Journal of Experimental Botany, Vol. 53, No. 370, pp. 855-864,
April 15, 2002
© 2002 Oxford University Press
Original Papers |
The regulation of nitrate and ammonium transport systems in plants
1 University of British Columbia, 6270 University Blvd, Vancouver, V6T1Z4, Canada
2 Division of Life Sciences, University Of Toronto, 1265 Military Trail, Scarborough, Ontario, M1C 1A4 Canada
3 Environmental Biology, RSBS, Australian National University, GPO Box 475, Canberra ACT 2601, Australia
4 School of Biology, University of St Andrews, St Andrews KY16 9TH, UK
Received 21 August 2001; Accepted 3 December 2001
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Abstract |
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 Top Abstract IntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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Inorganic nitrogen concentrations in soil solutions vary across several orders of magnitude among different soils and as a result of seasonal changes. In order to respond to this heterogeneity, plants have evolved mechanisms to regulate
and 
influx. In addition, efflux analysis using 13N has revealed that there is a co-ordinated regulation of all component fluxes within the root, including biochemical fluxes. Physiological studies have demonstrated the presence of two high-affinity transporter systems (HATS) for and one HATS for in roots of higher plants. By contrast, in Arabidopsis thaliana there exist seven members of the NRT2 family encoding putative HATS for and five members of the AMT1 family encoding putative HATS for . The induction of high-affinity transport and Nrt2.1 and Nrt2.2 expression occur in response to the provision of , while down-regulation of these genes appear to be due to the effects of glutamine. High-affinity transport and AMT1.1 expression also appear to be subject to down-regulation by glutamine. In addition, there is evidence that accumulated and may act post-transcriptionally on transporter function. The present challenge is to resolve the functions of all of these genes. In Aspergillus nidulans and Chlamydomonas reinhardtii there are but two high-affinity transporters and these appear to have undergone kinetic differentiation that permits a greater efficiency of absorption over the wide range of concentration normally found in nature. Such kinetic differentiation may also have occurred among higher plant transporters. The characterization of transporter function in higher plants is currently being inferred from patterns of gene expression in roots and shoots, as well as through studies of heterologous expression systems and knockout mutants. Key words: Ammonium, AMT1, flux regulation, nitrate, Nrt2.
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Introduction |
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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Inorganic ions accumulated in plant cells serve nutritional, osmotic, signalling, and storage functions. Insufficient ion accumulation as well as excess accumulation may therefore compromise these functions. While vacuolar reserves may buffer the cytoplasm against short-term perturbations, in laboratory studies when external sources of ions are removed vacuolar reserves are typically exhausted within a few days (Glass, 1975
; Lee et al., 1990; van der Leij et al., 1998). Under field conditions vacuolar reserves may be even more limited. When vacuolar reserves are consumed to sustain cytosolic functions, there is a need to replace their osmotic and charge-balancing function by means of alternative solutes, be they inorganic or organic. Hence vacuolar buffering of cytosolic ion concentrations is not achieved without consequences and, typically, plant roots respond to perturbations of external supply or internal demand long before vacuolar reserves are exhausted. This raises the interesting issue of the signal pathways between vacuole and cytoplasm required to initiate these responses; virtually unexplored territory.
Given that both and commonly serve as sources of N for plant growth and that they share some metabolic pathways, it is perhaps not surprising to find that they possess features in common: (1) both ions are actively absorbed into root cells at low external concentrations; (2) influx measurements indicate the presence of two high-affinity transport systems (HATS) for (one constitutive and the other inducible) and one HATS for 
(3) influx of both ions is responsive to plant N status and subject to diurnal regulation; (4) molecular studies indicate the presence of seven HATS for and five for in A. thaliana; and (5) some of the genes encoding transporters are subject to transcriptional regulation through inductive effects of , while some of those encoding and transporters are subject to down-regulating effects of glutamine. Notwithstanding these similarities there are also distinct differences in the characteristics of and uptake, as well as differences among species in the extent of their utilization of these different nitrogen sources.
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Soil heterogeneity |
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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Heterogeneity of soil nutrient availability is potentially the most important perturbing effect upon plant nutrient status. In addition, seasonal and diurnal changes in growth rates and plant demand for resources are also substantial. In this paper, the main focus will be upon flux regulation in response to perturbations of external supply and, in particular, the responses of the HATS for
and to these perturbations. In the context of these effects that would displace the plant from steady state, ion fluxes are regulated by feedback from various cellular parameters that serve to counteract such changes.
According to data compiled previously, and concentrations of agricultural soils range across three to four orders of magnitude (Wolt, 1994). The situation is even more variable in natural soils (Jackson and Caldwell, 1993). In addition, specific habitats (e.g. mature forests, arctic tundra) may be characterized by nitrogen profiles dominated by ammonium or amino acids, rather than . Many species occupying such habitats have become specialists, absorbing or amino acids in preference to (Kielland, 1994; Kronzucker et al., 1997; Nasholm et al., 1998, 2000). Even when exceeds by as much as 10-fold, uptake may still greatly exceed that of in field and laboratory studies (Gessler et al., 1998). In a study of nitrogen absorption by tomato (MY Siddiqi et al., unpublished data), it was demonstrated that 50% of plant N was absorbed as , even though this ion represented only 10% of available N, the remaining 90% being . In the context of this variability of N supply plants have evolved numerous mechanisms (physiological/biochemical, developmental and life history-based strategies) that enable them to optimize nitrogen acquisition. Included among the physiological adaptations, are the ‘up-regulation’ of nitrogen uptake under conditions of N-limitation, but also the restriction of nitrogen uptake under conditions of N excess. The latter presumably serves to minimize potentially harmful osmotic or specific ion effects.
