Characterisation of a two component high affinity nitrate uptake system in Arabidopsis

51 Pages·2006·0.29 MB·English

Plant Physiology Preview. Published on September 29, 2006, as DOI:10.1104/pp.106.085209
1 RUNNING TITLE: Two component high affinity uptake in Arabidopsis
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Characterisation of a two component high
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affinity nitrate uptake system in Arabidopsis;
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physiology and protein-protein interaction.
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9 Mathilde Orsel, Franck Chopin1, Olivier Leleu1, Susan J
10 Smith, Anne Krapp1, Françoise Daniel-Vedele1, Anthony J
11 Miller*.
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14 Crop Performance and Improvement Division,
15 Rothamsted Research,
16 Harpenden, Hertfordshire, AL5 2JQ, UK
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18 1Unité de la Nutrition Azotée des Plantes, Institut National de la Recherche
19 Agronomique, Route de St. Cyr, F-78026 Versailles cedex, France
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22 *Corresponding author:
23 Dr Tony Miller
24 Crop Performance and Improvement Division,
25 Rothamsted Research,
26 Harpenden,
27 Hertfordshire, AL5 2JQ
28 UK
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31 Email address: [email protected]
32 Fax number [phone]
33 Telephone [phone] ext 2248.
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36 Manuscript = 11972 words, 1 Table and 7 Figures.
37
38 [Supplementary data, 7 Figures].
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41 Key words: Arabidopsis, high affinity nitrate transport, root, two-component
42 transport system
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Copyright 2006 by the American Society of Plant Biologists
1 ABSTRACT
2 The identification of a family of the NAR2-type genes in higher plants showed that
3 there was a homologue in Arabidopsis, AtNAR2.1 (Tong et al., 2005, Plant J 41: 442-
4 450). These genes encode part of a two component nitrate high affinity transport
5 system (HATS). As the Arabidopsis NRT2 gene family of nitrate transporters has been
6 characterized we tested the idea that AtNAR2.1 and AtNRT2.1 are partners in a two
7 component HATS. Results using the yeast split ubiquitin system and Xenopus oocyte
8 expression showed that the two proteins interacted to give a functional high affinity
9 transport system. The growth and nitrogen physiology of two Arabidopsis gene knock
10 out mutants, atnrt2.1-1 and atnar2.1-1 one for each partner protein were compared.
11 Both types of plants had lost HATS activity at 0.2 mM nitrate, but the effect was more
12 severe in atnar2.1-1 plants. The relationship between plant N status and nitrate
13 transporter expression revealed a pattern that was characteristic of nitrogen deficiency
14 that was again stronger in atnar2.1-1. Plants resulting from a cross between both
15 mutants (atnrt2.1-1 x atnar2.1-1) showed a phenotype like that of the atnar2.1-1
16 mutant when grown in 0.5 mM nitrate. Lateral root assays also revealed growth
17 differences between the two mutants confirming that atnar2.1-1 had a stronger
18 phenotype. To show that the impaired HATS did not result from the decreased
19 expression of AtNRT2.1 we tested if constitutive root expression of a tobacco gene,
20 NpNRT2.1 that has previously been shown to complement the atnrt2.1-1, can restore
21 HATS to the atnar2.1-1 mutant. These plants did not recover wild type nitrate HATS.
22 Taken together these results all show that AtNAR2.1 is essential for HATS of nitrate
23 in Arabidopsis.
24
25 (275 words)
26
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1 INTRODUCTION
2 Nitrogen (N) is the most important inorganic nutrient in plants, and its
3 availability is a limiting factor for plant growth in most agricultural systems. Limiting
4 N supply influences shoot-root allocation of resources, favouring root system
5 development for exploration of a larger soil volume. Severe N deficiency leads to a
6 general inhibition of plant growth. Nitrate is the main N source available in arable soil
7 and in this form it can act directly as a signal that regulates shoot-root allocation and
8 modification of the root system architecture (Scheible et al., 1997; Zhang and Forde,
9 2000).
