Directory UMM :Data Elmu:jurnal:P:PlantScience:Plant Science_BioMedNet:021-040:
254
The molecular physiology of ammonium uptake and retrieval
Nicolaus von Wirén*‡, Sonia Gazzarrini*, Alain Gojon† and Wolf B Frommer*
Plants are able to take up ammonium from the soil, or through
symbiotic interactions with microorganisms, via the root
system. Using functional complementation of yeast mutants, it
has been possible to identify a new class of membrane
proteins, the ammonium transporter/methylammonium
permease (AMT/MEP) family, that mediate secondary active
ammonium uptake in eukaryotic and prokaryotic organisms. In
plants, the AMT gene family can be subdivided according to
their amino-acid sequences into three subfamilies: a large
subfamily of AMT1 genes and two additional subfamilies each
with single members (LeAMT1;3 from tomato and AtAMT2;1
from Arabidopsis thaliana). These transporters vary especially
in their kinetic properties and regulatory mechanism. Highaffinity transporters are induced in nitrogen-starved roots,
whereas other transporters may be considered as the ‘work
horses’ that are active when conditions are conducive to
ammonium assimilation. The expression of several AMTs in root
hairs further supports a role in nutrient acquisition. These
studies provide basic information that will be needed for the
dissection of nitrogen uptake by plants at the molecular level
and for determining the role of individual AMTs in nutrient
uptake and potentially in nutrient efficiency.
Addresses
*Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen,
Morgenstelle 1, D-72076 Tübingen, Germany
† Biochimie et Physiologie Moléculaire des Plantes, Ecole Nationale
Supérieure d'Agronomie de Montpellier — Institut National de la
Recherche Agronomique — Centre National de la Recherche
Scientifique, Place Viala, F-34060 Montpellier, France
‡ Corresponding author: e-mail: vonwiren@uni-tuebingen.de
Current Opinion in Plant Biology 2000, 3:254–261
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviations
AMT
ammonium transporter
Km
Michalis–Menten constant
MEP
methylammonium permease
SAT
symbiosome ammonium transporter
Introduction: a question of the right mixture
NH4+
NO3–
In most soils,
and
are the predominant sources
of N that are available for plant nutrition. Although the average NH4+ concentrations of soils are often 10–1000 times
lower than those of NO3– (and rarely exceed 50 µM) [1], the
difference in soil concentrations does not necessarily reflect
the uptake ratio of each N source. Indeed, the role of NH4+
in plant nutrition has probably been underestimated,
because most plants preferentially take up NH4+ when both
forms are present — even if NH4+ is present at lower concentrations than NO3– (see below). NH4+ requires less
energy for uptake and assimilation than NO3–, mainly
because NO3– has to be reduced prior to assimilation [2].
Optimal plant growth is, however, usually achieved when N
is supplied in both forms [3]. The exclusive supply of N as
NH4+ is harmful to many plant species, and can cause poor
root and shoot growth, and reduced mineral cation contents
relative to those of plants receiving NO3– or NH4NO3 nutrition [1]. In part, growth depression is directly related to
NH4+ uptake, as the assimilation of NH4+ is accompanied by
about equimolar H+ production. These protons are excreted,
most probably due to an increased H+-ATPase activity leading to an acidification of the rhizosphere and thus to
repressed cation uptake. Another reason for the restricted
growth of plants receiving exclusively NH4+-N may be related to the absence of NO3–. Nitrate is not only an important
osmoticum, but also an essential counter-ion for cation
translocation in the xylem, and a signal that induces the
expression of genes involved in N uptake, N assimilation,
organic acid metabolism and starch synthesis [4•]. Moreover,
changes in the N source can modify the hormonal balance in
the xylem sap, thereby affecting the growth of the shoot
[5,6]. Following the transfer of plants from mixed N nutrition
to NH4+ alone, newly formed leaves are smaller and have
fewer cells than those formed before the change in N nutrition; these changes coincide with a strong decline in
cytokinin concentrations in the xylem sap [5]. In addition,
exclusive NH4+ nutrition increases the xylem concentrations
of abscisic acid, a hormone that potentially contributes to the
stunted growth phenotype [6]. Thus, plants benefit from
mixed N nutrition, not only because NO3– usually is more
readily available than NH4+ but also because of the anionic
character, storage and signalling properties of NO3–.