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Physiological characterization of and uptake
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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Measurements of 13
influx and net uptake by several groups have revealed the presence of three transport systems for and two for (reviewed in Glass and Siddiqi, 1995). In roots of species examined for its presence, a low capacity, constitutively expressed, high-affinity transport system (cHATS) allows entry of from low external . The extent of this flux varies among and within species (Siddiqi et al., 1989; King et al., 1993; Kronzucker et al., 1995; Zhuo et al., 1999). Following first exposure to there is a rapid increase of an inducible high-affinity influx (iHATS), which is followed (after several h) by an equally rapid down-regulation of this flux (Siddiqi et al., 1989; Zhuo et al., 1999). There are significant differences in the response time to applied among species. For example, in Picea glauca, it was necessary to expose plants to for 3 d in order to induce peak 13 influx (Kronzucker et al., 1995). Both and 
are capable of inducing this flux (Siddiqi et al., 1992; Aslam et al., 1993).
Several studies have demonstrated that the provision of to N-deprived roots may initially increase uptake prior to down-regulating the flux, and the term induction has also been applied to this initial increase of influx (see Kronzucker et al., 1998, for references and discussion). However, in these studies high-affinity influx was already high (de-repressed) before exposure to , and it has been demonstrated that, in rice, the increase of influx resulting from pretreatment was relatively small (25–40%) (Kronzucker et al., 1998). By comparison, a 30-fold increase of 13 influx was recorded in Klondike barley following pretreatment with (Siddiqi et al., 1990). Kronzucker et al. concluded that the evidence did not support a true inductive effect of (Kronzucker et al., 1998).
At nitrate and ammonium concentrations between 
200 to 500 µM, low-affinity transporter systems (LATS) for these ions become apparent. These were evident in earlier studies (Doddema and Telkamp, 1979; Ullrich et al., 1984), but were largely overlooked, in part because the measurement of and uptake at high concentration by depletion methods was typically insufficiently sensitive to characterize these transporters. A perplexing feature of these high capacity low-affinity transporters has been their linear concentration responses (Pace and McClure, 1986; Ullrich et al., 1984), that were earlier suggested to result from diffusive fluxes. However, although fluxes via LATS are typically thermodynamically ‘downhill’ (Ullrich et al., 1984; Wang et al., 1993), the LATS for was shown to be active even at high external concentration and mediated, like the iHATS, by a proton:nitrate symport (Glass et al., 1992).
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Homeostatic processes for nitrogen uptake |
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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As outlined above, the uptake of both
and is subject to down-regulation as tissue N levels approach some upper limit. As early as 1906, Brezeale demonstrated, using hydroponic wheat plants, that withholding K, P, N, Ca or S for 18 h resulted in several-fold increases in rates of absorption of the particular nutrient that had been withheld (Brezeale, 1906). As far as is known, this is the first documented evidence of the physiological regulation of ion uptake by plant roots. Clement et al., using ryegrass as a model system, established that, when available concentrations were maintained from 14.3 µM to 14.3 mM, plant growth was only modestly affected and tissue nitrogen concentration remained essentially constant (Clement et al., 1978a). The up-regulation of nitrate fluxes first observed by Brezeale forms an important component of the processes responsible for achieving nitrogen homeostasis (Brezeale, 1906), while adjustments in growth rate may also be critical under some circumstances (Ingestad and Lund, 1979). While transport shows the same general homeostatic propensity (Wang et al., 1993; Rawat et al., 1999), the potential toxicity of elevated ambient concentrations severely limits the range of concentration over which adaptation is possible. In a study of 13 fluxes across the plasma membranes of barley roots, Britto et al. showed that at 10 mM external , active efflux rose to 76% of the value of influx (Britto et al., 2001). Simultaneously, root respiration increased by 40%, and was not diminished by treatment with the GS inhibitor methionine sulphoximine (MSX), indicating that the respiratory increase was not associated with increased assimilation of , but with active extrusion. In summary, while high-affinity fluxes are effectively regulated, transport via the low-affinity system is poorly regulated, resulting in considerable futile cycling of across the plasma membrane as well as toxic effects of excessive accumulation (Britto et al., 2001).
Studies of the many component and fluxes that occur in plant cells are severely limited, even in single-celled organisms by cellular compartmentation. In multicellular plants fluxes to and from roots via xylem and phloem further complicate the situation. Therefore, for technical reasons involving the ease of measurement, the emphasis in studies of the mechanisms responsible for ion fluxes and their regulation has been upon the influx step (
oc) across the plasma membrane. Nevertheless, there is evidence to suggest that efflux from cytosol to cell wall (co), fluxes across the tonoplast (cv and vc), from cytosol to xylem (cx), as well as fluxes to biochemical pathways appear to be co-ordinated. The use of efflux analysis to estimate the half-lives (t0.5) for 13 and 13 residence within the cytosolic compartment, has revealed that t0.5 values are virtually independent of prior nitrogen provision (Siddiqi et al., 1991; Wang et al., 1993; Britto and Kronzucker, 2001). Figure 1 shows data for 13 efflux from roots of barley grown under steady-state conditions with various concentrations of nitrate for 7 d prior to labelling with 13 and subsequent measurement of 13 efflux into non-labelled solutions of the same concentration (Britto and Kronzucker, 2001). Despite the wide range of concentrations used and the substantial changes of measured fluxes, the rate constants for 13 efflux were essentially identical (0.0408, 0.0400, 0.0417, 0.0418, and 0.04908 min-1 for plants grown in 10, 1, 0.1, 0.01, and 0 mM , respectively). In a study of the effect of perturbing external on 13 efflux from barley roots, Britto and Kronzucker showed that when external concentration was increased or decreased, respectively, from 1 mM to either 10 mM or to 100 µM, there was initially a rapid increase or decrease, respectively, of 13 efflux (Britto and Kronzucker, 2001). Yet, despite this initial perturbation of tracer efflux, rate constants for this flux were restored to their original values within minutes as shown in Fig. 2. Such results point to a precise integration of all component fluxes that impact upon cytosolic ion concentrations.
