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Update on Phosphorus Uptake

Phosphorus Uptake by Plants: From Soil to Cell

Daniel P. Schachtman*, Robert J. Reid, and S.M. Ayling

Departments of Botany (D.P.S., R.J.R.), and Soil Science (S.M.A.), University of Adelaide, SA 5005, Australia

P is an important plant macronutrient, making up about0.2% of a plants dry weight. It is a component of keymolecules such as nucleic acids, phospholipids, and ATP,and, consequently, plants cannot grow without a reliablesupply of this nutrient. Pi is also involved in controllingkey enzyme reactions and in the regulation of metabolicpathways (Theodorou and Plaxton, 1993). After N, P is thesecond most frequently limiting macronutrient for plantgrowth. This update focuses on P in soil and its uptake byplants, transport across cell membranes, and compartmen-tation and redistribution within the plant. We will concen-trate on P in higher plants, although broadly similar mech-anisms have been shown to apply in algae and fungi.

P IN SOIL

Although the total amount of P in the soil may be high,it is often present in unavailable forms or in forms that areonly available outside of the rhizosphere. Few unfertilizedsoils release P fast enough to support the high growth rates

of crop plant species. In many agricultural systems inwhich the application of P to the soil is necessary to ensureplant productivity, the recovery of applied P by crop plantsin a growing season is very low, because in the soil morethan 80% of the P becomes immobile and unavailable forplant uptake because of adsorption, precipitation, or con-version to the organic form (Holford, 1997).

Soil P is found in different pools, such as organic andmineral P (Fig. 1). It is important to emphasize that 20 to80% of P in soils is found in the organic form, of whichphytic acid (inositol hexaphosphate) is usually a majorcomponent (Richardson, 1994). The remainder is found inthe inorganic fraction containing 170 mineral forms of P(Holford, 1997). Soil microbes release immobile forms of Pto the soil solution and are also responsible for the immo-

bilization of P. The low availability of P in the bulk soillimits plant uptake. More soluble minerals such as K movethrough the soil via bulk flow and diffusion, but P ismoved mainly by diffusion. Since the rate of diffusion of Pis slow (1012 to 1015 m2 s1), high plant uptake ratescreate a zone around the root that is depleted of P.

Plant root geometry and morphology are important formaximizing P uptake, because root systems that havehigher ratios of surface area to volume will more effectivelyexplore a larger volume of soil (Lynch, 1995). For this reason

mycorrhizae are also important for plant P acquisition, sincefungal hyphae greatly increase the volume of soil that plantroots explore (Smith and Read, 1997). In certain plant spe-cies, root clusters (proteoid roots) are formed in response toP limitations. These specialized roots exude high amounts oforganic acids (up to 23% of net photosynthesis), which acid-ify the soil and chelate metal ions around the roots, resultingin the mobilization of P and some micronutrients (Mar-schner, 1995).

Pi UPTAKE ACROSS THE PLASMA MEMBRANEAND TONOPLAST

The uptake of P poses a problem for plants, since theconcentration of this mineral in the soil solution is low butplant requirements are high. The form of P most readilyaccessed by plants is Pi, the concentration of which rarelyexceeds 10 m in soil solutions (Bieleski, 1973). Therefore,plants must have specialized transporters at the root/soilinterface for extraction of Pi from solutions of micromolarconcentrations, as well as other mechanisms for transport-

ing Pi across membranes between intracellular compart-ments, where the concentrations of Pi may be 1000-foldhigher than in the external solution. There must also beefflux systems that play a role in the redistribution of thisprecious resource when soil P is no longer available oradequate.

The form in which Pi exists in solution changes accord-ing to pH. The pKs for the dissociation of H

3PO

4 into

H2PO

4

and then into HPO4

2 are 2.1 and 7.2, respectively.Therefore, below pH 6.0, most Pi will be present as themonovalent H

2PO

4

species, whereas H3PO

4and HPO

4

2

will be present only in minor proportions. Most studies onthe pH dependence of Pi uptake in higher plants havefound that uptake rates are highest between pH 5.0 and 6.0,where H

2PO

4

dominates (Ullrich-Eberius et al., 1984: Fu-rihata et al., 1992), which suggests that Pi is taken up as themonovalent form.

Under normal physiological conditions there is a re-quirement for energized transport of Pi across the plasmamembrane from the soil to the plant because of the rela-tively high concentration of Pi in the cytoplasm and thenegative membrane potential that is characteristic of plantcells. This energy requirement for Pi uptake is demon-strated by the effects of metabolic inhibitors, which rapidlyreduce Pi uptake. The precise mechanics of membranetransport are still not clear, although cotransport of Pi withone or more protons is the favored option based on the

following observations.

