Major advances in our understanding of the transport of inorganic nutrient ions across plant plasma membranes have emerged from recent studies on the control of the dominant H+-pumping ATPase and from identification of a range of new transporters for divalent cations, potassium, phosphate and nitrate. In many cases, multiple transporter isoforms have been described. An appreciation of the physiological roles of these transporters demands combined genetic and physiological approaches, which, in the case of an outward rectifying K+channel, have already been used to yield an intriguing insight into root-mediated K+release into the xylem. In this review we attempt to place some of those developments in a physiological context.
Addresses
The Plant Laboratory, Department of Biology, University of York, PO Box 373, York YO1 5YW, UK
*e-mail: [email protected]
Current Opinion in Plant Biology1999, 2:236–243 http://biomednet.com/elecref/1369526600200236 © Elsevier Science Ltd ISSN 1369-5266
Abbreviations
HAK high affinity K+transporter
KIRC K+selective inward rectifying channel KORC K+selective outward rectifying channel LCT low affinity cation transporter
PMF protonmotive force ZIP ZRT, IRP-like protein
Introduction
Plant growth depends on the acquisition and appropriate partitioning of inorganic solutes from the soil. The prin-cipal inorganic nutrients are potassium, nitrogen, phosphate and sulphate [1]. The latter three nutrients are metabolised and incorporated into organic com-pounds, while K+plays a key role in generating osmotic pressure and in compensating net negative charge of cytosolic constituents [2]. Other essential nutrients, which are absorbed to a lesser extent, include divalent cations required as enzyme cofactors (iron, zinc, copper) or for signal transduction (calcium).
Absorption of all nutrients is energised, ultimately, by pro-ton pumps [3]. H+-ATPases are ubiquitous in plant plasma membranes and generate an electrochemical potential dif-ference for H+ (i.e. protonmotive force [PMF]). The electrical component of the PMF — the membrane poten-tial — is typically of the order –150 mV, and tends to drive cation uptake requiring only the presence of a pore, or channel. By contrast, accumulation of anionic nutrients requires additional energization, which is provided by cou-pling their transport to the re-uptake of H+.
This simplistic picture for cation and anion transport — although broadly correct — requires refinement. When present at very low concentrations, transport of cations such as K+will require energisation by both components of the PMF [4]. In such cases transport requires the operation of more than one class of transport system. Furthermore, many of the proteins which catalyse transport for a given ion are expressed as a variety of isoforms. These findings demand that the significance of multiple pathways for ion transport be considered in a physiological context. New insights are also emerging into the mechanisms of control of the activities of H+-pumping ATPases, which underlie plasma membrane energisation.
Important families of transporters for ammonium and sul-phate have been identified some years ago and have been discussed previously [5]. In this review we consider the properties of recently-identified transporters for a number of nutrient ions, including divalent cations, K+, Pi and nitrate. Their physiological roles are considered in relation to their expression patterns and, where studied, their kinetic properties.
Regulation of H
+transport
The H+-ATPase reaction cycle includes a phosphorylat-ed intermphosphorylat-ediate state. Both the phosphorylation and ATP-binding domains are conserved. The H+-ATPase exhibits a long hydrophilic extension around the car-boxyl-terminus (Figure 1). Truncation of the carboxyl-terminus elicits constitutive activity of the enzyme, indicating that the carboxyl-terminus is a regu-latory domain [6]. It has also been known for many years that activity of the H+-ATPase is increased by the fungal toxin fusicoccin [7], and that one potential site of inter-action of fusicoccin is with 14-3-3 proteins [8]. 14-3-3 proteins are ubiquitous in all eukaryotic organisms where they play essential roles in cell signalling, possibly by modulating protein phosphorylation. Only recently, how-ever, has the mode of action of fusicoccin on the H+-ATPase been explained in a molecular context, that has general relevance to the control of H+ pumping at the plasma membrane.
Co-purification of an active H+-ATPase and a 14-3-3 pro-tein from fusicoccin-treated tissue, but not from control tissue, indicates that direct interaction between the H+ -ATPase and 14-3-3 proteins is mediated by fusicoccin [9•,10•,11]. Furthermore, cleavage of a 10 kDa fragment from the carboxy-terminal of the H+-ATPase abolished interaction with the 14-3-3 protein, reinforcing the notion that fusicoccin-induced stimulation is related to relief of auto-inhibition by the carboxy-terminal domain [9•]. Expression in yeast of one isoform of the H+
-Plasma membrane transport in context — making sense out
of complexity
ATPase from Arabidopsis thaliana, AHA2, has provided further evidence for functional interactions between the enzyme and 14-3-3 proteins [12••]. Thus, fusicoccin binding activity was conferred by the carboxy-terminal domain of AHA2, but only in the presence of 14-3-3 pro-teins. The fusicoccin binding site appears to be shared between the H+-ATPase and the 14-3-3, although whether an endogenous small molecule fulfils this func-tion in plantais not known.
