235
Nitrate transport: a key step in nitrate assimilation
Françoise Daniel-Vedele∗, Sophie Filleur and Michel Caboche
The nitrate assimilation pathway has been the matter of
intensive research during the past decade. Many genes
involved in low and high affinity nitrate uptake have been
identified in fungi, algae and, more recently, in plants. The
plant genes so far isolated are transcriptionally regulated; their
inducibility by nitrate seems to be a common feature, shared
by their homologs in fungi and algae. A number of questions
remain to be elucidated regarding the physiological roles of
these transporters and the regulation of their expression.
Addresses
Laboratoire de Biologie Cellulaire, INRA, Route de St Cyr, 78026
Versailles Cedex, France
∗e-mail: [email protected]
Current Opinion in Plant Biology 1998, 1:235–239
http://biomednet.com/elecref/1369526600100235
 Current Biology Ltd ISSN 1369-5266
Abbreviations
HATS
high affinity transport system
LATS
low affinity transport system
NiR
nitrite reductase
NR
nitrate reductase
Introduction
Nitrate is the major source of nitrogen for the vast majority
of plants. It is first reduced to nitrite by nitrate reductase
(NR), nitrite being further reduced into ammonium by
nitrite reductase (NiR). Nitrate assimilation has been the
matter of many studies and has been reviewed by Hoff et
al. [1], Crawford [2] and more recently by Daniel-Vedele
and Caboche [3] and Campbell [4]. Of particular interest,
is the study of the post-transcriptional control of nitrate
reductase activity. This enzyme represents a key step in
nitrate metabolism which involves phosphorylation.
Nitrate itself also has to be considered as a signal; recent
data suggest that nitrate may be a signal for the regulation
of carbon metabolism, for example by modulating the
expression of genes involved in the biosynthesis of organic
acids [5•]. Nitrate also functions as a morphogenetic
signal, governing shoot:root balance [6••]. Very recently,
the Arabidopsis thaliana ANR1 gene was isolated on the
basis of its nitrate inducibility. This gene belongs to the
MADS box family, whose members code for transcription
factors mostly expressed in flowers, affecting the floral
organ identity. ANR, specifically expressed in roots, was
shown to be a key determinant of developmental plasticity
in A. thaliana roots in response to nitrate [7••].
The duality of nitrate, as both a nutrient and a signal,
reinforces the need to increase our knowledge of the
two primary steps in nitrate acquisition, namely nitrate
sensing and nitrate uptake. In this review, we will begin
by describing the physiology of nitrate uptake and then
go on to discuss the genes involved in low and high
affinity transport, and how nitrate transport and reduction
is regulated. We will then highlight the many questions
that have been brought to light by recent advances in these
areas.
Physiology of nitrate uptake
Nitrate is the common form of inorganic nitrogen in most
soils and the available concentration can vary enormously
depending on pH and oxygen availability [8]. Active
nitrate transport across the plasmalemma of epidermal
and cortical cells of the root is the first step in nitrate
acquisition and use.
Net uptake of nitrate is the difference between its
influx and efflux across the plasma membrane. On the
basis of physiological studies of nitrate influx, three
different types of electrogenic H+/NO3− symporters have
been postulated to exist [9]. At high external nitrate
concentration, a low affinity transport system (LATS,
Km >0.5 mM) operates and appears to be constitutively
expressed and essentially unregulated. At low external
concentrations (0–0.5 mM), two high affinity transport
systems (HATS, 5<Km < 200 µM) operate, one of these
being constitutive whereas the other is induced by nitrate
[10]. Nitrate efflux may also be a substrate-inducible
process that requires the synthesis of both RNA and
protein(s). Unlike the uptake system, however, the efflux
system, once induced, is relatively stable in the presence
of inhibitors of RNA and protein synthesis as long as
nitrate or nitrite is present [11].
Feedback regulation of nitrate uptake appears to be highly
complex, and nitrate is capable of exercising feedback
regulation of its own influx [12]. Finally, nitrate uptake
is regulated at the whole plant level by nitrogen demand
[13], presumably mediated by a signal such as amino acids
translocated from shoots to roots.