10 Many physiological investigations on nitrate uptake by roots led to the
11 conclusion that plants have developed three types of transport system to cope with the
12 variations in nitrate concentrations in cultivated soils (Glass and Siddiqi, 1995). Two
13 saturable high affinity transport system (HATS) are able to take up nitrate at low
14 external concentration (1 µM to 1 mM). The constitutive system (cHATS) is available
15 even if the plants have never been supplied with nitrate. The inducible system
16 (iHATS) is strongly stimulated by nitrate in the external medium. The low affinity
17 transport system (LATS) displays linear kinetics and its contribution to global nitrate
18 uptake becomes significant at external nitrate concentrations above 1 mM.
19 Many studies on the molecular basis of root nitrate uptake systems revealed
20 two genes families, NRT1 and NRT2, potentially coding for nitrate transporters
21 involved in the LATS and HATS systems (Crawford and Glass, 1998; Forde, 2000;
22 Orsel et al., 2002). The chl1 line, affected in the Arabidopsis thaliana AtNRT1.1 gene,
23 was the first mutant characterised for a deficiency in the nitrate LATS system (Tsay et
24 al., 1993). Extensive studies revealed a complex role for AtNRT1.1, but no phenotype
25 related to plant growth (Guo et al., 2001; Guo et al., 2003). The first members of the
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1 NRT2 family were identified in lower eukaryotes. The crnA (nrtA) gene from
2 Aspergillus nidulans was identified from a mutant resistant to chlorate, a toxic nitrate
3 analogue (Unkles et al., 1991; Unkles et al., 2001). Two other genes CrNRT2.1 and
4 CrNRT2.2 were identified in the green algae Chlamydomonas reinhardtii using
5 deletion strains deficient in nitrate uptake (Quesada et al., 1994). Most members of
6 the NRT2 family in higher plants were subsequently identified based on their
7 sequence homology to nrtA and CrNRT2.1 (Daniel-Vedele et al., 1998). So far, in
8 each species studied, the NRT2 genes are a small multigenic family. The complete
9 genome analysis of A. thaliana revealed 7 NRT2 members that are differentially
10 expressed in plant tissues with a pattern that can depend on the external nitrogen
11 supply (Orsel et al., 2002; Okamoto et al., 2003).
12 AtNRT2.1, the first A. thaliana NRT2 gene to be identified, was cloned on the
13 basis of differential induction under nitrate versus glutamine supply (Filleur and
14 Daniel-Vedele, 1999). Expression analysis showed that AtNRT2.1 expression is
15 perfectly coordinated with nitrate HATS regulation; induction by low external nitrate
16 concentration and sudden N-deprivation (Filleur and Daniel-Vedele, 1999; Lejay et
17 al., 1999). Root uptake analysis of a knockout mutant atnrt2.1-1 (atnrt2a) for both
18 genes AtNRT2.1 and AtNRT2.2, provided the first functional evidence supporting the
19 role of NRT2 genes in nitrate inducible HATS (Filleur et al., 2001). Further
20 investigation of the atnrt2.1-1 mutant has shown that HATS deficiency had a strong
21 impact on plant growth under low nitrate (0.2 mM), but not higher concentrations
22 (6 mM) (Orsel et al., 2004). The shoot biomass and nitrate content of the mutants
23 were strongly decreased compared to the wild type, but the root growth was
24 maintained, leading to a decreased shoot/root ratio, a characteristic feature of
25 N-limited plants. Recent studies have highlighted the role of AtNRT2.1 in the root
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1 architecture response to low nitrate availability, especially in lateral root initiation
2 (Little et al., 2005; Remans et al., 2006).