Emerging classes of divalent cation transporter
The molecular identity of plant uptake systems for divalent cations has, until recently, been a mystery. Several develop-ments in the past few years have, however, resulted in the characterisation of divalent cation transporters, predomi-nantly via functional complementation of yeast mutants.An iron-regulated metal transporter gene (IRT1), encoding a protein with eight putative transmembrane spans, has
Figure 1
The modulation of H+-ATPase activity by fusicoccin (FC) and 14-3-3 proteins. The ATPase activity shifts from low to high after binding of both FC plus 14-3-3 to the carboxyl-terminus (Ct) of the pump. Neither the ATPase nor the 14-3-3 protein is capable of FC binding on its own (see [3]).
Ct
Low-activity state High-activity state
FC
14-3-3
Current Opinion in Plant Biology
Figure 2
Genes that encode transport systems in the plant plasma membrane and their putative functions as discussed in the text. CDPK; calcium/calmodulin domain protein kinase.
NRT1 NRT2
Vacuole
Root symplast
AHA
APT PHT LePT MePT
Root apoplast
ZIP
H+?K+
H+
K+
K+
Zn2+
Na+/K+
Na+
Ca2+
Ca2+
Ca2+
nH+?Pi H+? NO 3–
KUP HAK AtKT
SKOR
Guard cell
HKT LCT1
ATP Cd2+/
Ca2+?
ADP
CDPK KAT
ATP ADP
Xylem
been identified in Arabidopsis [13]. Yeast complementation experiments suggest that IRT1 is involved in Fe2+ trans-port. Expression is most marked in roots, and is induced by iron deficiency. The IRT1 gene is a member of a family which includes Zn2+ transporters encoded by the ZRT genes of yeast, known as the ZIP (for ZRT, IRP-like pro-tein) family (Figure 2).
In the case of Fe, transporter activity in delivering the ion to the root cell is often supplemented by secretion of chelators which mobilise Fe3+from the soil. Further insights into the mechanism of Fe uptake have emerged from the recent identification of a membrane-bound ferric-chelate reductase in Arabidopsis( see Note added in proof). At the root surface, Fe3+→Fe2+ reduction reduces affinity for the chelator, thereby increasing the concentration of Fe2+for subsequent uptake. The FRO2 gene was identified on the basis of homology with human and yeast plasma membrane enzymes that transfer electrons from FADH2 to an acceptor on the opposing side of the membrane via two intramembrane haem groups. FRO2transcripts accumulate in roots in response to Fe deficiency. Furthermore, FRO2 complements mutants which are defective in ferric chelate reductase activity. The deduced structure of FRO2 contains a cytosolic binding site for FAD and conserved intramembrane His residues thought to be involved in coordination of the two haems.
In complementation studies using yeast zrt mutants, four genes (ZIP1–4), which encode transporters involved in Zn2+uptake, were identified in Arabidopsis [14••]. Each of these transporters is predicted to possess 7–9 transmem-brane spans, and expression of two (ZIP1 and ZIP3) is preferential in root tissue and enhanced by Zn2+starvation. These findings suggest that ZIP1 and ZIP3 are involved in
Zn2+uptake from the soil. The differential substrate speci-ficity and expression patterns of ZIP isoforms might point to their discrete physiological roles. Competition experi-ments with other divalent cations also indicate that ZIP1 and ZIP3 are distinctly Zn2–-specific, while ZIP2 exhibits equal or greater affinity for Cu2+and Cd2+.
Sequence analysis of ZIP family members demonstrates a degree of conservation in putative transmembrane spans IV, V and VI, possibly associated with an intramembrane metal binding site [15•]. Furthermore, in the majority of members, a possible heavy metal binding site has been identified in the extramembrane loop between transmem-brane spans III and IV. This conserved sequence has the form HXHXH, a motif found in another, otherwise unre-lated family of heavy metal transporters from bacteria.