The relatively low soil concentrations of NH4+, its preferential uptake compared to NO3–, and its negative effects on
plant growth when supplied as the only N source emphasise the need to understand how plant roots regulate NH4+
uptake and internal NH4+ concentrations. Thus, the aim of
this review is to focus on recent progress made in characterising the molecular basis of NH4+ transport in plants.
Kinetic components in ammonium uptake by
roots
The net uptake of NH4+ by plant roots is the difference
between concomitant influx and efflux of this ion [7]. NH4+
influx can be measured by short-term labelling (using either
13NH + or 15NH +) because efflux of the tracer increases with
4
4
time of exposure. Concentration-dependent influx of NH4+
into intact plant roots exhibits biphasic kinetics that can be
separated into at least two distinct components. At 90% similarity at the aminoacid level, whereas the sequence of AtAMT1;1 shows
about 80% homology to AtAMT1;3 and AtAMT1;5.
AtAMT1;2 and AtAMT1;4 are more distantly related with
~70% similarity to the other AtAMTs (Figure 1).
Recently, a new gene, given the preliminary name
AtAMT2;1 (accession number AC003028), was identified
from A. thaliana. The protein encoded by AtAMT2;1 has a
sequence that shares greater similarity with those of the
yeast Mep proteins than with those of the A. thaliana
AMT1 transporters. Although data on the function and
regulation of AtAMT2;1 are not yet available, it is speculated that this gene is a member of a new subfamily
exhibiting specific functions (see ‘Update’).
Additional AMT1 homologs have been isolated from other
plant species: one from rice (OsAMT1;1, [40]), and three from
tomato (LeAMT1;1-3, [41,42••]). In contrast to LeAMT1;1
and LeAMT1;2, which share 76% amino acid sequence similarity, LeAMT1;3 is the most distantly related AMT1
member described so far (
The molecular physiology of ammonium uptake and retrieval
Nicolaus von Wirén*‡, Sonia Gazzarrini*, Alain Gojon† and Wolf B Frommer*
Plants are able to take up ammonium from the soil, or through
symbiotic interactions with microorganisms, via the root
system. Using functional complementation of yeast mutants, it
has been possible to identify a new class of membrane
proteins, the ammonium transporter/methylammonium
permease (AMT/MEP) family, that mediate secondary active
ammonium uptake in eukaryotic and prokaryotic organisms. In
plants, the AMT gene family can be subdivided according to
their amino-acid sequences into three subfamilies: a large
subfamily of AMT1 genes and two additional subfamilies each
with single members (LeAMT1;3 from tomato and AtAMT2;1
from Arabidopsis thaliana). These transporters vary especially
in their kinetic properties and regulatory mechanism. Highaffinity transporters are induced in nitrogen-starved roots,
whereas other transporters may be considered as the ‘work
horses’ that are active when conditions are conducive to
ammonium assimilation. The expression of several AMTs in root
hairs further supports a role in nutrient acquisition. These
studies provide basic information that will be needed for the
dissection of nitrogen uptake by plants at the molecular level
and for determining the role of individual AMTs in nutrient
uptake and potentially in nutrient efficiency.
Addresses
*Zentrum für Molekularbiologie der Pflanzen, Universität Tübingen,
Morgenstelle 1, D-72076 Tübingen, Germany
† Biochimie et Physiologie Moléculaire des Plantes, Ecole Nationale
Supérieure d'Agronomie de Montpellier — Institut National de la
Recherche Agronomique — Centre National de la Recherche
Scientifique, Place Viala, F-34060 Montpellier, France
‡ Corresponding author: e-mail: vonwiren@uni-tuebingen.de
Current Opinion in Plant Biology 2000, 3:254–261
1369-5266/00/$ — see front matter
© 2000 Elsevier Science Ltd. All rights reserved.
