-starved nrt2.4 mutants, nitrate uptake under low external supply and nitrate content in shoot phloem exudates was decreased. In the absence of NRT2.1 and NRT2.2, loss of function of NRT2.4 (triple mutants) has an impact on biomass production under low nitrate supply. Together, our results demonstrate that NRT2.4 is a nitrate transporter that has a role in both roots and shoots under N starvation.     

Potential transceptor AtNRT1.13 modulates shoot architecture and flowering time in a nitrate-dependent manner

Compared with root development regulated by external nutrients, less is known about how internal nutrients are monitored to control plasticity of shoot development. In this study, we characterize an Arabidopsis thaliana transceptor, NRT1.13 (NPF4.4), of the NRT1/PTR/NPF family. Different from most NRT1 transporters, NRT1.13 does not have the conserved proline residue between transmembrane domains 10 and 11; an essential residue for nitrate transport activity in CHL1/NRT1.1/NPF6.3. As expected, when expressed in oocytes, NRT1.13 showed no nitrate transport activity. However, when Ser 487 at the corresponding position was converted back to proline, NRT1.13 S487P regained nitrate uptake activity, suggesting that wild-type NRT1.13 cannot transport nitrate but can bind it. Subcellular localization and β-glucuronidase reporter analyses indicated that NRT1.13 is a plasma membrane protein expressed at the parenchyma cells next to xylem in the petioles and the stem nodes. When plants were grown with a normal concentration of nitrate, nrt1.13 showed no severe growth phenotype. However, when grown under low-nitrate conditions, nrt1.13 showed delayed flowering, increased node number, retarded branch outgrowth, and reduced lateral nitrate allocation to nodes. Our results suggest that NRT1.13 is required for low-nitrate acclimation and that internal nitrate is monitored near the xylem by NRT1.13 to regulate shoot architecture and flowering time.

Nitrate transporter/transceptor NRT1.13 monitors xylem 12 nitrate level to regulate shoot architecture and flowering time.     

The Arabidopsis Nitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance

Long-distance transport of nitrate requires xylem loading and unloading, a successive process that determines nitrate distribution and subsequent assimilation efficiency. Here, we report the functional characterization of NRT1.8, a member of the nitrate transporter (NRT1) family in Arabidopsis thaliana. NRT1.8 is upregulated by nitrate. Histochemical analysis using promoter-β-glucuronidase fusions, as well as in situ hybridization, showed that NRT1.8 is expressed predominantly in xylem parenchyma cells within the vasculature. Transient expression of the NRT1.8:enhanced green fluorescent protein fusion in onion epidermal cells and Arabidopsis protoplasts indicated that NRT1.8 is plasma membrane localized. Electrophysiological and nitrate uptake analyses using Xenopus laevis oocytes showed that NRT1.8 mediates low-affinity nitrate uptake. Functional disruption of NRT1.8 significantly increased the nitrate concentration in xylem sap. These data together suggest that NRT1.8 functions to remove nitrate from xylem vessels. Interestingly, NRT1.8 was the only nitrate assimilatory pathway gene that was strongly upregulated by cadmium (Cd²⁺) stress in roots, and the nrt1.8-1 mutant showed a nitrate-dependent Cd²⁺-sensitive phenotype. Further analyses showed that Cd²⁺ stress increases the proportion of nitrate allocated to wild-type roots compared with the nrt1.8-1 mutant. These data suggest that NRT1.8-regulated nitrate distribution plays an important role in Cd²⁺ tolerance.     

The Arabidopsis Nitrate Transporter NRT1.7, Expressed in Phloem,Is Responsible for Source-to-Sink Remobilization of Nitrate

Several quantitative trait locus analyses have suggested that grain yield and nitrogen use efficiency are well correlated with nitrate storage capacity and efficient remobilization. This study of the Arabidopsis thaliana nitrate transporter NRT1.7 provides new insights into nitrate remobilization. Immunoblots, quantitative RT-PCR, β-glucuronidase reporter analysis, and immunolocalization indicated that NRT1.7 is expressed in the phloem of the leaf minor vein and that its expression levels increase coincidentally with the source strength of the leaf. In nrt1.7 mutants, more nitrate was present in the older leaves, less ¹⁵NO₃⁻ spotted on old leaves was remobilized into N-demanding tissues, and less nitrate was detected in the phloem exudates of old leaves. These data indicate that NRT1.7 is responsible for phloem loading of nitrate in the source leaf to allow nitrate transport out of older leaves and into younger leaves. Interestingly, nrt1.7 mutants showed growth retardation when external nitrogen was depleted. We conclude that (1) nitrate itself, in addition to organic forms of nitrogen, is remobilized, (2) nitrate remobilization is important to sustain vigorous growth during nitrogen deficiency, and (3) source-to-sink remobilization of nitrate is mediated by phloem.     