A completely different type of transporter involved in cation uptake has been identified in wheat roots and shoots. The LCT1 (for low affinity cation transporter) gene was originally identified by virtue of its ability to restore growth of a K+uptake-deficient yeast in low millimolar K+ concentrations [16]. LCT1 was originally shown to trans-port K+ and Na+ in yeast; however, further work demonstrated an ability also to transport Ca2+ and Cd2+ [17••]. Although the physiological function of LCT1 is unknown, this is the first transport system identified as a candidate for mediating Ca2+uptake by plant cells.
Our knowledge of the pathways for divalent cation transport across the plasma membrane therefore remains fragmentary, yet, significant advances can be expected in the coming years, not only from complementation screens, but also from bioinformatic approaches to genome sequences.
Table 1
Gene products involved in the uptake and translocation of K+in plants.
Protein Species Type Expression Inhibitors Function Reference
AKT1 A. thaliana Channel Root cortex Cs+/TEA/Ba2+ Low and high [18••]
affinity uptake
SKOR A. thaliana Channel Root pericycle Translocation [25••]
Xylem parenchyma to shoot
HvHAK1–2 Barley Carrier Root Na+/NH
4+ High affinity uptake [19••]
Wheat (Kmof 27 µM)
Rice
A. thaliana
AtKUP1–4 A. thaliana Carrier Flower/leaf Cs+ High affinity uptake [21•]
(AtKT1–2) Root/stem (Kmof 22 µM) with
low affinity component
AtKUP1 A. thaliana Carrier Root Cs+/Ba2+ Dual affinity uptake [20•]
(Kms of 44 µM and 11mM)
HKT1 Wheat Carrier Root High affinity uptake [22–24]
Barley (Kmof 3 µM), low
affinity Na+uptake
Potassium transport
To maintain the essential roles that K+ plays in cellular homeostasis requires sophisticated means to move K+ at key locations throughout the plant, which include the soil/root interface, the xylem, and cells involved in move-ment, for example in guard cells, which are responsible for the opening and closing of stomatas.
K+transport at the soil/root interface
The process of K+uptake in plant roots has been a major focus of plant physiology. Conventionally, K+accumulation at various ambient K+concentrations is perceived as occur-ring at the root cell plasma membrane through two or more independent influx systems with respectively high and low affinities. Traditionally, low affinity K+uptake is believed to be mediated by K+selective ion channels, a notion sup-ported by both patch clamp characterisation of root K+ channels and yeast complementation studies [2]. Thermodynamic constraints require energisation of high affinity K+uptake via carriers [4].
The convenient notion of a channel and carrier mediated low and high affinity K+uptake is rapidly crumbling in the face of recent data. Functional analysis of the role of AKT1 (a K+selective inward rectifying channel or KIRC) in plan-ta has been advanced [18••] by identifying an AKT1 null mutant (akt1-1) from an Arabidopsis T-DNA mutagenised population. Surprisingly, differences in growth between wild type and akt1-1 were apparent only in the presence of millimolar ammonium and with K+levels of >1 mM. In the presence of ammonium, Rb+ uptake from external solu-tions was reduced, but progressively so at lower Rb+ concentrations. A reduced Rb+influx in the mutant also occurred in the absence of ammonium conditions where no phenotype was observed. The combined results of this study indicate that, in the presence of ammonium, the AKT1 channel plays a role in high affinity K+ uptake. They also indicate that AKT1 may not be the predomi-nant low affinity K+influx pathway in A. thalianaroots, as at 1 mM external Rb+ concentrations the mutant shows around 70% of the wild-type uptake.
With a sufficiently negative membrane potential, the influx of K+, even from lower micromolar concentrations, can proceed ‘down hill’ and this obviates the requirement
for an energised carrier mechanism. Nevertheless, a large number of carrier type K+ transporters has now been identified (Table 1, Figure 2). In barley, HvHAK was cloned [19••] using degenerate primers for conserved regions of the Schwanniomyces HAK (high affinity K+) transporter. Complementation of K+ transport deficient yeast restored high affinity Rb+uptake, which, in agree-ment with the observations of Hirsch et al. [18••], was sensitive to ammonium. The HvHAK1 protein has 12 putative membrane spanning regions and is a member of a large conserved family of transporters that are highly K+ selective and probably function as H+ coupled systems [19••]. The barley member of this family shows homolo-gy to HAK/KUP transporters found in bacteria (Escherichia coli), fungi (Schwanniomyces occidentalis), plants (Lyphopyrum, wheat, rice and Arabidopsis) and ani-mals (Homo sapiens). HvHAK1 transcript is exclusively found in roots and highly induced by K+starvation, which is in agreement with the frequently observed induction of high affinity K+uptake in intact plants.