* Corresponding author; e-mail [email protected].

edu.au; fax 61882323297.

Plant Physiol. (1998) 116: 447453

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The addition of Pi to starved roots results in both depo-larization of the plasma membrane and acidification of thecytoplasm (Ullrich and Novacky, 1990). The depolarizationindicates that Pi does not enter simply as H

2PO

4

orHPO

4

2, both of which would lead to membrane hyperpo-larization. From these results it is likely that Pi is co-transported with positively charged ions. Cotransport of Piwith a cation involving a stoichiometry of more than 1C/H

2PO

4

or more than 2 C/HPO4

2 would result in anet influx of positive charge and hence lead to the observedmembrane depolarization. The cytoplasmic acidificationassociated with Pi transport would suggest that the cationis H, but acidification would occur regardless of the na-ture of the cation if the transported species were H

2PO

4

,

since it would undergo a pH-dependent dissociation in thecytoplasm to HPO4

2 and H. To verify H cotransportrequires simultaneous or at least comparable measure-ments of Pi influx and the change induced in cytoplasmicpH. Estimates of the cytoplasmic buffering capacity wouldthen allow calculation of the Pi-associated H flux, fromwhich the stoichiometry could be deduced.

Pi uptake across the plasma membrane in animal cellsnormally involves cotransport with Na. Na-energized,high-affinity Pi uptake systems have also been found incyanobacteria and green algae. In some organisms, such asSaccharomyces cerevisiae, both Na- and H-dependent Piuptake systems have been described (Roomans et al., 1977).Dependence of Pi uptake on Na has not yet been demon-

strated in higher plants, but this may be partly because fewstudies have actually tested this possible mode of ener-gized Pi uptake.

Transfer of Pi from the cytoplasm to the vacuole involvesa different set of thermodynamic parameters to those ap-plying to the plasma membrane, mainly because of themillimolar concentrations in the cytoplasm and vacuolecompared with the micromolar concentrations in the soil.Few estimates of cytosolic and vacuolar Pi concentrationsare available. However, when maize was grown at Pi con-centrations similar to those found in soils (i.e. 10 m), theroot cell cytoplasmic Pi concentration was estimated to behigher than the vacuolar concentration (Lee and Ratcliffe,1993). Soybean leaf cell cytoplasmic Pi concentrations were

also found to be higher than concentrations in the vacuole

when plants were grown in solutions containing 50 to 100m Pi (Lauer et al., 1989). Since the membrane potential ofthe vacuole is usually slightly positive with respect to thecytoplasm under these realistic conditions, Pi transfer tothe vacuole need not be energized.

In plants supplied with higher concentrations of P, Pi

appears to be close to electrochemical equilibrium acrossthe tonoplast. In one of the few studies in which tonoplasttransport has been examined, Pi uptake into vacuoles iso-lated from P-sufficient barley leaves was shown to follow amonophasic, almost linear concentration dependence up toat least 20 mm, and was independent of ATP supply(Mimura et al., 1990). However, in vacuoles isolated fromPi-starved cells, Pi uptake rates were found to be muchhigher and ATP dependent, despite the fact that the lowerPi concentrations in the vacuoles would favor passive Piaccumulation. This suggests the de-repression or activationof a second transporter in the tonoplast in response to Pistarvation.

The concentration dependence of Pi uptake in vacuoles

from Pi-starved cells has not been reported; a biphasicresponse would support the presence of a second trans-porter that might play an important role in maintaining Pihomeostasis when the Pi supply is limited. The process ofvacuolar Pi mobilization following Pi starvation is likely torequire energy-dependent transport across the tonoplast,the mechanism of which is not understood, although anH/H

2PO

4

symport would be thermodynamically feasi-ble. There is clearly a great deal more to understand aboutthe specific mechanisms of vacuolar Pi transport in higherplants and the role these mechanisms play in bufferingcytoplasmic Pi concentration.

MULTIPLE Pi TRANSPORTERS

The question of whether there are several Pi transporterswith different functional characteristics in plant cell mem-

branes or only one transporter with characteristics thatvary with internal Pi status or external concentration has

been addressed using kinetic analysis of uptake. In thistype of analysis a transporters affinity (K

m) for a particular

mineral is estimated by measuring the rate of uptake atdifferent external concentrations of an ion. Results fromkinetic studies have been variously interpreted to supportthe existence of only one uptake system in barley roots(Drew and Saker, 1984) or up to seven in maize roots

(Nandi et al., 1987). The most common interpretation ofthese kinetic studies is that two Pi uptake systems exist,one with a high affinity and activity that is either increasedor de-repressed by Pi starvation, and one with a loweraffinity and activity that is constitutive. Estimates of theK

m

for high-affinity uptake range from 3 to 7 m, whereas forlow-affinity transporters the Km estimates are more vari-able, from 50 to 330 m in several different tissues andplant species (Ullrich-Eberius et al., 1984; McPharlin andBieleski, 1987; Furihata et al., 1992).