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Plants were grown for 7 d under steady-state conditions with respect to nitrate provision. Roots were then loaded with 13
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during elution (from Britto and Kronzucker, 2001Several studies using 13
and 13 have demonstrated that co increases as external ion concentration increases (Siddiqi et al., 1991; Wang et al., 1993) and that net transfer of nitrogen from vacuole to cytosol (vc–cv) increases (van der Leij et al., 1998), and from cytosol to stele (cx) decreases (Kronzucker et al., 1998), as external ion concentrations decrease. Nevertheless, these fluxes have not been quantified in the same detail that has characterized measurements of oc, nor have genes yet been cloned that encode these transport systems. Likewise there is a lack of detailed studies of the fluxes of and into leaf cells. Having noted the paucity of information concerning fluxes other than the root influx step, the remainder of this paper, will focus on the regulation of high-affinity and influx across the plasma membrane of root cells. |
Induction and down-regulation of influx |
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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It is evident from a number of different studies that only
or among potential products of nitrogen assimilation are capable of inducing influx by the iHATS (Tompkins et al., 1978; Behl et al., 1988; Siddiqi et al., 1992; Tischner et al., 1993; Guy and Heimer, 1993; Henriksen and Spanswick, 1993). Nevertheless, as low-N plants accumulate N, the influxes of both and are subsequently down-regulated (Lee and Rudge, 1986; Morgan and Jackson, 1988; Siddiqi et al., 1989; Kronzucker et al., 1995; Glass and Siddiqi, 1995; Forde and Clarkson 1999). Prior to the cloning of genes that encoded and transporters, two hypotheses emerged to explain this down-regulation. On the one hand it was proposed that accumulated or themselves, as opposed to their downstream metabolites, were responsible for down-regulation of fluxes. This was based upon inverse correlations between accumulated or and N fluxes in wild-type plants. This conclusion was supported by the results of experiments in which nitrate reductase (NR) was blocked by tungstate treatment in Lemna gibba and Helianthus annuus (Ingemarsson et al., 1987; De la Haba et al., 1990) or by mutation in barley (Warner and Huffaker, 1989; Siddiqi et al., 1989; King et al., 1993). Incapacitating NR failed to impact upon induction or down-regulation of influx, suggesting that itself was responsible for these effects. Likewise effects of MSX application (Ryan and Walker, 1994; King et al., 1993; Feng et al., 1994; Glass et al., 1997) suggested that itself was responsible for down-regulating influx. On the other hand convincing support for effects of down-stream metabolites has been provided by experiments in which exogenously applied amino acids strongly inhibited both and influx, and by several studies in which MSX application blocked down-regulation (Lee and Rudge, 1986; Morgan and Jackson, 1988; Lee et al., 1992; Muller and Touraine, 1992; Rodgers and Barneix, 1993). The contradictory nature of these findings is exemplified by studies on maize and sorghum (Feng et al., 1994). While 15 influx was stimulated by MSX treatment in maize, in sorghum influx was inhibited. Likewise, Glass et al. observed that, in low-N rice plants, the effects of MSX were consistent with down-regulation of influx by end-products of assimilation while in high-N plants itself appeared to be involved (Glass et al., 1997). Unfortunately, given that MSX has been used in so many of these studies, it must be acknowledged that cytosolic may reach as high as 80 mM when assimilation is blocked by this compound (Lee and Ratcliffe, 1991). These are clearly abnormal conditions. As will be evident below, the results of molecular studies has provided some clarification of this question at the transcript level. |
Genes encoding putative high-affinity and transporters
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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The cloning of genes encoding putative high-affinity
transporters belonging to the NRT2 family of genes (see Forde, 2000, for a recent review) and putative high-affinity transporters of the AMT1 family of genes (see Howitt and Udvardi, 2000, for a recent review), has allowed investigations of the regulation of high-affinity and influx to proceed to the transcript level. As was the case for induction of uptake, only or were capable of inducing the accumulation of NRT2 transcript. Moreover transcript accumulation followed the same general patterns as had been observed for the induction of uptake/influx, namely induction over a period of up to 3 h or more followed by down-regulation (Trueman et al., 1996; Quesada et al., 1997; Amarasinghe et al., 1998; Filleur et al., 1999; Zhuo et al., 1999). In NR mutants, high levels of accumulation and increased NRT2 transcript abundance suggested that while is responsible for inducing gene expression, it is down-stream metabolites that are responsible for down-regulation (Krapp et al., 1998; Filleur and Daniel-Vedele, 1999; Lejay et al., 1999). Likewise, in barley roots tungstate treatment to block NR caused increased NRT2 transcript abundance (Vidmar et al., 2000). Several reports have documented the down-regulation of NRT2 transcript abundance in response to pretreatment with or amino acids (Quesada et al., 1997; Krapp et al., 1998; Zhuo et al., 1999). Unfortunately, exogenous application of amino acids or provides little information concerning the N pools that might be responsible for these effects. Differences in uptake or assimilation of applied amino acids, as well as their inter-conversion obscure the sources of observed effects. In addition, exogenous application of various amino acids was shown to increase root [] up to 6-fold in rice (Wang, 1994; Kumar et al., unpublished results). Another important consideration is whether or not a particular amino acid is a typical/major component of xylem and phloem-translocated N, since cycling/recycling of amino acids within the vascular system has been proposed as the basis of communicating plant N status to roots so that N uptake may be regulated according to plant N demand (Cooper and Clarkson, 1989; Marschner et al., 1997; Glass et al., 2001). By providing various nitrogen sources (, , and/or amino acids) in the presence and absence of inhibitors of assimilation, for example, tungstate (
) to block nitrate reductase, MSX to block glutamine synthetase, and azaserine (AZA) to block glutamate synthase, this confusion can be resolved. In barley, combining results based on the effects of exogenous applications of amino acids with data from inhibitor studies (Fig. 3) demonstrated that NRT2 transcript abundance was most strongly correlated with root glutamine concentrations (Vidmar et al., 2000). Thus, increasing root glutamine by pretreatment with AZA virtually eliminated 13 influx and NRT2 transcript in both A. thaliana and in H. vulgare (Zhuo et al., 1999; Vidmar et al., 2000).