Genes involved in low affinity nitrate
transport (LATS)
Chlorate, a nitrate analog which is toxic when reduced
to chlorite by nitrate reductase, has been widely used
to select for mutants impaired in nitrate uptake or
metabolism. The genes encoding nitrate transporters have
been renamed according to the recommended nomenclature: Nrt (nitrate transport) 1 or 2 (for low and high
affinity respectively): member number, plant abbreviation.
In higher plants, the first mutants defective in nitrate
transport were identified in Arabidopsis thaliana, and were
all found to belong to the same complementation group
termed chl1. A screening of chlorate resistant plants among
236
Physiology and metabolism
T-DNA tagged populations allowed Tsay and collaborators
[14] to isolate a disrupted allele of Chl1, renamed
Nrt1:1At according to the agreed nomenclature [15], and
to demonstrate its role in chlorate toxicity. Nrt1:1At
protein contains 12 putative transmembrane-spanning
segments. The expression of Nrt1:1At is preferentially
located in root cells, and is pH dependent and nitrate
inducible. In addition, expression of the Nrt1:1At protein
in Xenopus oocytes exposed to nitrate at acidic pH leads to
a depolarization of the membrane potential, which is also
the initial response observed when plant cells are exposed
to nitrate, to an inward current or to an accumulation of
nitrate.
The role of Nrt1:1At in low affinity nitrate uptake is still
a matter of debate. Nitrate uptake, measured by nitrate
depletion in the medium, is found to be lower in the chl1
mutant than in the wild-type or in transgenic chl1 mutant
plants constitutively expressing 35S:: Nrt1:1At only when
these different genotypes are grown on an ammonium
nitrate containing medium. When plants are grown on
potassium nitrate alone, however, the depletion rates for
the wild-type and the mutant are similar [16••]. This
surprising result is confirmed by short-term measurements
of 13NO3− influx [17]. Clearly, the presence of ammonium
during the growth period is required to reveal differences
between the two genotypes. One explanation for these
findings is the possible existence of a second, low affinity
nitrate transporter, encoded by an ammonium-repressible
gene. The two-gene model for the LATS nitrate uptake
system would explain the apparent conflict between the
observed constitutive low affinity nitrate transport and the
nitrate inducibility of Nrt1:1At expression.
Recently, Nrt1:1At was used as a heterologous probe
to isolate homologues from tomato [18]. Interestingly,
two different but homologous genes were isolated from
a root hair-specific tomato cDNA library: Nrt1:1Le is
expressed in root hairs as well as other root tissues under
all nitrogen treatments applied and may correspond to
a constitutively expressed LATS. In contrast, Nrt1:2Le
mRNA accumulation is restricted to root hairs that
had been exposed to nitrate. Functional studies remain
to be performed, however, to determine the transport
characteristics of these two proteins.
Genes involved in high affinity nitrate
transport (HATS)
Recent progress made in isolating genes involved in the
HATS in higher plants originates from the molecular
analysis of nitrate uptake in fungi and algae.
The crnA mutant of Aspergillus nidulans was isolated on
the basis of chlorate resistance: this mutant is defective
in nitrate uptake at the conidiospore and young mycelium
stages. The corresponding gene was isolated [19] and it
encodes a 507 amino acid protein, containing 12 putative
membrane-spanning domains [20]. In the green alga,
Chlamydomonas reinhardtii, restoration of nitrate transport
in a mutant strain was achieved by co-transformation with
the genes nar-2 and Nrt2:1Cr or nar-2 and Nrt2:2Cr but not
with single plasmids containing the individual genes. In
addition, in contrast with nar-2 whose function is not yet
identified, the deduced amino acid sequences of Nrt2:1Cr
and Nrt2:2Cr showed significant similarity with the A.
nidulans CRNA protein (Figure 1 [21]). Physiological
studies of the complemented strains suggest that three
HATS operate in C. reinhardtii: one, encoded by a gene
sharing strong similarities with the other members of the
family (A Quesada, J Hidalgo, E Fernandez, unpublished
data), is specific for nitrite; a second one is encoded by
nar2/Nrt2:2Cr specific for nitrate; and a third one encoded
by nar2/Nrt2:1Cr, which is bispecific for both anions [22].