3 In C. reinhardtii two types of genes involved in nitrate transport (CrNRT2 and
4 CrNAR2) are located within a nitrate-regulated gene cluster. Mutants deleted in this
5 genomic region recovered high affinity nitrate uptake activity only after
6 transformation with plasmids carrying CrNAR2 and either CrNRT2.1 or CrNRT2.2,
7 but not with any of these genes individually (Quesada et al., 1994). The co-injection
8 of two different types of mRNA in Xenopus oocytes revealed that both gene family
9 products were required for functional nitrate uptake (Zhou et al., 2000). Now
10 numerous genes belonging to the NAR2 family have been identified in plants among
11 many different plants species, including AtNAR2.1 in Arabidopsis (Tong et al., 2005).
12 More recently, two AtNAR2 genes have been identified in Arabidopsis; AtNAR2.1
13 (AtNRT3.1) and AtNAR2.2 (AtNRT3.2) with accession numbers At5g50200 and
14 At4g24720, respectively (Okamoto et al., 2006). We will use the former gene naming
15 system throughout this paper. No evidence for the expression of the second gene,
16 AtNAR2.2 was found in the databases (ESTs, cDNA or microarray data) but using
17 several sets of specific primers in a sensitive assay it was found to be just detectable
18 (Okamoto et al., 2006). In contrast, AtNAR2.1 was strongly expressed in the roots in
19 the AtGenExpress Affymetrix experiments (Weigel et al., 2004).
20 The presence of both NRT2 and NAR2 families in plant genomes suggested a
21 more general relevance for the NRT2/NAR2 transport model and functional two
22 component nitrate transport was reconstituted in Xenopus oocytes using the barley
23 genes HvNRT2.1/HvNAR2.3 (Tong et al., 2005). As deletion mutants are easier to
24 obtain in Arabidopsis than barley we have investigated the functional identity of a two
25 component high affinity nitrate transport system in planta using this model. Our
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1 starting hypothesis was that AtNRT2.1/AtNAR2.1 constitutes a two component system.
2 We began by testing this hypothesis in two different heterologous expression systems
3 and then moved to a more detailed investigation of the AtNAR2.1/AtNRT2 system by
4 studying a new isolated knock out mutant for AtNAR2.1. The atnar2.1-1 T-DNA
5 insertion mutant isolated and used for this analysis has recently been reported in a
6 parallel but independent study (Atnrt3.1-2). Comparing the atnar2.1-1 mutant and
7 wild type plants (WT), this work concluded that nitrate HATS system in higher plants
8 requires a functional NAR2 gene (Okamoto et al., 2006). We are reporting how we
9 have extended this previous work by comparing the growth and nitrogen physiology
10 of atnar2.1-1 and atnrt2.1-1 mutants with WT plants. The expression of nitrate
11 transporters was first characterised as being induced by the substrate (Crawford and
12 Glass, 1998; Forde, 2000) but in common with other nutrients such as phosphate and
13 sulphate it is becoming apparent that N starvation can increase the expression of some
14 family members (Orsel et al., 2003). We therefore checked the relationship between
15 plant N status and the expression and activity of the AtNAR2.1/AtNRT2 system.
16 Finally, as root morphology changes provide a convenient and sensitive assay for
17 identifying Arabidopsis phenotypes we have used this method to investigate the
18 function of the two component system.