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Using A. thaliana as the model system, Rawat et al. demonstrated that up-regulation and down-regulation of 13
influx (following removal and restoration of exogenous N, respectively) was strongly correlated with AMT1.1 transcript abundance (Rawat et al., 1999). In the presence of MSX, provision caused root [] to increase 27-fold, while root glutamine levels remained at the original (N-deprived) level. Concurrent measurements of 13 influx and Northern analysis revealed that despite this increase of root [], transcript abundance and influx remained almost at control (N-starved) levels. These results strongly suggest that glutamine is pivotal in regulating AMT1 transcript abundance.
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Multiple members of the Nrt2 and Amt1 families |
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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In the study of barley NRT2 genes by Trueman et al., it was suggested that there might be as many as 8–10 homologues in this species (Trueman et al., 1996
). Following completion of the Arabidopsis genome sequencing project, it is now apparent that there are seven homologues in A. thaliana. A major task to be resolved is the individual functions of these genes. Work in the senior author's laboratory has been directed toward this goal, using A. thaliana as a model system. Under the conditions of this growth system, in which plants are grown hydroponically in open vessels, it has been possible to detect expression of all seven NRT2 homologues in roots and shoots using RT-PCR (Okamoto et al., unpublished data). Based upon the number of PCR cycles required and quantities of template RNA provided, it appears that AtNRT2.1 and AtNRT2.2 are the most abundantly expressed genes. In roots these genes are expressed at roughly 10 times the levels of all other genes whether in roots or shoots. The seven genes have been grouped into three categories according to their responses to nitrate feeding in plants previously deprived of for a period of 7 d before resupplying this ion. Category No. 1 includes AtNRT2.1 and AtNRT2.2, genes whose expression in roots increased 3–5-fold following provision of 1 mM . Both genes are subsequently down-regulated, presumably by a gradual increase of tissue glutamine. In shoots expression levels of these genes increased by less than 50% in response to provision, but, as in roots, this increase was followed by substantial down-regulation. Category No. 2 contains genes that are constitutively expressed, showing virtually no response to provision of . In both roots and shoots AtNRT2.5 and AtNRT2.6 show this pattern while for AtNRT2.3 this pattern was restricted to roots. In shoots, AtNRT2.3 expression levels doubled by 48 h. Category No. 3 contains AtNRT2.4 and AtNRT2.7, genes that are immediately down-regulated following exposure to (Okamoto et al., unpublished results). Interestingly, when AtNRT2.1 and AtNRT2.2 were first cloned from plants grown for several days with 1 mM KNO3 (Zhuo et al., 1999), it was stated that AtNRT2.2 was expressed at substantially lower levels than AtNRT2.1. However, it is apparent from these time-course studies (Okamoto et al., unpublished data) that, following initial exposure to , AtNRT2.2 transcript abundance is roughly equivalent to that of AtNRT2.1, however, by 12 h AtNRT2.2 transcript abundance is substantially reduced compared to AtNRT2.1. Based on the high levels of AtNRT2.1 and AtNRT2.2 transcript abundance in roots and the correspondence between the patterns of changes in transcript abundance and high-affinity influx, these genes are good candidates for encoding iHATS. Recently, Filleur et al. have isolated a T-DNA insertional mutant of A. thaliana disrupted in adjoining AtNRT2.1 and AtNRT2.2 genes (Filleur et al., 2001). High-affinity transport in this mutant was reduced to 27% of wild-type rates. Thus it can be concluded that AtNRT2.1 and AtNRT2.2 make major contributions to the iHATS. The extent to which the remaining transport is due to other NRT2 genes or to NRT1 (low-affinity transport) is presently unknown (Wang et al., 1998).
If both AtNRT2.1 and AtNRT2.2 genes encode iHATS in roots, an important question is what (if any) differential roles these transporters might serve. Some suggestive answers to this question may be provided by comparisons with NRT2 genes of other organisms. In Aspergillus nidulans only two functional NRT2 genes appear to exist, and all four genotypes (wild type, double mutant and two single mutants) have been characterized with respect to 13 influx kinetics (Unkles et al., 2001). Hoffstee plots of 13 influx indicate that both transporters contribute to influx in wild-type strains, although the transporters show distinct kinetic differentiation. The NrtA (originally crnA) transporter has a high Vmax and high Km (564 nmol mg-1 DW h-1 and 96.3 µM, respectively) while the second transporter (NrtB) has a low Vmax and low Km (141 nmol mg-1 DW h-1 and 11 µM, respectively). Interestingly the corresponding transporters in Chlamydomonas reinhardtii also possess widely different Km values for uptake (1.6 and 11 µM, respectively), but differ only slightly in Vmax values (9.0 and 5.6 µmol h-1 mg-1 chlorophyll, respectively (Galvan et al., 1996). This kinetic differentiation presumably enables the organism to access efficiently over a much wider range of concentration than would be possible by means of a single transporter. The A. nidulans double mutant is incapable of using as sole source of N at concentrations up to 250 mM or of absorbing 13 at concentrations up to 500 µM. Continued exposure to leads to down-regulation of 13 influx in wild-type strains. This is due to down-regulation of NrtA, activity (Vmax values were 564±67 and 300±71 nmol mg-1 DW h-1 at 6 h and 16 h, respectively). By contrast, 13 influx via the NrtB protein was unaffected by duration of exposure to (Vmax values were 141±6 and 162±26 nmol mg-1 DW h-1 at 6 and 16 h, respectively). This difference in response to duration of exposure among the strains may be due to slower accumulation of and products of assimilation that would normally down-regulate gene expression in mutant strains expressing only the NrtB protein. Thus, by default, gene mutation is partially compensated for.