Characterization of the Arabidopsis Nitrate Transporter NRT1.6 Reveals a Role of Nitrate in Early Embryo Development

This study of the Arabidopsis thaliana nitrate transporter NRT1.6 indicated that nitrate is important for early embryo development. Functional analysis of cDNA-injected Xenopus laevis oocytes showed that NRT1.6 is a low-affinity nitrate transporter and does not transport dipeptides. RT-PCR, in situ hybridization, and β-glucuronidase reporter gene analysis showed that expression of NRT1.6 is only detectable in reproductive tissue (the vascular tissue of the silique and funiculus) and that expression increases immediately after pollination, suggesting that NRT1.6 is involved in delivering nitrate from maternal tissue to the developing embryo. In nrt1.6 mutants, the amount of nitrate accumulated in mature seeds was reduced and the seed abortion rate increased. In the mutants, abnormalities (i.e., excessive cell division and loss of turgidity), were found mainly in the suspensor cells at the one- or two-cell stages of embryo development. The phenotype of the nrt1.6 mutants revealed a novel role of nitrate in early embryo development. Interestingly, the seed abortion rate of the mutant was reduced when grown under N-deficient conditions, suggesting that nitrate requirements in early embryo development can be modulated in response to external nitrogen changes.     

Arabidopsis NRT1.5 Is Another Essential Component in the Regulation of Nitrate Reallocation and Stress Tolerance

Nitrate reallocation to plant roots occurs frequently under adverse conditions and was recently characterized to be actively regulated by Nitrate Transporter1.8 (NRT1.8) in Arabidopsis (Arabidopsis thaliana) and implicated as a common response to stresses. However, the underlying mechanisms remain largely to be determined. In this study, characterization of NRT1.5, a xylem nitrate-loading transporter, showed that the mRNA level of NRT1.5 is down-regulated by salt, drought, and cadmium treatments. Functional disruption of NRT1.5 enhanced tolerance to salt, drought, and cadmium stresses. Further analyses showed that nitrate, as well as Na(+) and Cd(2+) levels, were significantly increased in nrt1.5 roots. Important genes including Na(+)/H(+) exchanger1, Salt overly sensitive1, Pyrroline-5-carboxylate synthase1, Responsive to desiccation29A, Phytochelatin synthase1, and NRT1.8 in stress response pathways are steadily up-regulated in nrt1.5 mutant plants. Interestingly, altered accumulation of metabolites, including proline and malondialdehyde, was also observed in nrt1.5 plants. These data suggest that NRT1.5 is involved in nitrate allocation to roots and the consequent tolerance to several stresses, in a mechanism probably shared with NRT1.8.     

Characterization of the Nrt2.6 Gene in Arabidopsis thaliana: A Link with Plant Response to Biotic and Abiotic Stress

The high affinity nitrate transport system in Arabidopsis thaliana involves one gene and potentially seven genes from the NRT1 and NRT2 family, respectively. Among them, NRT2.1, NRT2.2, NRT2.4 and NRT2.7 proteins have been shown to transport nitrate and are localized on the plasmalemma or the tonoplast membranes. NRT2.1, NRT2.2 and NRT2.4 play a role in nitrate uptake from soil solution by root cells while NRT2.7 is responsible for nitrate loading in the seed vacuole. We have undertaken the functional characterization of a third member of the family, the NRT2.6 gene. NRT2.6 was weakly expressed in most plant organs and its expression was higher in vegetative organs than in reproductive organs. Contrary to other NRT2 members, NRT2.6 expression was not induced by limiting but rather by high nitrogen levels, and no nitrate-related phenotype was found in the nrt2.6-1 mutant. Consistently, the over-expression of the gene failed to complement the nitrate uptake defect of an nrt2.1-nrt2.2 double mutant. The NRT2.6 expression is induced after inoculation of Arabidopsis thaliana by the phytopathogenic bacterium Erwinia amylovora. Interestingly, plants with a decreased NRT2.6 expression showed a lower tolerance to pathogen attack. A correlation was found between NRT2.6 expression and ROS species accumulation in response to infection by E. amylovora and treatment with the redox-active herbicide methyl viologen, suggesting a probable link between NRT2.6 activity and the production of ROS in response to biotic and abiotic stress.     

