PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of plantsigLink to Publisher's site
 
Plant Signal Behav. 2016 May; 11(5): e1176819.
Published online 2016 April 18. doi:  10.1080/15592324.2016.1176819
PMCID: PMC4973756

The Arabidopsis nitrate transporter NPF7.3/NRT1.5 is involved in lateral root development under potassium deprivation

ABSTRACT

Plants have evolved a large array of transporters and channels that are responsible for uptake, source-to-sink distribution, homeostasis and signaling of nitrate (NO3), which is for most plants the primary nitrogen source and a growth-limiting macronutrient. To optimize NO3 uptake in response to changing NO3 concentrations in the soil, plants are able to modify their root architecture. Potassium is another macronutrient that influences the root architecture. We recently demonstrated that the Arabidopsis NO3 transporter NPF7.3/NRT1.5, which drives root-to-shoot transport of NO3, is also involved in root-to-shoot translocation of K+ under low NO3 nutrition. Here, we show that K+ shortage, but not limiting NO3 supply, causes in nrt1.5 mutant plants an altered root architecture with conspicuously reduced lateral root density. Since lateral root development is influenced by auxin, we discuss a possible involvement of NPF7.3/NRT1.5 in auxin homeostasis in roots under K+ deprivation.

KEYWORDS: Arabidopsis thaliana, NPF7.3/NRT1.5, nitrate transporter, potassium transporter, root development, lateral root, nutrient signaling, nutrient deficiency, ion homeostasis

In most soils, nitrate (NO3) is the major inorganic nitrogen source for higher plants, and its uptake, translocation, assimilation and remobilization determine plant productivity in terms of plant health and yield formation.1 In Arabidopsis thaliana members of four protein families accomplish NO3 uptake and allocation to individual plant organs.2 The largest of these families is the NPF (nitrate transporter1/peptide transporter - NRT1/PTR) family3 which consists of 53 members in Arabidopsis thaliana. Two NPF proteins were shown to function as bidirectional NO3 influx/efflux transporters involved in root-to-shoot translocation of this important nutrient. NPF6.3/CHL1/NRT1.1 (subsequently termed NRT1.1) is a nitrate transceptor and auxin transporter that controls various physiological and gene regulatory responses to NO3 and represses lateral root development under NO3 deficiency.4,5 The other is the NPF7.3/NRT1.5 protein (subsequently termed NRT1.5) which was characterized as a low-affinity bidirectional NO3 transporter driving xylem loading and root-to-shoot translocation of NO3.6

We recently found that NRT1.5 is also involved in potassium (K+) homeostasis in shoots by affecting K+ root-to-shoot translocation in a NO3 dependent manner.7 Lately the same phenomenon was reported in a different study.8 Lack of NRT1.5 in roots results in K+ deficiency in shoots under low NO3 nutrition, whereas the whole root elemental composition remains unchanged. Our results indicate that NRT1.5 is an important component linking the NO3 and K+ signaling pathways.

It is well known that the availability and balance of nutrients, particularly nitrate, phosphate and potassium, affects the root system development and architecture by modulating growth rate and morphology of primary roots (PR), lateral roots (LR) and root hairs.4,9-13 For example, low NO3 availability conditionally stimulates10 or represses LR elongation,4,13 whereas K+ deficiency causes shortening of primary root (PR) as well as total root length.14 Potassium is the most abundant and important cation in plants and is critically involved in cell expansion processes like stomatal opening and cell growth in roots.15 The K+ nutritional status is linked to the ethylene, cytokinin, jasmonic acid and auxin signaling pathways, thus leading to developmental and physiological alterations.16-18 Moreover, it presumably influences plant disease resistance.19 When grown under limiting K+ supply, total root size was reduced in 26 investigated natural Arabidopsis accessions, but the degree of reduction and the root architectural response to K+ limitation strongly differed between accessions.20