The AMT1 family of high-affinity transporters contains five members, of which AtAMT1.1, AtAMT1.2 and AtAMT1.3 have been studied in detail (Gazzarini et al., 1999). All three genes are expressed in roots, while only AMT1.1 is expressed in significant amounts in leaves. By measuring 14C-methylamine uptake by Saccharomyces cerevisiae mutants expressing these genes individually, it was possible to estimate Km values of 0.5 µM for the AtAMT1.1, transporter and 40 µM for the AtAMT1.2 and AtAMT1.3 transporters. During N starvation, transcript abundance of AtAMT1.1 increased 7-fold during 24 h (Rawat et al., 1999). In a comparative study of root AtAMT1.1, AtAMT1.2 and AtAMT1.3 expression in response to N deprivation, it was shown that AtAMT1.1 increased 5-fold within 72 h, compared to a 2-fold increase in AtAMT1.3 and no change in AtAMT1.2 transcript abundance (Gazzarini et al., 1999). In tomato, LeAMT1.1 and LeAMT 1.2 transporters are expressed in roots, while LeAMT1.3 is preferentially expressed in shoots (von Wiren et al., 2000). Levels of LeAMT1.1 transcript in tomato roots also increased over time under conditions of N-deprivation and this was associated with a decline of glutamine and pool sizes (von Wiren et al., 2000). By contrast, and perhaps contrary to expectation, LeAMT1.2 transcript abundance increased following re-supply of or . This response may account for the initial stimulation of influx that was discussed above following resupply of N to N-starved plants (Kronzucker et al., 1998). LeAMT1.3 was not detected in roots.
A T-DNA insertional mutant has recently been isolated from Arabidopsis that fails to express AtAMT1.1 mRNA (Glass et al., 2001). Surprisingly, since AMT1.1 shows the strongest response to N-deprivation and also had the highest affinity for (at least when expressed heterologously in S. cerevisiae) disruption of this gene function reduced 13 influx by only 20–30% (Glass et al., 2001). It is possible that, because of reduced uptake and thereby reduced negative feedback effects on transcript abundance of other AMT genes, there was compensation for the disruption of AtAMT1.1. This isssue is currently being explored.
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Diurnal effects on and uptake
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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There is now abundant evidence to confirm that
and uptake display characteristic diurnal patterns (Clement et al., 1978b; Macduff et al., 1997; Peuke and Jeschke, 1998; Gazzarini et al., 1999; Tischner, 2000). In the study by Clement et al., peak uptake occurred in the late afternoon while minimum uptake rates occurred at the end of the dark period or even in the first hours of daylight (Clement et al., 1978b). It is notable that the amplitude of the diurnal pattern and the absolute values of the flux declined substantially during the course of the greenhouse study (Clement et al., 1978b). This was associated with the onset of poor weather and a 75% reduction of irradiance. This may account for the low amplitude of the diurnal pattern reported in many growth chamber studies where plants are generally maintained under low irradiance. For example, in soybeans maintained on a 9/15 h light/dark regimen, uptake of 15 was reduced by only 6% in the dark compared to the light period (Rufty et al., 1984). It has been suggested that reduced uptake associated with darkness may be countered by exogenously applied carbohydrates (Sehtiya and Goyal, 2000). Thus, in barley and maize, 1% sucrose additions caused 31% and 70% increases of uptake, respectively, in the light, while in dark-grown plants the values were 38% for both barley and maize. Nevertheless, given that dark-grown seedlings should have been substantially more carbohydrate-depleted than light-grown plants, it is surprising that the sucrose effect was actually less (maize) or similar (barley) in dark-grown plants.
uptake in Phleum, Festuca and Arabidopsis also exhibits a diurnal periodicity, gradually increasing to a peak level toward the end of daylight hours (Macduff et al., 1997; Gazzarini et al., 1999), and the amplitude of the diurnal pattern of , and K+ uptake was highest on high irradiance days (Macduff et al., 1997).
Molecular studies have demonstrated that diurnal patterns of N uptake are correlated with diurnal patterns of transcript abundance for the high-affinity NRT2 and AMT1 genes (Lejay et al., 1999; Ono et al., 2000; von Wiren et al., 2000; Matt et al., 2001). In A. thaliana, NRT2.1 expression in roots increased in daylight hours and declined in the first hours of the dark period, this night-time reduction being prevented by additions of sucrose (Lejay et al., 1999). In roots of A. thaliana, all three members of the AMT1 family exhibited diurnal variation, with AtAMT1.3 expression showing the strongest correlation with diurnal patterns of 15 uptake. In leaves of tomato, LeAMT1.2 and LeAMT1.3 showed a reciprocal diurnal pattern of expression with LeAMT1.3 transcript being highest in darkness.
The conclusion that C and N metabolism are tightly linked is inescapable (Coruzzi and Bush, 2001). In the study by Matt et al., the activities of various enzymes involved in nitrogen metabolism and their transcript abundances, including the high-affinity nitrate transporter, as well as concentrations of various metabolites (, amino acids, sugars and 2-oxoglutarate) were measured during a diurnal cycle in tobacco (Matt et al., 2001). Based upon the correspondence between root sugar levels and NRT2 transcript abundance (and a lack of correspondence with other metabolites) the authors concluded that root sugars were responsible for the diurnal pattern of NRT2 expression. It is intriguing to consider whether the effects of carbohydrate supply might act directly or indirectly on nitrogen pools and/or transcript abundances. For example, when carbohydrate supply to the root limits N assimilation and/or growth, accumulation of N metabolites might reduce expression of transporter genes or even act directly upon the transporters. Furthermore, the study by Matt et al. acknowledged that the observed correlations between NRT2 expression and root sugar levels were based upon whole root analyses (Matt et al., 2001). Clearly, cytosolic metabolite concentrations might have provided a different conclusion.
In summary, a high degree of heterogeneity with respect to soil N availability and diurnal and seasonal variation in plant requirements for N impose a need to regulate N fluxes across the plasma membrane of plant roots in order to optimize plant N capture. The need to integrate/co-ordinate N acquisition from several potential soil N sources (, and amino acids) suggests that regulation might be most effective if a common end-product of assimilation such as glutamine were to serve as the source of negative feedback. Experiments listed above indicate that this may be the case. Nevertheless, there is no reason to assume that, in addition to the clearly demonstrated regulation by transcript abundance, there will not be post-transcriptional regulation by other nitrogen sources. Indeed preliminary evidence for such effects has already been presented (Fraisier et al., 2000; Vidmar et al., 2000; Rawat et al., 1999).