CRNA, Nrt2:1Cr and Nrt2:2Cr share a common structural
motif of 12 transmembrane α-helical segments and a
number of conserved sequence motifs [23]. They belong
to the major facilitator superfamily which contains five
clusters of transport proteins, including drug-resistant
proteins, sugar facilitators, facilitators for Kreb’s cycle
intermediates, phosphate ester–phosphate antiporters and
a distinct group of oligosaccharide–proton symporters [23].
A PCR approach using degenerate oligonucleotides derived from these conserved motifs allowed Trueman
and collaborators to isolate barley cDNAs (Nrt2:1Hv and
Nrt2:2Hv) encoding polypeptides similar to the algal genes
Nrt2:1Cr and Nrt2:1Cr. They have lesser, but significant,
similarity to the CRNA protein (Figure 1). As in the case
of Nrt1:1At, these genes are expressed preferentially in
barley roots and are induced by nitrate [24••]. Recently,
a full length cDNA, Nrt2:1Np, was isolated from the
dicot species Nicotiana plumbaginifolia, also using a PCR
approach [25]. The expression of this gene is root-specific,
nitrate-inducible and negatively regulated by nitrogen
metabolites. This last observation is especially striking as
it suggests that the high affinity transporter is under the
same regulatory control operating on nitrate and nitrite
reduction. Furthermore, nitrate inducibility of Nrt2:1Np in
plants starved of a nitrogen source is only transient; after a
rapid initial induction, the Nrt2:1Np level decreases even
though the internal nitrate concentration remains high (A
Krapp et al., unpublished data). This suggests that the
process of nitrate uptake by root cells is tightly controlled
by the plant nitrogen status.
In contrast to algae, there is not yet direct evidence for
the physiological role of the plant Nrt2 genes. A first
interesting correlation, however, comes from the study of
a methylammonium resistant mutant of N. plumbaginifolia
[26]. This mutant, MeaR, is impaired in the negative
feedback regulation of nitrate uptake by methylammonium. It is also impaired in the repression mechanism
of Nrt2:1Np expression by methylammonium (A Krapp
et al., unpublished data). A Hansenula polymorpha nitrate
transport mutant was recently isolated [27] and, consistent
Nitrate transport: a key step in nitrate assimilation Daniel-Vedele, Filleur and Caboche
237
Figure 1
I
S
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214
212
235
235
219
207
208
H V VM G L
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232
230
253
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238
283
287
NL A SL
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309
307
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330
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361
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VAR
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385
383
406
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456
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59
57
80
80
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58
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79
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63
49
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136
134
157
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141
128
129
213
211
234
234
218
206
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L
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231
229
252
252
237
282
286
GM A N I
GM A N I
GM A N L
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308
306
329
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315
360
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FV SR
FV SR
F I SR
FV SR
FV SR
VVNR
HVH P
384
382
405
405
392
439
444
P
P
-
- - AT
- - AT
VKG S
VKGT
- - AE
455
453
479
479
468
507
507
507
507
530
530
507
Current Opinion in Plant Biology
Sequence alignment and comparison of higher plants, algae and fungi CRNA related proteins: Nrt2:1Hv and Nrt2:2Hv: H. vulgare sequences
[24••]; Nrt2:1Np: N. plumbaginifolia sequence [25]; Nrt2:1At: A. thaliana sequence (this work); Nrt2:1Cr: C. reinhardtii sequence [21];
Nrt2:1Hp: H. polymorpha sequence [27]; CRNA : A. nidulans sequence [20]. The alignment was performed using the PILEUP program,
identities of at least five residues among seven are boxed and shaded, gaps generated in the alignment are indicated by dots. Stretches of
sequence homology, represented by shaded areas do not correspond only to membrane spanning regions defined by Trueman et al. [24••], but
rather are dispersed throughout the protein.
with a nitrate transport function, the Nrt2:1Hv cDNA from
barley was able to complement this yeast mutant (M Dunn
and B Forde, abstract 682, 5th International Congress
of Plant Molecular Biology, Singapore, 21–27 September
1997).