Drechsler et al. recently discovered that nrt1.5 mutant plants, when growing under low N fertilization conditions, exhibit a pleiotropic shoot phenotype and early leaf senescence symptoms, that are caused by K+ deficiency in the shoot organs.7 However, the NRT1.5 gene is mainly expressed in root pericycle cells which surround the xylem.6 This prompted us to investigate root development of nrt1.5 mutants under varying NO3 and K+ supply. Arabidopsis Col-0 and nrt1.5 seeds were germinated for 5 days on 0.5 × MS medium (containing ~30 mM N) and then transferred to modified 0.5 × MS medium with reduced N and K+ concentrations. On medium containing 1 mM K+, reducing the NO3 concentration from 2 mM to 0 mM resulted in wild-type as well as nrt1.5 seedlings in a dramatic impairment of shoot development, decreased chlorophyll biosynthesis and accumulation of anthocyanins (Fig. 1A).7 Shoot and root fresh weight and root architecture of wild-type and nrt1.5 mutants are virtually identical though (Fig. 1A–C). Similarly, on medium containing 2 mM K+ and 1 mM NO3, mutant and wild-type seedlings are phenotypically indistinguishable and show no morphological abnormalities in shoots or roots (Fig. 2A).

Figure 1.
Nitrate deficiency does not affect root development of Arabidopsis nrt1.5 mutant seedlings. Seeds were surface-sterilized, distributed on 0.5 × MS plates containing 1% (w/v) sucrose and 0.3% (w/v) Gelrite (pH 5.8) and stratified for 2 days ...
Figure 2.
Potassium deficiency affects root development of Arabidopsis nrt1.5 mutant seedlings. Seeds were germinated and grown as described in Fig. 1 except that the medium contained 1 mM NO3 and varying amounts of K+. (A) Potassium-dependent ...

Remarkably, in Col-0 wild-type plants the absence of NO3 does not affect LR density (Fig. 1C). This contrasts with the phenotype of seedlings that were sown and germinated on medium lacking NO3. Under these conditions LR density is significantly reduced, although the LR primordia density is not altered.4,13 At low NO3 supply NRT1.1 prevents auxin accumulation in LR primordia, thereby repressing development of LRs.4 In our experiment, after 5 days germination on 0.5 × MS medium the LRs have emerged already before the plants are transferred to NO3 free medium, suggesting that under NO3 deficiency NRT1.1 suppresses the outgrowth of LR primordia but not or only marginally the elongation of emerged LRs. This conclusion is consistent with the findings of Linkohr and colleagues that the LR elongation response to varying NO3 concentrations is not affected in auxin-resistant mutants and thus wild-type auxin signaling is not required for mediating LR elongation.10

Other than NO3 deficiency, lowering the K+ concentration in the medium to less than 1 mM perturbs development of the nrt1.5 mutants more severely than wild-type plants. The mutant's roots and shoots gain only 50% and 60% fresh weight relative to wild-type, respectively (Fig. 2B). Notably, the root architecture of the mutants is conspicuously altered under K+ deficiency. The primary roots are slightly, but significantly shorter and the lateral root density is reduced to approximately 60% (Fig. 2A,C). We have previously shown that expression of NRT1.5 by the PHO1 promoter, which has a comparable root specific activity pattern as the NRT1.5 promoter but was more active in the transgenic plants, successfully complements the K+ deficiency phenotype in shoots of nrt1.5-5 plants.7 We therefore examined outgrowth and development of LRs in two independent PHO1p:NRT1.5 complementation lines on medium with 0 mM K+/1.0 mM NO3 supply. Indeed, after seven days the LR density of both PHO1p:NRT1.5 lines is identical to wild-type plants (Fig. 3). These results demonstrate that under K+ limiting conditions, but not under NO3 deficiency, the nitrate transporter NRT1.5 is required for normal root development.

Figure 3.
The K+ deficiency-dependent nrt1.5 root phenotype is complemented by NRT1.5 expression. nrt1.5-5 mutant plants were transformed with a PHO1 promoter:NRT1.5 construct (PHO1p:NRT1.5) as described previously.7 Seeds were germinated and grown on medium containing ...

Lin and colleagues reported that NRT1.5 is predominantly expressed in root pericycle cells adjacent to the protoxylem but they did not investigate LR development in detail.6 Considering the reduced LR density in nrt1.5 mutants under K+ deficiency, we reexamined the activity and tissue specificity of the NRT1.5 promoter in primary and lateral roots. In ten independent Arabidopsis Col-0 transformants carrying the ß-glucuronidase (uidA) gene under the control of a 2.4 kb NRT1.5 promoter fragment, we observed largely identical GUS staining patterns. In 15 days old seedlings, NRT1.5 promoter activity is detected throughout the PR vascular cylinder, while the root cortex is not stained (Fig. 4A). GUS staining is most intense at lateral root primordia (Fig. 4A–B), whereas in the newly developing lateral roots at first the vascular tissue, root cap, elongation zone and meristematic zone are not stained (Fig. 4B, black arrowheads). With increasing age, NRT1.5 promoter activity spreads from the lateral root primordia also along the lateral root vasculature (Fig. 3B, white arrowhead). These results suggest that transport processes regulated or performed by NRT1.5 are important determinants for lateral root outgrowth.