From the soil to the seeds: the long journey of nitrate in plants

Under temperate climates and in cultivated soils, nitrate is the most important source of nitrogen (N) available for crops and, before its reduction and assimilation into amino acids, must enter the root cells and then move in the whole plant. The aim of this review is to provide an overall picture of the numerous membrane proteins that achieve these processes by being localized in different compartments and in different tissues. Nitrate transporters (NRT) from the NRT1 and NRT2 families ensure the capacity of root cells to take up nitrate, through high- and low-affinity systems (HATS and LATS) depending on nitrate concentrations in the soil solution. Other members of the NRT1 family are involved subsequently in loading and unloading of nitrate to and from the xylem vessels, allowing its distribution to aerial organs or its remobilization from old leaves. Once in the cell, nitrate can be stored in the vacuole by passing through the tonoplast, a step that involves chloride channels (CLC) or a NRT2 member. Finally, with the exception of one NRT1 member, the transport of nitrite towards the chloroplast is still largely unknown. All these fluxes are controlled by key factors, the 'major tour operators' like the internal nutritional status of the plant but also by external abiotic factors.     

The Arabidopsis ATNRT2.7 nitrate transporter controls nitrate content in seeds

In higher plants, nitrate is taken up by root cells where Arabidopsis thaliana NITRATE TRANSPORTER2.1 (ATNRT2.1) chiefly acts as the high-affinity nitrate uptake system. Nitrate taken up by the roots can then be translocated from the root to the leaves and the seeds. In this work, the function of the ATNRT2.7 gene, one of the seven members of the NRT2 family in Arabidopsis, was investigated. High expression of the gene was detected in reproductive organs and peaked in dry seeds. beta-Glucuronidase or green fluorescent protein reporter gene expression driven by the ATNRT2.7 promoter confirmed this organ specificity. We assessed the capacity of ATNRT2.7 to transport nitrate in Xenopus laevis oocytes or when it is expressed ectopically in mutant plants deficient in nitrate transport. We measured the impact of an ATNRT2.7 mutation and found no difference from the wild type during vegetative development. By contrast, seed nitrate content was affected by overexpression of ATNRT2.7 or a mutation in the gene. Finally, we showed that this nitrate transporter protein was localized to the vacuolar membrane. Our results demonstrate that ATNRT2.7 plays a specific role in nitrate accumulation in the seed.     

Root-to-shoot transport of sulfate in Arabidopsis. Evidence for the role of SULTR3;5 as a component of low-affinity sulfate transport system in the root vasculature

Xylem transport of sulfate regulates distribution of sulfur in vascular plants. Here, we describe SULTR3;5 as an essential component of the sulfate transport system that facilitates the root-to-shoot transport of sulfate in the vasculature. In Arabidopsis (Arabidopsis thaliana), SULTR3;5 was colocalized with the SULTR2;1 low-affinity sulfate transporter in xylem parenchyma and pericycle cells in roots. In a yeast (Saccharomyces cerevisiae) expression system, sulfate uptake was hardly detectable with SULTR3;5 expression alone; however, cells coexpressing both SULTR3;5 and SULTR2;1 showed substantial uptake activity that was considerably higher than with SULTR2;1 expression alone. The V(max) value of sulfate uptake activity with SULTR3;5-SULTR2;1 coexpression was approximately 3 times higher than with SULTR2;1 alone. In Arabidopsis, the root-to-shoot transport of sulfate was restricted in the sultr3;5 mutants, under conditions of high SULTR2;1 expression in the roots after sulfur limitation. These results suggested that SULTR3;5 is constitutively expressed in the root vasculature, but its function to reinforce the capacity of the SULTR2;1 low-affinity transporter is only essential when SULTR2;1 mRNA is induced by sulfur limitation. Consequently, coexpression of SULTR3;5 and SULTR2;1 provides maximum capacity of sulfate transport activity, which facilitates retrieval of apoplastic sulfate to the xylem parenchyma cells in the vasculature of Arabidopsis roots and may contribute to the root-to-shoot transport of sulfate.     