In addition to regulating influx across root plasma membranes, internal redistributions to vacuole and to xylem suggest that there is a need for integration of all component fluxes as well as for the integration of amino acid fluxes involved in nutrient cycling within plants. Thus far, the focus of attention in studies of inorganic N uptake at the physiological and molecular levels has been upon the regulation of root plasma membrane transporters. It is to be anticipated that future physiological and molecular studies will include fluxes to subcellular compartments and between major organs of the plant (such as fluxes from root to xylem, xylem to shoot) and leaf uptake of inorganic N.
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Acknowledgments |
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The work undertaken by the authors was financed by grants from NSERC to ADM Glass, who gratefully acknowledges this support. In addition we gratefully acknowledge the provision of 13N by the UBC TRIUMF cyclotron.
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5 Present address: Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.
6 To whom correspondence should be addressed. Fax: +16048226089. E-mail: aglass{at}interchange.ubc.ca
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Abbreviations |
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NR, nitrate reductase; NiR, nitrite reductase; GS, glutamine synthetase; Gln, glutamine; Glu, glutamate..
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TopAbstractIntroductionSoil heterogeneityPhysiological characterization...Homeostatic processes for...Induction and down-regulation of...Genes encoding putative high...Multiple members of the...Diurnal effects on and...References |
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Amarasinghe BHRR, Debruxelles GL, Braddon M, Onyeocha I, Forde BG, Udvardi MK. 1998. Regulation of GMNRT2 expression and nitrate transport activity in soybean (Glycine max). Planta 206, 44–52.[Web of Science][Medline]
Aslam M, Travis RL, Huffaker RC. 1993. Comparative induction of nitrate and nitrite uptake and reduction systems by ambient nitrate and nitrite in intact roots of barley (Hordeum vulgare L.) seedlings. Plant Physiology 102, 811–819.[Abstract]
Behl R, Tischner R, Raschke K. 1988. Induction of a high-capacity nitrate uptake mechanism in barley roots prompted by nitrate uptake through a constitutive low-capacity mechanism. Planta 176, 235–240.[Web of Science]
Brezeale JF. 1906. The relation of sodium to potassium in soil and solution cultures. Journal of American Chemical Society 28, 1013–1025.
Britto DT, Kronzucker HJ. 2001. Constancy of nitrogen turnover kinetics in the plant cell: Insights into the integration of subcellular N fluxes. Planta 213, 175–181.[Web of Science][Medline]
Britto DT, Siddiqi MY, Glass ADM, Kronzucker HJ. 2001. Futile membrane ion cycling: a new cellular hypothesis to explain ammonium toxicity in plants. Proceedings of National Academy of Sciences, USA 98, 4255–4258.
Clement CR, Hopper MJ, Jones LHP. 1978a. The uptake of nitrate by Lolium perenne from flowing nutrient solution. I. Effect of nitrate concentration. Journal of Experimental Botany 29, 453–464.
Clement CR, Hopper MJ, Jones LHP. 1978b. The uptake of nitrate by Lolium perenne from flowing nutrient solution. II. Effect of light, defoliation and relationship to CO2 flux. Journal of Experimental Botany 29, 1173–1183.
Cooper HD, Clarkson DT. 1989. Cycling of amino-nitrogen and other nutrients between shoots and roots in cereals—a possible mechanism integrating shoot and root in the regulation of nutrient uptake. Journal of Experimental Botany 40, 753–762.
Coruzzi G, Bush DR. 2001. Nitrogen and carbon nutrient and metabolite signalling in plants. Plant Physiology 125, 61–64.
De la Haba P, Aguera E, Maldonado JM. 1990. Differential effects of ammonium and tungsten on nitrate and ammonium uptake and reduction by sunflower plants. Plant Science 70, 21–26.
Doddema H, Telkamp GP. 1979. Uptake of nitrate by mutants of Arabidopsis thaliana, disturbed in uptake or reduction of nitrate. II. Kinetics. Physiologia Plantarum 45, 332–338.
Feng J, Volk RJ, Jackson WA. 1994. Inward and outward transport of ammonium in roots of maize and sorghum: contrasting effects of methionine sulphoximine. Journal of Experimental Botany 45, 429–439.
Filleur S, Daniel-Vedele F. 1999. Expression analysis of a high-affinity nitrate transporter isolated from Arabidopsis thaliana by differential display. Planta 207, 461–469.[Web of Science][Medline]
Filleur S, Dorbe MF, Cerezo M, Orsel M, Granier F, Gojon A, Daniel-Vedele F. 2001. An Arabidopsis T-DNA mutant affected in Nrt2 genes is impaired in nitrate uptake. FEBS Letters 489, 220–224.[Web of Science][Medline]
Forde BG. 2000. Nitrate transporters in plants: structure, function and regulation. Biochimica et Biophysica Acta 1465, 219–236.[Medline]
Forde BG, Clarkson DT. 1999. Nitrate and ammonium nutrition of plants: physiological and molecular perspectives. Advances in Botanical Research 30, 1–90.
Fraisier V, Gojon A, Tillard P, Daniel-Vedele F. 2000. Constitutive expression of a putative high-affinity nitrate transporter in Nicotiana plumbaginifolia: evidence for post-transcriptional regulation by a reduced nitrogen source. The Plant Journal 23, 489–496.[Web of Science][Medline]
Galvan A, Quesada A, Fernandez E. 1996. Nitrate and nitrite are transported by different specific transport systems and by a bispecific transporter in Chlamydomonas reinhardtii. Journal of Biological Chemistry 271, 2088–2092.
Gazzarini S, Lejay L, Gojon A, Ninnemann O, Frommer WB. 1999. Three functional transporters for constitutive, diurnally regulated and starvation-induced uptake of ammonium into Arabidopsis roots. The Plant Cell 11, 937–947.
Gessler A, Schneider S, Von Sengbusch D, Weber P, Hanemann U, Huber C, Rothe A, Kreutzer K, Rennenberg H. 1998. Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees. New Phytologist 138, 275–285.
Glass ADM. 1975. The regulation of potassium absorption of barley roots. Plant Physiology 56, 377–380.