Abbreviations
AMT
ammonium transporter
Km
Michalis–Menten constant
MEP
methylammonium permease
SAT
symbiosome ammonium transporter
Introduction: a question of the right mixture
NH4+
NO3–
In most soils,
and
are the predominant sources
of N that are available for plant nutrition. Although the average NH4+ concentrations of soils are often 10–1000 times
lower than those of NO3– (and rarely exceed 50 µM) [1], the
difference in soil concentrations does not necessarily reflect
the uptake ratio of each N source. Indeed, the role of NH4+
in plant nutrition has probably been underestimated,
because most plants preferentially take up NH4+ when both
forms are present — even if NH4+ is present at lower concentrations than NO3– (see below). NH4+ requires less
energy for uptake and assimilation than NO3–, mainly
because NO3– has to be reduced prior to assimilation [2].
Optimal plant growth is, however, usually achieved when N
is supplied in both forms [3]. The exclusive supply of N as
NH4+ is harmful to many plant species, and can cause poor
root and shoot growth, and reduced mineral cation contents
relative to those of plants receiving NO3– or NH4NO3 nutrition [1]. In part, growth depression is directly related to
NH4+ uptake, as the assimilation of NH4+ is accompanied by
about equimolar H+ production. These protons are excreted,
most probably due to an increased H+-ATPase activity leading to an acidification of the rhizosphere and thus to
repressed cation uptake. Another reason for the restricted
growth of plants receiving exclusively NH4+-N may be related to the absence of NO3–. Nitrate is not only an important
osmoticum, but also an essential counter-ion for cation
translocation in the xylem, and a signal that induces the
expression of genes involved in N uptake, N assimilation,
organic acid metabolism and starch synthesis [4•]. Moreover,
changes in the N source can modify the hormonal balance in
the xylem sap, thereby affecting the growth of the shoot
[5,6]. Following the transfer of plants from mixed N nutrition
to NH4+ alone, newly formed leaves are smaller and have
fewer cells than those formed before the change in N nutrition; these changes coincide with a strong decline in
cytokinin concentrations in the xylem sap [5]. In addition,
exclusive NH4+ nutrition increases the xylem concentrations
of abscisic acid, a hormone that potentially contributes to the
stunted growth phenotype [6]. Thus, plants benefit from
mixed N nutrition, not only because NO3– usually is more
readily available than NH4+ but also because of the anionic
character, storage and signalling properties of NO3–.
The relatively low soil concentrations of NH4+, its preferential uptake compared to NO3–, and its negative effects on
plant growth when supplied as the only N source emphasise the need to understand how plant roots regulate NH4+
uptake and internal NH4+ concentrations. Thus, the aim of
this review is to focus on recent progress made in characterising the molecular basis of NH4+ transport in plants.
Kinetic components in ammonium uptake by
roots
The net uptake of NH4+ by plant roots is the difference
between concomitant influx and efflux of this ion [7]. NH4+
influx can be measured by short-term labelling (using either
13NH + or 15NH +) because efflux of the tracer increases with
4
4
time of exposure. Concentration-dependent influx of NH4+
into intact plant roots exhibits biphasic kinetics that can be
separated into at least two distinct components. At 90% similarity at the aminoacid level, whereas the sequence of AtAMT1;1 shows
about 80% homology to AtAMT1;3 and AtAMT1;5.
AtAMT1;2 and AtAMT1;4 are more distantly related with
~70% similarity to the other AtAMTs (Figure 1).
Recently, a new gene, given the preliminary name
AtAMT2;1 (accession number AC003028), was identified
from A. thaliana. The protein encoded by AtAMT2;1 has a
sequence that shares greater similarity with those of the
yeast Mep proteins than with those of the A. thaliana
AMT1 transporters. Although data on the function and
regulation of AtAMT2;1 are not yet available, it is speculated that this gene is a member of a new subfamily
exhibiting specific functions (see ‘Update’).
Additional AMT1 homologs have been isolated from other
plant species: one from rice (OsAMT1;1, [40]), and three from
tomato (LeAMT1;1-3, [41,42••]). In contrast to LeAMT1;1
and LeAMT1;2, which share 76% amino acid sequence similarity, LeAMT1;3 is the most distantly related AMT1
member described so far (