Figure 4.
NRT1.5 promoter activity in Col-0 seedling roots. A 2.4 kb genomic fragment upstream of the inferred initiation codon of NRT1.5 was PCR-amplified from the BAC clone F5D14 (Arabidopsis Biological Resource Center, ABRC) and fused with the uidA gene ...

In Arabidopsis the potassium transporter HAK5 and the shaker-family potassium channel AKT1 are the two principal K+ uptake systems at moderate to low K+ supply.21 At very low external K+ concentrations (<10 µM) the high-affinity K+ transporter HAK5 is the only protein capable of taking up K+.22 By K+ deprivation HAK5, but not AKT1, is transcriptionally upregulated in Col-0 roots.23,24 Drechsler et al. found that HAK5 is approximately 3-fold downregulated in roots of nrt1.5 mutant plants.7 It is therefore conceivable that at limiting external K+ supply the reduced expression of HAK5 in nrt1.5 roots leads to K+ shortage in the roots. This shortage could indirectly be responsible for the decreased LR density phenotype of nrt1.5 plants at low K+ supply by repressing the K-dependent expression of the auxin signaling-modulating transcription factor MYB77.25 Indeed, the phenotype of myb77 mutants resembles the nrt1.5 phenotype at low K+ supply.26 Auxin is known to control LR development through multiple auxin-signaling components27 and it is therefore tempting to speculate that NRT1.5 could be involved in K+-dependent hormone homeostasis. NRT1.1 is known to control LR primordia outgrowth by preventing auxin accumulation in LR primordia at low NO3 supply. In roots of adult nrt1.5-5 mutant plants hydroponically grown under low NO3 supply NRT1.1 expression is not altered.7 To investigate whether the knock-out of NRT1.5 affects NRT1.1 expression under K+ limitation, we determined NRT1.1 expression in roots of the Col-0 and nrt1.5-5 plants shown in Fig. 2A by qRT-PCR. However, NRT1.1 expression is not significantly altered in nrt1.5-5 relative to wild-type plants in 0 mM K+/1.0 mM NO3 or 2.0 mM K+/1.0 mM NO3, respectively (data not shown). It is therefore unlikely that the decreased LR density phenotype of nrt1.5 plants at low K+ supply is caused indirectly via modulation of auxin distribution by NRT1.1. Whether NRT1.5 is directly or indirectly involved in auxin transport remains to be investigated in future studies.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Acknowledgments

We thank Nikolaus von Wirén and Alexis Kasaras for helpful comments on the manuscript.

Funding

This research was supported by a grant from the Deutsche Forschungsgemeinschaft (FOR948, KU715/10-2) and fellowships of the China Scholarship Council to Y.Z. and the Studienstiftung des deutschen Volkes to N.D.