Nitrate-Dependent Control of Shoot K Homeostasis by the Nitrate Transporter1/Peptide Transporter Family Member NPF7.3/NRT1.5 and the Stelar K+ Outward Rectifier SKOR in Arabidopsis

Root-to-shoot translocation and shoot homeostasis of potassium (K) determine nutrient balance, growth, and stress tolerance of vascular plants. To maintain the cation-anion balance, xylem loading of K⁺ in the roots relies on the concomitant loading of counteranions, like nitrate (NO₃⁻). However, the coregulation of these loading steps is unclear. Here, we show that the bidirectional, low-affinity Nitrate Transporter1 (NRT1)/Peptide Transporter (PTR) family member NPF7.3/NRT1.5 is important for the NO₃⁻-dependent K⁺ translocation in Arabidopsis (Arabidopsis thaliana). Lack of NPF7.3/NRT1.5 resulted in K deficiency in shoots under low NO₃⁻ nutrition, whereas the root elemental composition was unchanged. Gene expression data corroborated K deficiency in the nrt1.5-5 shoot, whereas the root responded with a differential expression of genes involved in cation-anion balance. A grafting experiment confirmed that the presence of NPF7.3/NRT1.5 in the root is a prerequisite for proper root-to-shoot translocation of K⁺ under low NO₃⁻ supply. Because the depolarization-activated Stelar K⁺ Outward Rectifier (SKOR) has previously been described as a major contributor for root-to-shoot translocation of K⁺ in Arabidopsis, we addressed the hypothesis that NPF7.3/NRT1.5-mediated NO₃⁻ translocation might affect xylem loading and root-to-shoot K⁺ translocation through SKOR. Indeed, growth of nrt1.5-5 and skor-2 single and double mutants under different K/NO₃⁻ regimes revealed that both proteins contribute to K⁺ translocation from root to shoot. SKOR activity dominates under high NO₃⁻ and low K⁺ supply, whereas NPF7.3/NRT1.5 is required under low NO₃⁻ availability. This study unravels nutritional conditions as a critical factor for the joint activity of SKOR and NPF7.3/NRT1.5 for shoot K homeostasis.The macronutrient potassium (K) is essential for plant growth and development because of its crucial roles in various cellular processes (i.e. regulation of enzyme activities), stabilization of protein synthesis, and neutralization of negative charges. In addition, it is a major component of the cation-anion balance and osmoregulation in plants, thereby influencing cellular turgor, xylem and phloem transport, pH homeostasis, and the setting of membrane potentials (Maathuis, 2009; Marschner, 2012; Sharma et al., 2013). K⁺ uptake and distribution in Arabidopsis (Arabidopsis thaliana) are accomplished by a total of 71 membrane proteins that have been assigned to five gene families: the Shaker and Tandem-Pore K⁺ channels (now also including the inward-rectifier K-like (K_ir-like) channels), the K⁺ uptake permeases (KUP/HAK/KT), the K⁺ transporter (HKT) family, and the cation proton antiporters (CPA; Gierth and Mäser, 2007; Gomez-Porras et al., 2012; Sharma et al., 2013).Root xylem loading is a key step for the delivery of nutrients to the shoot (Poirier et al., 1991; Engels and Marschner, 1992a; Gaymard et al., 1998; Takano et al., 2002; Park et al., 2008). Root-to-shoot translocation of K⁺ is mediated by the voltage-dependent Shaker family K⁺ channel Stelar K⁺ Outward Rectifier (SKOR). The gene is primarily expressed in pericycle and root xylem parenchyma cells, and it is down-regulated upon K shortage and in response to treatments with the phytohormones abscisic acid, cytokinin, and auxin. Such gene expression changes are thought to control K⁺ secretion into the xylem sap and K⁺ reallocation through the phloem to adjust root K⁺ transport activity to K⁺ availability and shoot demand (Pilot et al., 2003). SKOR is activated upon membrane depolarization, and it is in a closed state when the driving force for K⁺ is inwardly directed. It elicits outward K⁺ currents, facilitating the release of the cation from the cells into the xylem. The voltage dependency of the channel is modulated by the external K⁺ concentration to minimize the risk of an undesired K⁺ influx under high K⁺ availability (Johansson et al., 2006). Root-to-shoot K⁺ transfer was strongly reduced in the knockout mutant skor-1, resulting in a decreased shoot K content, whereas the root K content remained unaffected (Gaymard et al., 1998).Root xylem loading is subject to the maintenance of a cation-anion balance, and nitrate (NO₃⁻) is the quantitatively most