By searching for HAK/KUP homologues in A. thaliana EST databases, several groups isolated a number of HAK/KUP isoforms (AtKUP 1–4). The Rb+and K+ trans-port capacity of AtKUP1 was analysed in homologous and heterologous expression systems [20•,21•]. In both expression systems AtKUP1 mediated high affinity Rb+ transport with a micromolar Kmindicating a role in high affinity K+transport in vivo. In addition to high affinity K+ transport, AtKUP1 expressing yeast cells showed a rise in Rb+uptake from millimolar concentrations [20•]. The latter suggests that HAK/KUP type proteins may also contribute to low affinity K+ transport. Expression patterns of AtKUP1 differ in various reports (Table 1) and expression of at least one isoform (AtKUP3) is up regu-lated upon K+-starvation [21•].
A class of putative high affinity K+transporters, not relat-ed to HAK/KUP, is providrelat-ed by HKT1 type mechanisms [22]. Originally cloned from wheat, HKT1 catalyses K+:Na+(µM ambient Na+) or Na+:Na+(mM ambient Na+) symport. Homologues of HKT1 have been found in bar-ley [23], rice [24] and A. thaliana[22] and in both barley and wheat mRNA levels increase in K+starvation condi-tions [23]. The physiological relevance of HKT1 remains
Table 2
Gene products involved in the uptake of phosphate in plants.
Protein Species Expression Transport Km Reference
APT1–2 (APT1–4) A. thaliana Root/low level in leaf – [33,35•]
PHT1 A. thaliana Root 3µM [34]
StPT1–2 Potato Root/tuber/leaf flower 280 and 130µM [31•]
LePT1 Tomato Root/mature leaf 31µM [30•]
to be clarified but may be in providing a pathway for Na+ entry into the plant [24].
K+release into the xylem
SKOR [25••] is an Arabidopsis K+selective channel and a member of the Shaker gene family (Figure 3). Shaker type channels are K+selective, voltage gated outward rectifying channels that were first identified in Drosophila shaker mutants. In spite of the large degree of homology to KAT and AKT inward rectifying channels, SKOR mediates K+ efflux. How such similar primary protein structures result in K+channels with contrasting voltage sensitivity remains to be explained. Expression patterns and gene disruption mutants imply a clearly defined function for SKOR in xylem loading of K+: GUS constructs with the SKOR pro-moter show expression restricted to the root pericycle and xylem parenchyma cells. Disruption of the SKOR gene did
not affect root K+levels but caused a decrease in xylem sap K+and a reduction in shoot K+contents, consistent with a function in K+release into the xylem. Exposure of plants to the stress hormone ABA, which is believed to play a role in ion translocation to the shoot, resulted in a rapid decrease of SKOR transcript.
K+transport in guard cells
The first plant K+channels were identified in guard cells which control stomatal aperture by osmotic swelling and shrinking caused by the movement of large amounts of K+. Via the membrane voltage, either inward or outward rectifying channels are activated to allow a sustained inward or outward flux of K+ for stomatal opening or closing, respectively. A variety of stimuli such as ABA, CO2 and oxidative stress promote stomatal closure, probably via a raise in cytosolic Ca2+. A direct target for cytosolic Ca2+
Figure 3
Model according to Doyle et al. 1998 [44] for the permeation of K+ions through an inward rectifying K+channel with a pore region similar to that of the Shakerfamily K+ channels. Ions traverse a narrow pore within the channel, which forms the selectivity filter (SF) and can hold two ions simultaneously. The large aqueous cavity helps to stabilise cation(s) in the middle of the membrane by the dipole action of the depicted helices. SF
Current Opinion in Plant Biology
– –
Figure 4
Generalised model for the topology of high affinity phosphate carriers. Transmembrane spans show an internal repeat and are arranged in a typical 6+6 structure characteristic of many carriers belonging to the major facilitator superfamily. The large cytoplasmic loop contains a putative phosphorylation site for kinase C. COOH
NH2
has now been described in Vicia faba guard cells [26] as a Ca2+ dependent protein kinase that phosphorylates and down regulates KAT1, the predominant KIRC in guard cell plasma membranes.