In the future, two parallel approaches should give further
insights into the function of the plant Nrt genes. The
physiological characterization of transgenic plants specifically overexpressing or underexpressing these genes will
be of great interest. Loss-of-function mutations affecting
these genes would be extremely useful, but cannot be
easily screened for, due to the redundancy of such genes;
two copies exist in N. plumbaginifolia [25], and between 7
and 10 related genes are found in barley [24••]. This goal
can be achieved in the model species Arabidopsis thaliana,
however, using a reverse genetic approach with PCR
experiments performed on a T-DNA insertional library.
For this purpose, we have recently isolated by differential
display a full length cDNA from A. thaliana, Nrt2:1At,
which we presume to code for one of the HATS elements
(Figure 1; S Filleur, F Daniel-Vedele, unpublished data).
Regulation of nitrate transport and reduction
It has been known for a long time that the nitrate
assimilatory pathway is under tight regulation by the
available nitrate and reduced nitrogen. Several of the
LATS- and HATS-related genes, apart from being root
specific, are also inducible by nitrate, and there is
evidence that at least one HATS-related gene, Nrt2:1Np,
is also repressible by reduced nitrogen ([25], A Krapp
et al., unpublished data). This suggests that regulatory
mechanisms are shared by components of the transport
system and of the assimilatory pathway.
In algae, the Nrg1 and Nrg2 genes are proposed to be
responsible for a specific negative control by ammonium
(or its derivatives) of the expression of proteins involved
in nitrate assimilation, namely NR, NiR and nitrate
238
Physiology and metabolism
transporters [28]. It will be interesting to test whether
MeaR loci are related to these regulatory elements.
The search for such regulatory loci has led to the
identification of AreA/Nit2 and NirA/Nit4 in fungi [29].
In these organisms, the regulation of NR and NiR
genes is governed, on the one hand, by pathway-specific
control genes, NiRA/Nit4 for A. nidulans and Neurospora
respectively, involved in nitrate inducibility. On the other
hand, AreA/Nit2, the major positive control genes are
involved in N-metabolite repression.
It has turned out to be more of a challenge to identify their
plant homologs. Putative homologs of Nit2 were isolated
[30] from tobacco, but their role in nitrate metabolism is
not yet proven. A functional complementation approach
aimed at identifying a homolog of the yeast Gln3 gene,
which is related to AreA and Nit2, led to the finding
of a new class of genes named RGA1 (restore growth
on ammonium 1) and RGA2 [31•] in Arabidopsis. These
genes are members of a multigene family, a member of
which, SCARECROW, is involved in regulating pattern
formation in roots [32]. More recently, RGA2 and RGA1
were found to be, respectively, the GAI [33•] and RGA
[34] genes involved in giberellin signal transduction. Work
is, therefore, under progress in our laboratory to evaluate
the putative role of RGA1 and RGA2 in N metabolite
regulation.
Conclusion: genes involved in nitrate
transport, the beginning of a long story?
Nrt1 and Nrt2 genes define two classes of membrane
proteins, probably involved in low and high affinity nitrate
transport respectively. At the molecular level, it is still
unclear whether they function alone as nitrate transporters
or as part of a more sophisticated transport system
involving other polypeptides. A number of questions
remain unanswered regarding their physiological roles and
their regulation.
What are the molecular components of the constitutive
low affinity transport system? Is there an Nrt1 related
gene such as the Nrt1:1Le in tomato? In A. thaliana, an
Nrt1:1At homolog, NTL1, fits the criteria of a second,
constitutive low affinity nitrate transporter [16••]. It
has also been shown that Nrt1:1At is able to transport
peptides (T Miller, personal communication). Is this latter
observation of physiological significance? Finally, are these
low affinity transporters also involved in nitrate efflux?
The observation of differential turnover rates for the
uptake and efflux does not support this hypothesis, but
suggests rather that influx and efflux are independent
processes [11]. It would be of great interest to identify
the proteins involved in nitrate efflux and to compare
their structural characteristics to that of NRT1 and NRT2
proteins.