important anion counterbalancing xylem loading of K⁺ (Engels and Marschner, 1993). Members of the Nitrate Transporter1 (NRT1)/Peptide Transporter (PTR) transporter family (NPF) play a prominent role in NO₃⁻ uptake and allocation in Arabidopsis (summarized in Krouk et al., 2010; Wang et al., 2012; and Léran et al., 2014). Two of them have recently been reported to control xylem NO₃⁻ loading and unloading. The low-affinity, pH-dependent bidirectional NO₃⁻ transporter NPF7.3/NRT1.5 (subsequently termed NRT1.5) mediates NO₃⁻ efflux from pericycle cells to the xylem vessels, whereas the low-affinity influx protein NPF7.2/NRT1.8 removes NO₃⁻ from the xylem sap and transfers it into xylem parenchyma cells (Lin et al., 2008; Li et al., 2010; Chen et al., 2012). Accordingly, the expression of both genes is oppositely regulated under various stress conditions (Li et al., 2010). In nrt1.5 mutants, NRT1.8 expression is increased, which is thought to enhance NO₃⁻ reallocation to the root (Chen et al., 2012).The NRT1.5 gene is mainly expressed in root pericycle cells close to the xylem, and the protein localizes to the plasma membrane. In nrt1.5 mutants, less NO₃⁻ is transported from the root to the shoot, and the NO₃⁻ concentration in the xylem sap is reduced. However, root-to-shoot NO₃⁻ transport is not completely abolished in these mutants, indicating the existence of additional xylem-loading activities for NO₃⁻ (Lin et al., 2008; Wang et al., 2012). The recent observation that NPF6.3/NRT1.1/CHL1 and NPF6.2/NRT1.4 are also capable of mediating bidirectional NO₃⁻ transport in Xenopus laevis oocytes might indicate that more NPF family members are contributing to xylem loading with NO₃⁻ (Léran et al., 2013).Electrophysiological studies with NRT1.5-expressing X. laevis oocytes revealed that NO₃⁻ excited an inward current at pH 5.5, which would be expected for a proton-coupled nitrate transporter with a proton to nitrate ratio larger than one (Lin et al., 2008). The inward currents elicited by exposure to nitrate were pH dependent, and Lin et al. (2008) observed that NRT1.5 can also facilitate nitrate efflux when the oocytes were incubated at pH 7.4. Lin et al. (2008) concluded that NRT1.5 can transport nitrate in both directions, presumably through a proton-coupled mechanism. Interestingly, a K⁺ gradient was not sufficient to drive NRT1.5-mediated NO₃⁻ export. However, the determination of root and shoot cation concentrations in the nrt1.5-1 mutant revealed that the amount of K⁺ translocated to the shoot was reduced when NO₃⁻ but not NH₄⁺ was supplied as the N source. Therefore, Lin et al. (2008) suggested a regulatory loop between NO₃⁻ and K⁺ at the xylem loading step.A close relationship between these two nutrients concerning uptake, translocation, recycling, and reduction (of NO₃⁻) has been described in physiological studies since the 1960s (e.g. Ben Zioni et al., 1971; Blevins et al., 1978; Barneix and Breteler, 1985), but only recently, common components in the NO₃⁻ and K⁺ uptake pathways were identified and led to the first ideas of how such a cross talk might be coordinated on the molecular level. The uptake activity of the K⁺ channel AKT1 as well as the affinity of the NO₃⁻ transporter NPF6.3/NRT1.1/CHL1 are both modulated by the activity of CALCINEURIN B-LIKE PROTEIN-INTERACTING PROTEIN KINASE23 (CIPK23), which itself is regulated by CALCINEURIN B-LIKE PROTEIN9 (CBL9) under both deficiencies (Xu et al., 2006; Ho et al., 2009). Yet, the details of this interaction in root K⁺ uptake, the (regulation of) xylem loading with K⁺ and NO₃⁻, and the involvement of SKOR and NRT1.5 in this process are unknown.In this study, we approached this problem by investigating the molecular and physiological responses of Arabidopsis wild-type (Columbia-0 [Col-0]), nrt1.5, and skor transfer DNA (T-DNA) insertion lines to varying NO₃⁻ and K⁺ regimes. The nrt1.5 mutant developed an early senescence phenotype under low NO₃⁻ nutrition, which could be attributed to a reduced K⁺ translocation to the shoot. The assessment of nrt1.5 and skor single- and double-knockout lines disclosed an interplay of the two proteins in the NO₃⁻-dependent control of shoot K homeostasis. The presented data indicate that SKOR mediates K⁺ root-to-shoot translocation under high NO₃⁻ and low K⁺ availability, whereas NRT1.5 is important for K⁺ translocation under low NO₃⁻ availability, irrespective of the K⁺ supply.     