Glass ADM, Britto DT, Kaiser BN, Kronzucker HJ, Kumar A, Okamoto M, Rawat SR, Siddiqi MY, Silim SM, Vidmar JJ, Zhuo D. 2001. Nitrogen transport in plants, with emphasis on the regulation of fluxes to match plant demand. Zeitschrift für Pflanzenernährung Bodenkunde 164, 199–207.
Glass ADM, Erner Y, Kronzucker HJ, Schjoerring JK, Siddiqi MY, Wang M-Y. 1997. Ammonium fluxes into plant roots: energetics, kinetics and regulation. Zeitschrift für Pflanzenernährung Bodenkunde 160, 261–268.
Glass ADM, Shaff JE, Kochian LV. 1992. Studies of the uptake of nitrate in barley. IV. Electrophysiology. Plant Physiology 99, 456–463.
Glass ADM, Siddiqi MY. 1995. Nitrogen absorption by plant roots. In: Srivastava HS, Singh RP, eds. Nitrogen nutrition in higher plants. New Delhi: Associated Publishing Company, 21–56.
Guy M, Heimer YM. 1993. On the inducibility of nitrate transport by tobacco cells. Physiologia Plantarum 89, 596–601.
Henriksen GH, Spanswick RM. 1993. Investigation of the apparent induction of nitrate uptake in barley (Hordeum vulgare L.) using selective microelectrodes: modulation of coarse regulation of -uptakeby exogenous application of downstream metabolites in the -assimilatory pathway. Plant Physiology 103, 885–892.[Abstract]
Howitt SM, Udvardi MK. 2000. Structure, function and regulation of ammonium transporters in plants. Biochimica et Biophysica Acta 1465, 152–171.[Medline]
Ingemarsson B, Oscarson P, af Ugglas M, Larsson C-M. 1987. Nitrogen utilization in Lemna. II. Studies of nitrate uptake using 13. Plant Physiology 85, 860–864.
Ingestad T, Lund AB. 1979. Nitrogen stress in birch seedlings. I. Growth technique and growth. Physiologia Plantarum 45, 137–148.
Jackson RB, Caldwell MM. 1993. The scale of nutrient heterogeneity around individual plants and its quantification with geostatics. Ecology 74, 612–614.[Web of Science]
Kielland K. 1994. Amino acid absorption by arctic plants: implications for plant nutrition and nitrogen cycling. Ecology 75, 2373–2383.[Web of Science]
King BJ, Siddiqi MY, Ruth TJ, Warner RL Glass ADM. 1993. Feedback regulation of nitrate influx in barley roots by nitrate, nitrite and ammonium. Plant Physiology 102, 1279–1286.[Abstract]
Krapp A, Fraisier V, Scheible WR, Quesada A, Gojon A, Stitt M, Caboche M, Daniel-Vedele F. 1998. Expression studies of NRT2-1NP, a putative high-affinity nitrate transporter: evidence for its role in nitrate uptake. The Plant Journal 14, 723–731.
Kronzucker HJ, Schjoering JK, Erner Y, Kirk GJD, Siddiqi MY, Glass ADM. 1998. Dynamic interactions between root influx and long-distance N translocation in rice: insights into feedback processes. Plant and Cell Physiology 39, 1287–1293.
Kronzucker HJ, Siddiqi MY, Glass ADM. 1995. Compartmentation and flux characteristics of nitrate in spruce. Planta 196, 674–682.
Kronzucker HJ, Siddiqi MY, Glass ADM. 1997. Root discrimination against soil nitrate and the ecology of forest succession. Nature 385, 59–61.
Lee RB, Purves JV, Ratcliffe RG, Saker LR. 1992. Nitrogen assimilation and the control of ammonium and nitrate absorption by maize roots. Journal of Experimental Botany 43, 1385–1396.
Lee RB, Ratcliffe RG. 1991. Observations on the subcellular distribution of the ammonium ion in maize root tissue using in vivo 14N-nuclear magnetic resonance spectroscopy. Planta 183, 359–367.[Web of Science]
Lee RB, Ratcliffe RG, Southon TE. 1990. Phosphorus-31 NMR measurements of the cytoplasmic and vacuolar inorganic phosphate content of mature maize roots: relationships with phosphorus status and phosphate fluxes. Journal of Experimental Botany 41, 1063–1078.
Lee RB, Rudge KA. 1986. Effects of nitrogen deficiency on the absorption of nitrate and ammonium by barley plants. Annals of Botany 57, 471–486.
Lejay L, Tillard P, Lepetit M, Olive FD, Filleur S, Daniel-Vedele F, Gojon A. 1999. Molecular and functional regulation of two uptake systems by N- and C-status of Arabidopsis plants. The Plant Journal 18, 509–519.[Web of Science][Medline]
MacDuff JH, Bakken AK, Dhanoa MS. 1997. An analysis of the physiological basis of commonality between diurnal patterns of , and K+ uptake by Phleum pratense and Festuca pratensis. Journal of Experimental Botany 48, 1691–1701.
Marschner H, Kirkby EA, Engels C. 1997. Importance of cycling and recycling of mineral nutrients within plants for growth and development. Botanica Acta 110, 65–273.
Matt P, Geiger M, Walch-Liu P, Engels C, Krapp A, Stitt M. 2001. The immediate cause of the diurnal changes of nitrogen metabolism in leaves of nitrate-replete tobacco: a major imbalance between the rate of nitrate reduction and the rates of nitrate uptake and ammonium metabolism during the first part of the light period. Plant, Cell and Environment 24, 177–190.
Morgan MA, Jackson WA. 1988. Inward and outward movement of ammonium in root systems: transient responses during recovery from nitrogen deprivation in the presence of ammonium. Journal of Experimental Botany 39, 179–191.
Muller B, Touraine B. 1992. Inhibition of uptake by various phloem-translocated amino acids in soybean seedlings. Journal of Experimental Botany 43, 617–623.
Nasholm T, Ekblad A, Nordin A, Giesler R, Hogberg M, Hogberg P. 1998. Boreal forest plants take up organic nitroge. Nature 392, 914–916.