One class of mutants impaired in constitutive high affinity
nitrate transport has been identified [35]. These mutants
do not affect inducible, high affinity or constitutive,
low affinity nitrate uptake. To which family does the
corresponding gene belong?
Is there, as found for algae, a two-component high affinity
transport system in plants? If so, does a homolog of the
nar-2 gene exist in plants?
Is there a specific role for each copy of the crnA related
genes found in N. plumbaginifolia or barley?
A common feature to all these plant genes is their preferential expression in roots. One additional challenge for
the future will be identifying the molecular components
of nitrate translocation in shoots and its storage in the
vacuole.
Note added in proof
The papers referred to in the text as (A Quesada, J
Hidalgo, E Fernandez, unpublished data) and (A Krapp
et al., unpublished data) have now been accepted for
publication [36,37].
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
•• of outstanding interest
1.
Hoff T, Truong HN, Caboche M: The use of mutants and
transgenic plants to study nitrate assimilation. Plant Cell
Environment 1994, 17:489-506.
2.
Crawford NM: Nitrate: nutrient and signal for plant growth. Plant
Cell 1995, 7:859-868.
3.
Daniel-Vedele F, Caboche M: Molecular analysis of nitrate
assimilation in higher plants. CR Acad Sci Paris 1996, 319:961968.
4.
Campbell WH: Nitrate reductase biochemistry comes of age.
Plant Physiol 1996, 111:355-361.
5.
•
Scheible WR, Gonzales-Fontes A, Lauerer M, Muller-Rober B,
Caboche M, Stitt M: Nitrate acts as a signal to induce organic
acid metabolism and repress starch metabolism in tobacco.
Plant Cell 1997, 9:1-17.
These authors have investigated the effects of nitrate on carbon and nitrogen
metabolism in wild-type tobacco plants and low nitrate reductase expressing
transformants. These experiments allowed the distinction between events
triggered by nitrate itself and changes produced more indirectly as a result of
nitrate assimilation. Nitrate initiates an extensive program of gene expression,
resulting in co-ordinate alterations in the activities of enzymes in several
metabolic pathways.
6.
••
Scheible WR, Lauerer M, Schulze ED, Caboche M, Stitt M:
Accumulation of nitrate in the shoot acts as a signal to
regulate shoot-root allocation in tobacco. Plant J 1997, 11:671691.
Tobacco genotypes with low expression of nitrate reductase resemble a
nitrate-deficient wild-type with respect to their growth rates and content of
organic nitrogenous compounds, but accumulate high levels of nitrate. Nitrate accumulation in the shoots is accompanied by inhibition of root growth
and an increase in the shoot:root ratio.
7.
••
Zhang H, Forde BG: An Arabidopsis MADS Box gene that
controls nutrient-induced changes in root architecture. Science
1998, 279:407-409.
In the course of screening Arabidopsis roots for nitrate-inducible genes, a
cDNA clone was identified that has homology to the MADS Box family of
transcription factors. The expression of the gene in antisense orientation
leads to an increased sensitivity of lateral roots to nitrate inhibition, when
nitrate is ubiquitously supplied, and to an inhibition of lateral root elongation
Nitrate transport: a key step in nitrate assimilation Daniel-Vedele, Filleur and Caboche
by localized applications of nitrate. These results demonstrate that nitrate
acts as a signal in the development of the plant root system.
8.
Marschner M: Mineral nutrition in higher plants, edn 2. London:
Academic Press; 1995.
9.
Glass ADW, Siddiqui MY: Nitrogen absorption by plant roots.
In Nitrogen Nutrition in Higher Plants. Edited by Srivastava HS,
Singh RP. New Dehli: Associate Publishers; 1995:21-56.
10.
Aslam M, Travis RL, Huffaker RC: Comparative kinetics and
reciprocal inhibition of nitrate and nitrite uptake in roots of
uninduced and induced barley seedlings. Plant Physiol 1992,
99:1124-1133.
11.
Aslam M, Travis RL, Rains WD: Evidence for substrate induction
of a nitrate efflux system in barley roots. Plant Physiol 1996,
112:1167-1175.