References

1. Xu G, Fan X, Miller AJ Plant nitrogen assimilation and use efficiency. Annual Rev Plant Biol 2012; 63:153-82; PMID:22224450; http://dx.doi.org/2405513910.1146/annurev-arplant-042811-105532 [PubMed] [Cross Ref]
2. Krapp A, David LC, Chardin C, Girin T, Marmagne A, Leprince AS, Chaillou S, Ferrario-Mery S, Meyer C, Daniel-Vedele F Nitrate transport and signalling in Arabidopsis. J Exp Botany 2014; 65:789-98; PMID:24532451; http://dx.doi.org/2405513910.1093/jxb/eru001 [PubMed] [Cross Ref]
3. Leran S, Varala K, Boyer JC, Chiurazzi M, Crawford N, Daniel-Vedele F, David L, Dickstein R, Fernandez E, Forde B, et al. A unified nomenclature of nitrate transporter 1/peptide transporter family members in plants. Trends Plant Sci 2014; 19:5-9; PMID:24055139; http://dx.doi.org/10.1016/j.tplants.2013.08.008 [PubMed] [Cross Ref]
4. Bouguyon E, Brun F, Meynard D, Kubes M, Pervent M, Leran S, Lacombe B, Krouk G, Guiderdoni E, Zazimalova E, et al. Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1. Nat Plants 2015; 1; https://dx.doi.org/10.1038/Nplants.2015.15 [PubMed] [Cross Ref]
5. Leran S, Munos S, Brachet C, Tillard P, Gojon A, Lacombe B. Arabidopsis NRT1.1 is a bidirectional transporter involved in root-to-shoot nitrate translocation. Mol Plant 2013; 6:1984-7; PMID:23645597; http://dx.doi.org/10.1093/mp/sst068 [PubMed] [Cross Ref]
6. Lin SH, Kuo HF, Canivenc G, Lin CS, Lepetit M, Hsu PK, Tillard P, Lin HL, Wang YY, Tsai CB, et al. Mutation of the Arabidopsis NRT1.5 nitrate transporter causes defective root-to-shoot nitrate transport. Plant Cell 2008; 20:2514-28; PMID:18780802; http://dx.doi.org/10.1105/tpc.108.060244 [PubMed] [Cross Ref]
7. Drechsler N, Zheng Y, Bohner A, Nobmann B, von Wirén N, Kunze R, Rausch C 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. Plant Physiol 2015; 169:2832-47; PMID:26508776; https://dx.doi.org/2062707510.1104/pp.15.01152 [PubMed] [Cross Ref]
8. Meng S, Peng JS, He YN, Zhang GB, Yi HY, Fu YL, Gong JM Arabidopsis NRT1.5 mediates the suppression of nitrate starvation-induced leaf senescence by modulating foliar potassium level. Mol Plant 2015; 9(3):461-70; PMID:26732494; https://dx.doi.org/2062707510.1016/j.molp.2015.12.015 [PubMed] [Cross Ref]
9. Forde B, Lorenzo H The nutritional control of root development. Plant Soil 2001; 232:51-68; http://dx.doi.org/10.1023/A:1010329902165 [Cross Ref]
10. Linkohr BI, Williamson LC, Fitter AH, Leyser HMO Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis. Plant J 2002; 29:751-60; PMID:12148533; http://dx.doi.org/2062707510.1046/j.1365-313X.2002.01251.x [PubMed] [Cross Ref]
11. Ingram PA, Malamy JE Root System Architecture. In: Kader JC, Delseny M, eds. Adv Botanical Res 2010; 55:75-117; PMID:20020426; https://dx.doi.org/2062707510.1016/s0065-2296(10)55002-x [Cross Ref]
12. Lopez-Bucio J, Cruz-Ramirez A, Herrera-Estrella L The role of nutrient availability in regulating root architecture. Curr Opin Plant Biol 2003; 6:280-7; PMID:12753979; http://dx.doi.org/2062707510.1016/S1369-5266(03)00035-9 [PubMed] [Cross Ref]
13. Krouk G, Lacombe B, Bielach A, Perrine-Walker F, Malinska K, Mounier E, Hoyerova K, Tillard P, Leon S, Ljung K, et al. Nitrate-regulated auxin transport by NRT1.1 defines a mechanism for nutrient sensing in plants. Dev Cell 2010; 18:927-37; PMID:20627075; http://dx.doi.org/10.1016/j.devcel.2010.05.008 [PubMed] [Cross Ref]
14. Gruber BD, Giehl RF, Friedel S, von Wirén N Plasticity of the Arabidopsis root system under nutrient deficiencies. Plant Physiol 2013; 163:161-79; PMID:23852440; http://dx.doi.org/2673404810.1104/pp.113.218453 [PubMed] [Cross Ref]
15. Dolan L, Davies J Cell expansion in roots. Curr Opin Plant Biol 2004; 7:33-9; PMID:14732439; http://dx.doi.org/2673404810.1016/j.pbi.2003.11.006 [PubMed] [Cross Ref]