Cytosolic Ca2+also affects the activity of a newly identified K+selective outward rectifying channel (KORC) [27]. In contrast to previously described KORCs that show sus-tained currents, this particular type of KORC rapidly inactivates. The physiological role of this channel remains to be elucidated, but may be in the transduction of signals during stomatal closing. Additional potential modulators of stomatal aperture that act on K+channels are agents that affect the polymerisation of actin filaments [28] and the osmolarity of solutions facing the membrane [29].
Phosphate uptake in plant roots
Phosphate is an essential macronutrient and plant cells con-trol cytosolic phosphate within narrow limits by balancing import and export. Rates of phosphate uptake depend on growth demand and phosphate supply, the latter frequent-ly being inadequate due to the low mobility of phosphate in the soil. At the prevailing soil pH, phosphate is most readily taken up by plants as H2PO4–. External concentra-tions of this nutrient are typically a few micromolar in physiological conditions, whereas cytosolic levels are a 1000-fold higher. Phosphate transport shows a pronounced pH optimum, is highly sensitive to uncouplers [30•] and is generally assumed to occur via H+coupled symport.
Several genes encoding high affinity phosphate trans-porters have been cloned (Table 2; [30•–32•,33,34,35•]). The proteins encoded by these genes are around 60 kD, and show high levels of homology to each other and to the high affinity phosphate transporters of yeast PHO84 [36] and mycorrhizal fungi GvPT [37]. APT1 and APT2 [35•] for example, are both expressed in Arabidopsis root tissue and share 99% identity in their coding regions; however, their promoter regions are very different, pointing to cell-type-specific or development-cell-type-specific expression for either gene.
Hydrophobicity plots show a common structure of 12 transmembrane spans that contain an internal repeat of 6 + 6 transmembrane spans (Figure 4) with a large central hydrophobic region protruding into the cytosol that carries a highly conserved putative kinase phosphorylation site. In some cases transcript levels were shown to be sensitive to the phosphate status of the plant (e.g. for APT1, APT2) whereas other genes appear to be constitutively expressed (e.g. for StPT2).
The functional analysis of LePT1 from tomato [30•] and of MtPT1 from Medicago [32•] was carried out in the yeast pho84 strain. In both cases, yeast growth was restored on micromolar phosphate and transport assays confirmed the restoration of phosphate uptake with an apparent Kmof 31
µM for LePT1 and 192 µM for MtPT1. Both Kmvalues are much higher than those established in intact plants and may
result from the use of Saccharomyces as expression system where the interaction of several proteins is necessary for the proper functioning of high affinity phosphate transport [36].
The relative contribution of these gene products to the overall uptake of phosphate by plants will be affected by the formation of mycorrhizae, which can drastically improve plant phosphate nutrition. The presence of myc-orrhizae will shift phosphate uptake to GvPT-type transporters at the soil/fungus interface and can reduce the amount taken up by the plant root to almost nil. The mechanism of phosphate transfer from fungus to root remains unknown but probably involves separate gene products as was shown for Medicago where MtPT transcript was significantly reduced after mycorrhiza formation [32•].
Nitrate uptake
Nitrate uptake from the soil has long been suspected, from kinetic studies, to involve an array of different trans-port systems. Thus, constitutive high affinity and non-saturable kinetic phases exist in roots along with a high affinity phase, which is induced by nitrate [38]. The properties of the transport systems that underlie this kinetic complexity are beginning to be elucidated with candidate genes falling into two families.
The first family of nitrate transporters is identified by a consensus motif (FYXXINXGSL), characteristic of the PTR family of peptide transporters present in yeast, plants and mammals. At least two members of this fami-ly are present in Arabidopsis: NRT1 (or CHL1) and NRT3 (or NLT3) [38]. NRT1 expression is inducible by nitrate, whereas that of NRT3 appears to be constitutive. Until recently both transport systems were thought to be involved only in low affinity transport. Careful analysis of one chl1 mutant line containing an active transposable element in CHL1, however, revealed defects in both low and high affinity transport [39••]. The extent to which CHL1 participates in high affinity transport depends critically on growth conditions: the presence of ammoni-um ions results in a marked contribution of CHL1 to high affinity uptake, whereas in the absence of ammoni-um no contribution is apparent. CHL1, therefore, appears to behave as a dual affinity transporter, a phe-nomenon that can be explained by random binding of H+ and nitrate to the carrier [40].
family exhibit differential expression patterns in tomato roots [42].