19
20 RESULTS
21 Protein-protein interaction between AtNRT2.1 and AtNAR2.1
22 The interaction of AtNRT2.1 and AtNAR2.1 proteins was tested using a
23 mating based split-ubiquitin system (mbSUS) that allows detection of interacting
24 membrane proteins (Obrdlik et al., 2004). Both CrNRT2.1 and AtNRT2.3 proteins
25 were used to assay the specificity of the interaction. The C. reinhardtii protein
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1 CrNRT2.1 is involved in a two component system with CrNAR2.1 (Quesada et al.,
2 1994) and has 55% sequence similarity with AtNRT2.1 (EMBOSS-Align, program
3 Needle, EBlossum 62). The Arabidopsis AtNRT2.3 protein has more than 76%
4 similarity with AtNRT2.1 but expression of the gene is almost undetectable in all
5 conditions (Orsel et al., 2002), making it an unlikely partner for AtNAR2.1. Each
6 potential partner was fused to the N (Nub) or C (Cub) terminal domain of the
7 ubiquitin protein. In mbSUS, the interaction between the two membrane-bound fusion
8 proteins leads to the reconstruction of the ubiquitin protease activity and release of the
9 PLV transcription factor fused to the Cub domain (Obrdlik et al., 2004). Two types of
10 reporter genes are under the control of the PLV transcription factor, allowing growth
11 tests on selective media (His and Ade auxotrophy) and ß-galactosidase activity assays
12 (LacZ).
13 The NRT2 cDNAs were cloned in the pNXgate and pXNgate plasmids and
14 tested in combination with AtNAR2.1 cDNA clones in the pMetYCgate. Growth of
15 diploid cells under selective conditions revealed interaction of AtNAR2.1-CubPLV
16 with NubG-AtNRT2.1 and NubG-AtNRT2.3, but not with NubG-CrNRT2.1 (Fig. 1a).
17 The NRT2-NubG constructs never showed any interaction in combination with
18 AtNAR2.1-CubPLV (data not shown). Testing the ß-galactosidase activity provides
19 more quantitative results (Fig. 1b). The activity detected when AtNAR2.1-CubPLV
20 was in combination with NubG-AtRNT2.1 was 2 and 16 times higher than in
21 combination with NubG-AtNRT2.3 and NubG-CrNRT2.1, respectively. These results
22 indicate that the strongest interaction occurs between AtNRT2.1 and AtNAR2.1,
23 much more than between any other NRT2 protein and AtNAR2.1. In the yeast system,
24 the AtNRT2.1 and AtNAR2.1 proteins can interact in a specific manner and form a
25 membrane complex.
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1
2 Expressing an AtNAR2.1/AtNRT2 nitrate transport system in Xenopus oocytes
3 In order to test if AtNAR2.1 and AtNRT2.1 constituted an Arabidopsis two
4 component nitrate transport system both mRNAs were tested in the Xenopus oocyte
5 heterologous system (Tong et al., 2005). Oocytes were injected with various
6 combinations of NRT2/NAR2 mRNA, including single injection with one or the other
7 mRNA. Oocytes were assayed for nitrate transport activity using enriched 15N nitrate
8 (Tong et al., 2005). Measurement of 15N enrichment in single oocytes showed that
9 only the combination of AtNRT2.1 and AtNAR2.1 provided a significant uptake
10 when compared with the water and single injected oocytes (Fig. 2). A very important
11 result is the negative result obtained with AtNRT2.3. Despite some sequence
12 similarity between AtNRT2.1 and AtNRT2.3 (see above), no functional nitrate uptake
13 system could be reconstituted with the AtNRT2.3/AtNAR2.1 combination in oocytes
14 (Fig. 2). These results show that only the co-injection of AtNAR2.1 together with
15 AtNRT2.1 mRNA results in high affinity nitrate transport in Xenopus oocytes.
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17 Comparing the growth and N pools between WT and the atnrt2.1-1 and
18 atnar2.1-1 mutants
19 The atnrt2.1-1 mutant, has already been shown to have altered HATS activity
20 and growth at low nitrate concentrations (Orsel et al., 2004). Plants were grown in
21 hydroponics with only nitrate as the N source at two different concentrations, high
22 (6 mM) and low (0.2 mM). At these two levels of concentration, the nitrate supply
23 was not limiting as the nutrient solutions were frequently replaced, wild type (WT)
24 growth was not significantly different (see supplementary data Figs. S1a to d). By
25 contrast, growth of the atnar2.1-1 mutant was severely decreased at the low nitrate
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1 concentration and this effect was stronger than that measured for atnrt2.1-1 plants
2 (see supplementary Fig. S1a). The shoot and root biomass of the atnar2.1-1 mutant
3 were only 10% and 48% respectively of the WT biomass (supplementary Figs. S1c,d),
4 and the shoot/root biomass ratio was strongly reduced to 0.9 (supplementary Fig.