12.
King BJ, Siddiqui MY, Ruth T, Warner RL, Glass AD: Feedback
regulation of nitrate influx in barley roots by nitrate, nitrite and
ammonium. Plant Physiol 1993, 102:1279-1286.
13.
Ismande J, Touraine B: N demand and the regulation of nitrate
uptake. Plant Physiol 1994, 105:3-7.
14.
Tsay YF, Schroeder JI, Feldmann KA, Crawford NM: The herbicide
sensitivity gene CHL1 of Arabidopsis encodes a nitrateinducible nitrate transporter. Cell 1995, 72:705-713.
15.
Caboche M, Campbell W, Crawford NM, Fernêndez E, Kleinhofs A,
Ida S, Mendel R, Omata T, Rothstein S, Wray J: Genes involved
in nitrate assimilation. Plant Mol Biol Rep 1994, 12:S45-S49.
Huang NC, Chiang CS, Crawford NM, Tsay YI: Chl1 encodes
a component of the low affinity nitrate uptake system in
arabidopsis and shows cell type-specific expression in roots.
Plant Cell 1996, 8:2183-2191.
In vivo uptake measurements show that the amount of nitrate taken up by
chl1 mutants was less than that in the wild-type plants and this uptake
deficiency was effectively corrected in 35S:CHL1 transgenic plants of the
chl1 mutant. CHL1 mRNA is found primarily in the epidermal cells near the
root tip and in cells beyond the epidermis when moving basipetally. The low
affinity transport system system of Arabidopsis is proposed to be comprised
of both inducible and constitutive elements encoded by two different genes.
homologs are members of a subclass within the major facilitator superfamily.
The expression of the barley gene was induced by nitrate but not by ammonium. This paper describes for the first time the molecular cloning of putative
high affinity nitrate transporters from higher plants.
25.
Quesada A, Krapp A, Trueman L, Daniel-Vedele F, Fernêndez
E, Forde B, Caboche M: PCR-identification of a Nicotiana
plumbaginifolia cDNA homologous to the high affinity nitrate
transporters of the crnA family. Plant Mol Biol 1997, 34:265274.
26.
Godon C, Krapp A, Leydecker MT, Daniel-Vedele F, Caboche
M: Methylammonium-resistant mutants of Nicotiana
plumbaginifolia are affected in nitrate transport. Mol Gen Genet
1996, 250:357-366.
27.
Perez MD, Gonzalez C, Avila J, Brito N, Siverio J: The YNT1
gene encoding the nitrate transporter in the yeast Hansenula
polymorpha is clustered with genes YNI1 and YNR1 encoding
nitrite reductase and nitrate reductase, and its disruption
causes inability to grow in nitrate. Biochem J 1997, 321:397403.
28.
Pietro R, Dubus A, Galvan A, Fernêndez E: Isolation and
characterization of two negative regulatory mutants for
nitrate assimilation in Chlamydomonas reinhardtii obtained by
insertional mutagenesis. Mol Gen Genet 1996, 251:461-471.
29.
Marzluf GA: Genetic regulation of nitrogen metabolism in the
fungi. Microbiol Mol Biol Rev 1997, 61:1-17.
30.
Daniel-Vedele F, Caboche M: A tobacco cDNA clone encoding
a GATA-1 zinc finger protein homologous to regulators of
nitrogen metabolism in fungi. Mol Gen Genet 1993, 240:365373.
16.
••
17.
Touraine B, Glass ADM: NO3− and Clo3− fluxes in the chl1-5
mutant of Arabidopsis thaliana. Does the Chl1-5 gene encode
a low affinity NO3− transporter? Plant Physiol 1997, 114:137144.
18.
Lauter FR, Ninnemann O, Bucher M, Riesmeier JW, Frommer WB:
Preferential expression of an ammonium transport and of two
putative nitrate transporters in root hairs of tomato. Proc Natl
Acad Sci USA 1996, 93:8139-8144.
19.
Unkles SE, Hawker KL, Grieve C, Campbell EI, Montague P,
Kinghorn JR: crnA encodes a nitrate transporter in Aspergillus
nidulans. Proc Natl Acad Sci USA 1991, 88:204-208.