Members of the second nitrate transporter family (NRT2) are nitrate inducible, belong to the major facilitator super-family and were identified originally on the basis of homologies to fungal and algal nitrate transporters [38]. Confirmation of a role in nitrate transport awaits their func-tional expression or the identification of mutants in these genes. The high degree of correlation between expression levels of the tobacco transporter NRT2 with high affinity nitrate transport activity, however, provides evidence for a function in inducible, high affinity nitrate transport [43•].
Conclusions and prospects
The number of transporters identified at a molecular level has increased dramatically over the past years. For any given nutrient, an array of potential transport path-ways is present at the plasma membrane, even within a single species.
At one level, this complexity reflects the presence of iso-forms that are often expressed on a tissue- or cell type-specific basis. Isoforms enable transport to be con-trolled at a transcriptional level in response to developmental or environmental signals. At a second level, different classes of transporter are apparent for ions like K+ and nitrate which may reflect the wide range of nutrient concentrations to which plants are exposed. Although the complexity of multiple transport pathways will doubtless increase as genomic information expands, we can expect marked advances in our understanding of the functional attributes of transporters with the combina-tion of reverse genetic and physiological approaches over the coming years.
Considerable effort and expense is required for the inor-ganic fertilisation of many agricultural systems. Many such systems are, indeed, over-fertilised, with devastating consequences for water courses which become eutrophic. Identification of those transporters which are pivotal in the uptake of inorganic nutrients from the soil and the subsequent redistribution of nutrients around the plant offers the exciting possibility of engineering more effi-cient strategies for fertiliser application. Such strategies should focus not only on the mechanisms for solute uptake, involving, for example, the affinity of the trans-porter for the ion. In addition the clear implication of the presence of multiple isoforms is that transcriptional and/or post-translational controls might differ, and elucidating the pathways leading to regulation of transporter activity can be predicted to become a new and major goal of stud-ies in nutrient acquisition.
Note added in proof
Further insights into the mechanism of Fe uptake have emerged from the recent identification of a membrane-bound ferric-chelate reductase in Arabidopsis[45••].
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
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Experiments carried out with sealed plasma membrane vesicles show that the fusicoccin-binding site faces the cytoplasmic surface of the membrane. Stabilisation of the labile ATPase–14-3-3 complex in plasma membranes could be achieved by fusicoccin treatment in vivoor in vitro. The carboxyl-terminus probably represents the binding domain for 14-3-3 homologues. 11. Oecking C, Hagemann K: Association of 14-3-3 proteins with the
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and a fusicoccin responsive system.Plant J1998, 13:661-671. By testing the fusicoccin binding activity of yeast membranes, the carboxy-termi-nal regulatory domain of AHA2 was found to be part of a functiocarboxy-termi-nal fusicoccin receptor, a component of which was the 14-3-3 protein. ATP hydrolytic activity of
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transporters. Expression in yeast reveals high specificity of two members of the family for Zn2+, and transcript levels are upregulated in roots on Zn2+starvation.
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Expression of LCT1 in yeast renders growth of yeast more sensitive to Cd2+.
Ion flux assays showed an increase in Cd2+uptake. Growth assays also
demonstrate a sensitivity of LCT1-expressing yeast cells to extracellular mil-limolar Ca2+concentrations. It is concluded that LCT1 can mediate uptake
of Ca2+and Cd2+in yeast and possibly also in plants.
18. Hirsch RE, Lewis BD, Spalding EP, Sussman MR: A role for the AKT1 •• potassium channel in plant nutrition.Science1998, 280:918-921. Reverse genetic screening identified an Arabidopsis thaliana mutant in which the AKT1 channel gene was disrupted. Roots of this mutant lacked inward-rectifying potassium channels and displayed reduced potassium (rubidium-86) uptake in the presence of ammonium.
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• Arabidopsis.Plant Cell1998, 10:63-67.
Characterisation of AtKUP3 — a K+transporter — in yeast. When expressed
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• gene encoding high-affinity potassium transport activity.Plant Cell1998, 10:51-62.
The identification of AtKUP from Arabidopsis thalianavia homology screening of non-plant Kup and of HAK1 potassium transporters from Escherichia coli
and Schwanniomyces occidentalis. AtKUP1 and AtKUP2 are able to comple-ment a potassium transport deficient E. coli triple mutant. Kinetic characterisa-tion and expression patterns are reported. Through sequence analysis a family specific signature sequence and metol-binding domains are described. 22. Schachtman DP, Schroeder JI: Structure and transport mechanism
of a high-affinity potassium uptake transporter from higher plants.Nature1994, 370:655-658.
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