5 S1b). There was no growth difference between the WT and both types of mutants
6 when the plants were supplied at the higher nitrate concentration (supplementary Figs.
7 S1a to d).
8 Analysis of shoot and root nitrate content revealed no differences between WT
9 and mutant plants when grown at high nitrate concentration (180 and 50 µmol g-1 FW
10 respectively for shoot and root) (see supplementary data Figs. S1e,f). When grown at
11 low nitrate concentration, the WT shoot and root nitrate content were decreased by
12 half compared to high nitrate, but in contrast nitrate was almost undetectable in either
13 shoot or root of the atnar2.1-1 mutant. For comparison and confirming earlier work
14 (Orsel et al., 2004), when grown at low nitrate concentration the shoot nitrate content
15 of the atnrt2.1-1 mutant was more decreased than the root nitrate content; actually
16 43% and 77% respectively of the WT.
17 The total amino acids of WT tissues were increased when plants were grown
18 at low nitrate concentration. Root amino acids increased by 162% of the value for WT
19 plants grown at the higher nitrate concentration (supplementary data Figs. S1g,h). In
20 contrast, growing at a lower nitrate concentration the shoot and root amino acids
21 concentrations were both decreased to 37% for atnar2.1-1 and 80% for atnrt2.1-1
22 mutants when compared with the values at higher nitrate. The mutant atnar2.1-1
23 showed only 40% and 29% respectively of the shoot and root WT amino acids
24 concentrations when grown at the lower nitrate concentration.
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1 Analysis of total N content revealed no significant difference between the
2 genotypes under high nitrate supply with around 6 % DW and 5 % DW respectively
3 for shoots and roots (see supplementary data Figs. S1i,j). But on the low nitrate
4 concentration, while the WT total N content was only slightly reduced, the dramatic
5 decrease of nitrate and amino acids content in the root and shoot of the atnar2.1-1
6 mutant led to decreased total N content, to only 2.2 and 2.7 %DW in shoot and root
7 respectively. The total N content of the atnar2.1-1 plants grown on low nitrate supply
8 is similar to values measured in WT after 10 d of N starvation (unpublished results).
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10 Comparing HATS activity between WT and the atnrt2.1-1 and atnar2.1-1
11 mutants
12 At the end of the experiment used to obtain biomass data (supplementary Fig.
13 S1), the same plants were used to measure nitrate influxes at 0.2 and 6 mM 15NO -
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14 external concentrations to discriminate between the activities of nitrate HATS and
15 LATS. Growing on 6 mM nitrate, WT and mutants showed the same HATS activity
16 (see supplementary data Fig. S2a). Grown at the lower nitrate concentration (0.2 mM
17 NO -), WT plants showed increased HATS activity, which reached
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18 120 µmol 15NO - h-1 g-1 root DW. This classical response of HATS to low nitrate
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19 supply was lost in the atnrt2.1-1 mutant, which showed only
20 25 µmol 15NO - h-1 g-1 root DW (21% of the WT 15NO - influx). For the atnar2.1-1
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21 mutant, the nitrate HATS activity was only 4 µmol 15NO - h-1 g-1 root DW, even lower
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22 than that measured for the atnrt2.1-1 mutant. This rate was only 3% of the WT 15NO -
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23 influx, and was less than the atnar2.1-1 HATS activity for plants growing on 6 mM
24 nitrate (20 µmol 15NO - h-1 g-1 root DW). To evaluate root 15NO - influx resulting
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25 from the activity of the LATS, nitrate influx at 0.2 mM 15NO - was subtracted from
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