16. Nam YJ, Tran LS, Kojima M, Sakakibara H, Nishiyama R, Shin R Regulatory roles of cytokinins and cytokinin signaling in response to potassium deficiency in Arabidopsis. PLoS One 2012; 7:e47797; PMID:23112848; https://dx.doi.org/2673404810.1371/journal.pone.0047797 [PMC free article] [PubMed] [Cross Ref]
17. Schachtman DP.. The Role of Ethylene in Plant Responses to K(+) Deficiency. Frontiers Plant Sci 2015; 6:1153; PMID:26734048; http://dx.doi.org/10.3389/fpls.2015.01153 [PMC free article] [PubMed] [Cross Ref]
18. Troufflard S, Mullen W, Larson TR, Graham IA, Crozier A, Amtmann A, Armengaud P. Potassium deficiency induces the biosynthesis of oxylipins and glucosinolates in Arabidopsis thaliana. BMC Plant Biol 2010; 10:172; PMID:20067625; http://dx.doi.org/10.1186/1471-2229-10-172 [PMC free article] [PubMed] [Cross Ref]
19. Amtmann A, Troufflard S, Armengaud P The effect of potassium nutrition on pest and disease resistance in plants. Physiologia Plantarum 2008; 133:682-91; PMID:18331404; http://dx.doi.org/2481076710.1111/j.1399-3054.2008.01075.x [PubMed] [Cross Ref]
20. Kellermeier F, Chardon F, Amtmann A Natural variation of Arabidopsis root architecture reveals complementing adaptive strategies to potassium starvation. Plant Physiol 2013; 161:1421-32; PMID:23329148; http://dx.doi.org/2481076710.1104/pp.112.211144 [PubMed] [Cross Ref]
21. Nieves-Cordones M, Aleman F, Martinez V, Rubio F. K+ uptake in plant roots. The systems involved, their regulation and parallels in other organisms. J Plant Physiol 2014; 171:688-95; PMID:24810767; http://dx.doi.org/10.1016/j.jplph.2013.09.021 [PubMed] [Cross Ref]
22. Rubio F, Aleman F, Nieves-Cordones M, Martinez V Studies on Arabidopsis athak5, atakt1 double mutants disclose the range of concentrations at which AtHAK5, AtAKT1 and unknown systems mediate K uptake. Physiologia Plantarum 2010; 139:220-8; PMID:20088908; http://dx.doi.org/1573490910.1111/j.1399-3054.2010.01354.x [PubMed] [Cross Ref]
23. Gierth M, Maser P, Schroeder JI. The potassium transporter AtHAK5 functions in K(+) deprivation-induced high-affinity K(+) uptake and AKT1 K(+) channel contribution to K(+) uptake kinetics in Arabidopsis roots. Plant Physiol 2005; 137:1105-14; PMID:15734909; http://dx.doi.org/10.1104/pp.104.057216 [PubMed] [Cross Ref]
24. Lagarde D, Basset M, Lepetit M, Conejero G, Gaymard F, Astruc S, Grignon C. Tissue-specific expression of Arabidopsis AKT1 gene is consistent with a role in K+ nutrition. Plant J 1996; 9:195-203; PMID:8820606; http://dx.doi.org/10.1046/j.1365-313X.1996.09020195.x [PubMed] [Cross Ref]
25. Shin R, Berg RH, Schachtman DP. Reactive oxygen species and root hairs in Arabidopsis root response to nitrogen, phosphorus and potassium deficiency. Plant Cell Physiol 2005; 46:1350-7; PMID:15946982; http://dx.doi.org/10.1093/pcp/pci145 [PubMed] [Cross Ref]
26. Shin R, Burch AY, Huppert KA, Tiwari SB, Murphy AS, Guilfoyle TJ, Schachtman DP. The Arabidopsis transcription factor MYB77 modulates auxin signal transduction. Plant Cell 2007; 19:2440-53; PMID:17675404; http://dx.doi.org/10.1105/tpc.107.050963 [PubMed] [Cross Ref]
27. Lavenus J, Goh T, Roberts I, Guyomarc'h S, Lucas M, De Smet I, Fukaki H, Beeckman T, Bennett M, Laplaze L. Lateral root development in Arabidopsis: fifty shades of auxin. Trends Plant Sci 2013; 18(8):450-8; PMID:23701908; https://dx.doi.org/10.1016/j.tplants.2013.04.006 [PubMed] [Cross Ref]
28. Clough SJ.. Floral dip: agrobacterium-mediated germ line transformation. Methods Mol Biol 2005; 286:91-102; PMID:15310915 [PubMed]

Articles from Plant Signaling & Behavior are provided here courtesy of Taylor & Francis