Nasholm T, Huss-Danell K, Hogberg P. 2000. Uptake of organic nitrogen in the field by four agriculturally important plant species. Ecology 81, 1155–1161.
Ono F, Frommer WB, von Wiren N. 2000. Coordinated diurnal regulation of low- and high-affinity nitrate transporters in tomato. Plant Biology 2, 17–23.
Pace GM, McClure PR. 1986. Comparison of nitrate uptake kinetic parameters across maize inbred lines. Journal of Plant Nutrition 9, 1095–1111.
Peuke AD, Jeschke WD. 1998. The effects of light on induction, time courses and kinetic patterns of net nitrate uptake in barley. Plant, Cell and Environment 21, 765–774.
Quesada A, Krapp A, Trueman LJ, Daniel-Vedele F, Fernandez E, Forde BG, Caboche M. 1997. PCR-identification of a Nicotiana plumbaginifolia cDNA homologous to the high-affinity nitrate transporters of the crnA family. Plant Molecular Biology 34, 265–274.[Web of Science][Medline]
Rodgers CO, Barneix AJ. 1993. The effect of amino acids and amides on the regulation of nitrate uptake by wheat seedlings. Journal of Plant Nutrition 16, 337–348.
Rawat SR, Silim SN, Kronzucker HJ, Siddiqi MY, Glass ADM. 1999. AtAMT1 gene expression and uptake in roots of Arabidopsis thaliana: evidence for regulation by root glutamine levels. The Plant Journal 19, 143–152.[Web of Science][Medline]
Rufty Jr TW, Israel DW, Volk RJ. 1984. Assimilation of 15 taken up by the plants in the light and in the dark. Plant Physiology 76, 769–775.
Ryan PR, Walker NA. 1994. The regulation of ammonia uptake in Chara australis. Journal of Experimental Botany 45, 1057–1067.
Sehtiya HL, Goyal SS. 2000. Comparative uptake of nitrate by intact seedlings of C-3 (barley) and C-4 (corn) plants: effect of light and exogenously supplied sucrose. Plant and Soil 227, 185–190.
Siddiqi MY, Glass ADM, Ruth TJ. 1991. Studies of the uptake of nitrate in barley. III. Compartmentation of . Journal of Experimental Botany 42, 1455–1463.
Siddiqi MY, Glass ADM, Ruth TJ, Fernando M. 1989. Studies of the regulation of nitrate influx by barley seedlings using 13. Plant Physiology 90, 806–813.
Siddiqi MY, Glass ADM, Ruth TJ, Rufty TW. l990. Studies of the uptake of nitrate in barley. I. Kinetics of 13 influx. Plant Physiology 93, 1426–1432.
Siddiqi MY, King BJ, Glass ADM. 1992. Effects of nitrate, chlorate and chlorite on nitrate uptake and nitrate reductase activity. Plant Physiology 100, 644–650.
Tischner R. 2000. Nitrate uptake and reduction in higher and lower plants. Plant, Cell and Environment 23, 1005–1024.
Tischner R, Waldeck B, Goyal SS, Rains WD. 1993. Effect of nitrate pulses on the nitrate-uptake rate, synthesis of mRNA coding for nitrate reductase, and nitrate-reductase activity in the roots of barley seedlings. Planta 189, 533–537.
Tompkins GA, Jackson WA, Volk RJ. 1978. Accelerated nitrate uptake in wheat seedlings: effects of ammonium and nitrite treatments and of six methyl purine and puromycin. Physiologia Plantarum 43, 166–171.
Trueman LJ, Richardson A, Forde BG. 1996. Molecular cloning of higher plant homologues of the high-affinity nitrate transporters of Chlamydomonas reinhardtii and Aspergillus nidulans. Gene 175, 223–231.[Web of Science][Medline]
Ullrich WR, Larsson M, Larsson C-M, Lesch S, Novacky A. 1984. Ammonium uptake in Lemna gibba G1, related membrane potential change and inhibition of anion uptake. Physiologia Plantarum 61, 369–376.
Unkles SE, Zhou D, Siddiqi MY, Kinghorn JR, Glass ADM. 2001. Apparent genetic redundancy facilitates ecological plasticity for nitrate transport. EMBO Journal 20, 6246–6255.[Web of Science][Medline]
van der Leij M, Smith SJ, Miller AJ. 1998. Remobilisation of vacuolar stored nitrate in barley root cells. Planta 205, 64–72.[Web of Science]
Vidmar JJ, Zhuo D, Siddiqi MY, Schjoerring JK, Touraine B, Glass ADM. 2000. Regulation of HvNRT2 expression and high-affinity nitrate influx in roots of Hordeum vulgare by ammonium and amino acids. Plant Physiology 123, 307–318.
von Wiren N, Lauter FR, Ninnemann O, Gillissen B, Walch-Liu P, Engels C, Jost W, Frommer WB. 2000. Differential regulation of three functional ammonium transporter genes by nitrogen in root hairs and by light in leaves of tomato. The Plant Journal 21, 167–175.[Web of Science][Medline]
Wang MY. 1994. Ammonium uptake by rice roots. PhD thesis, University of BC, Vancouver, Canada.
Wang MY, Siddiqi MY, Ruth TJ, Glass ADM. 1993. Ammonium uptake by rice roots. II. Kinetics of 13 influx across the plasmalemma. Plant Physiology 103, 1259–1267.[Abstract]
Wang R, Liu D, Crawford NM. 1998. The Arabidopsis CHL1 protein plays a major role in high-affinity nitrate uptake. Proceedings of National Academy of Sciences, USA 95, 15134–15239.
Warner RL, Huffaker RC. 1989. Nitrate transport is independent of NADH and NAD(P)H nitrate reductases in barley seedlings. Plant Physiology 91, 947–953.
Wolt JD. 1994. Soil solution chemistry: applications to environmental science and agriculture. New York: Wiley.
Zhuo D, Okamoto M, Vidmar JJ, Glass ADM. 1999. Regulation of a putative high-affinity nitrate transporter (Nrt2;1At) in roots of Arabidopsis thaliana. The Plant Journal 17, 563–568.[Web of Science][Medline]
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