A deletion in NRT2.1 attenuates Pseudomonas syringae-induced hormonal perturbation, resulting in primed plant defenses

For an efficient defense response against pathogens, plants must coordinate rapid genetic reprogramming to produce an incompatible interaction. Nitrate Trasnporter2 (NRT2) gene family members are sentinels of nitrate availability. In this study, we present an additional role for NRT2.1 linked to plant resistance against pathogens. This gene antagonizes the priming of plant defenses against the bacterial pathogen Pseudomonas syringae pv tomato DC3000 (Pst). The nrt2 mutant (which is deficient in two genes, NRT2.1 and NRT2.2) displays reduced susceptibility to this bacterium. We demonstrate that modifying environmental conditions that stimulate the derepression of the NRT2.1 gene influences resistance to Pst independently of the total level of endogenous nitrogen. Additionally, hormonal homeostasis seemed to be affected in nrt2, which displays priming of salicylic acid signaling and concomitant irregular functioning of the jasmonic acid and abscisic acid pathways upon infection. Effector-triggered susceptibility and hormonal perturbation by the bacterium seem to be altered in nrt2, probably due to reduced sensitivity to the bacterial phytotoxin coronatine. The main genetic and metabolic targets of coronatine in Arabidopsis (Arabidopsis thaliana) remain largely unstimulated in nrt2 mutants. In addition, a P. syringae strain defective in coronatine synthesis showed the same virulence toward nrt2 as the coronatine-producing strain. Taken together, the reduced susceptibility of nrt2 mutants seems to be a combination of priming of salicylic acid-dependent defenses and reduced sensitivity to the bacterial effector coronatine. These results suggest additional functions for NRT2.1 that may influence plant disease resistance by down-regulating biotic stress defense mechanisms and favoring abiotic stress responses.     

Nitrate transporter NPF7.3/NRT1.5 plays an essential role in regulating phosphate deficiency responses in Arabidopsis

AtNPF7.3/AtNRT1.5, which is a nitrate transporter that drives root-to-shoot transport of NO₃^?, is also involved in modulating the response to K⁺ deprivation in Arabidopsis by affecting root development and K⁺ transport. However, whether NPF7.3/NRT1.5 functions in regulating plant responses to deficiencies of other nutrients remains unknown. In this study, we found that the expression of AtNPF7.3/AtNRT1.5 was predominant in the roots and was substantially induced by phosphate (Pi) starvation. The atnrt1.5 mutants displayed conspicuously longer primary roots along with a significantly reduced lateral root density under Pi-deficient conditions than did the wild-type plants, and these morphological differences in the roots were eliminated to a certain extent by the ethylene synthesis antagonist Co²⁺. Further analyses revealed that the expression of important Pi starvation-induced genes, which are directly involved in Pi transport, mobilization and distribution, were significantly higher in the atnrt1.5 mutants than that in the wild-type plants under Pi-starvation conditions; therefore, the atnrt1.5 mutants retained higher tissue Pi concentrations. Taken together, our results suggest that NPF7.3/NRT1.5 is an important component in the regulation of phosphate deficiency responses in Arabidopsis.