20.
Unkles SE, Hawker KL, Grieve C, Campbell EI, Montague P,
Kinghorn JR: crnA encodes a nitrate transporter in Aspergillus
nidulans (correction). Proc Natl Acad Sci USA 1995, 92:3076.
21.
Quesada A, Galvan A, Fernêndez E: Identification of nitrate
transporter genes in Chlamydomonas reinhardtii. Plant J 1994,
5:407-419.
22.
Galvan A, Quesada A, Fernêndez E: Nitrate and nitrite are
transported by different specific transport system and by a
bispecific transporter in Chlamydomonas reinhardtii. J Biol
Chem 1996, 271:2088-2092.
23.
Trueman LJ, Onyeocha I, Forde B: Recent advances in
molecular biology of a family of eukaryotic high affinity nitrate
transporters. Plant Physiol Biochem 1996, 34:621-627.
24.
••
Trueman LJ, Richardson A, Forde B: Molecular cloning of higher
plants homologues of the high affinity nitrate transporters of
Chlamydomonas reinhardtii and Aspergillus nidulans Gene
1996, 175:223-231.
Degenerate oligonucleotides corresponding to crnA were used in PCR to
amplify related sequences from barley root RNA. Two full-length cDNAs were
isolated and found to code for hydrophobic proteins with twelve predicted
transmembrane domains. NARK from E. coli, CRNA, NRT2:1Cr and barley
239
31.
•
Truong HN, Caboche M, Daniel-Vedele F: Sequence and
characterization of two Arabidopsis thaliana cDNAs isolated
by functional complementation of a yeast gln3 gdh1 mutant.
FEBS Lett 1997, 410:213-218.
The expression of two Arabidopsis thaliana cDNAs was found to restore
growth on ammonium containing medium of a yeast strain, mutated in both
the Gdh1 gene encoding the NADP(H)-glutamate dehydrogenase and the
regulatory Gln3 gene. The two clones are homologous to each other and
define, with the SCARECROW gene, a new family of putative transcription
factors. Their expression in plants appears to be constitutive with respect to
nitrogen source or organ specificity.
32.
Di Laurenzio l, Wysocka-Diller J, Malamy JE, Pysh L, Helariutta Y,
Freshour G, Hahn MG, Feldmann K, Benfey P: The SCARECROW
gene regulates an asymmetric cell division that is essential for
generating the radial organization of the Arabidopsis root. Cell
1996, 86:423-433.
33.
•
Peng J, Carol P, Richards DE, King KE, Cowling R, Murphy GP,
Harberd NP: The Arabidopsis GAI gene defines a signaling
pathway that negatively regulates giberellin responses. Genes
Develop 1997, 11:3194-3205.
This reports the molecular cloning of GAI and a closely related gene GRS,
both of which are involved in giberellin signaling. The authors demonstrate
that, like the pathway that operates as a negative regulator of the ethylene
response, a negative and derepressible regulatory system is the mechanism
by which giberellin regulates plant development. It is rather surprising that
the GAI and GRS genes correspond to genes that are able to restore growth
on ammonium (RGA1 and RGA2) of a yeast mutant strain [31].
34.
Silverstone AL, Ciampaglio CN, Sun TP: The Arabidopsis RGA
gene encodes a transcriptional regulator repressing the
giberellin signal transduction pathway. Plant Cell 1998, 10:in
press.
35.
Wang R, Crawford NM: Genetic identification of a gene
involved in constitutive, high-affinity nitrate transport in higher
plants. Proc Natl Acad Sci USA 1996, 93:9297-9301.
36.
Quesada A, Hidalgo J, Fernandez E: Three Nr2 genes are
differentially regulated in Chlamydomonas reinhardtii. Mol Gen
Genet 1998, in press.
37.
Krapp A, Fraisier V, Scheible A, Queseda A, Gojon A, Stitt M,
Caboche M, Daniel-Vedele F: Expression studies of Nrt2:1Np,
a putative high affinity nitrate transporter: evidence for its role
in nitrate uptake. Plant J 1998, in press.