Recent advances in the molecular biology of putativeplasma membrane and tonoplast Pi transporters confirmthat plants have multiple transporters for Pi. Thus far, fourdifferent transporter genes have been cloned from Arabi-

dopsis, three from potato, and two from tomato. Putative

Figure 1. Plant acquisition of soil P.

448 Schachtman et al. Plant Physiol. Vol. 116, 1998

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plasma membrane or tonoplast phosphate transporters inhigher plants were cloned by probing the database of trans-lated expressed sequence tags with fungal phosphate trans-porter peptide sequences. This approach identified at leastthree expressed sequence tags from randomly sequencedArabidopsis cDNAs with translational products that weresimilar to the fungal phosphate-transporter proteins. Usingthe expressed sequence tags, full-length clones have beenisolated from cDNA and genomic libraries (Muchhal et al.,1996; Leggewie et al., 1997; Smith et al., 1997). One putativephosphate transporter gene was expressed in tobacco cells(Mitsukawa et al., 1997). High-affinity Pi uptake was de-tected in the cells in which this gene was overexpressed,demonstrating that at least one member of this gene familyencodes a high-affinity plasma membrane Pi transporter.

The proteins encoded by these genes contain large re-gions that are identical to each other (Table I). The genefamily appears to be clustered in the Arabidopsis genomewith at least three members (APT1, APT2, and AtPT4)

mapping to a specific region of chromosome 5 (Lu et al.,1997; Smith et al., 1997). These multiple Pi-transportergenes are differentially expressed. Some are strongly up-regulated by Pi starvation, whereas the expression of oth-ers is constitutive (Leggewie et al., 1997). In the cases of

APT1 and APT2, the deduced amino acid sequences are99% identical, which suggests that the proteins have thesame functional characteristics. Although these proteinsare almost identical, the promoter regions are completelydifferent and may contain specific information that con-trols the spatial expression of these genes in different celltypes, such as epidermal or cortical cells in the roots.

A cDNA encoding a Pi transporter from potato, which is

expressed in roots under conditions of Pi starvation, wascharacterized in the pho84 yeast mutant (Leggewie et al.,1997). TheK

mfor Pi uptake was 130 m, much higher than

would be expected if it were involved in Pi uptake fromsoils, where concentrations rarely exceed 10 m. Variousreasons were suggested (Leggewie et al., 1997) for the highKm

values, but perhaps the most interesting is that phos-phate transporters may contain a number of different pro-tein subunits. The normal function of phosphate transport-ers may require subunits that are absent when this plantcDNA is expressed in yeast. Genetic evidence from Saccha-romyces cerevisiaeindicates that several proteins containingputative membrane-spanning domains may interact toform a Pi-transporter complex (Bun-ya et al., 1991, 1996;

Yompakdee et al., 1996). Although the association betweenthese proteins has not been directly demonstrated, and oneprotein (Pho84) has been shown to be sufficient to catalyzephosphate transport in proteoliposomes (Berhe et al., 1995),the genetic evidence supports the idea that phosphatetransporters are comprised of multiple subunits.

In summary, kinetic and molecular data show thathigher plants have multiple transporters for Pi across cel-lular membranes. The molecular data show that there are atleast four genes that encode Pi transporters, and the kineticdata suggests the presence of two types of transporterswith different affinities for Pi. The recent advances in themolecular biology of these transporters provide powerfultools for understanding how their function is integratedinto plant physiological processes. More work will be re-quired to gain a comprehensive picture of the location(cellular and subcellular) and precise function of the mul-tiple phosphate transporters in plants.

COMPARTMENTATION OF P

Maintenance of stable cytoplasmic Pi concentrations isessential for many enzyme reactions. This homeostasis isachieved by a combination of membrane transport andexchange between various intracellular pools of P. Thesepools can be classified in a number of different ways. First,according to their location in physical compartments suchas the cytoplasm, vacuole, apoplast, and nucleus. The pHof these compartments will determine the form of Pi. Thesecond pKa for H

3PO

4is 7.2, so Pi in the cytoplasm will be

approximately equally partitioned between the ionic formsH

2PO

4

and HPO4

2, whereas in the more acidic vacuoleand apoplast, H

2PO

4

will be the dominant species. Sec-ond, by the chemical form of P, such as Pi, P-esters,P-lipids, and nucleic acids. The proportion of the total P ineach chemical form (except P in DNA) changes with tissuetype and age and in response to P nutrition. Third, accord-ing to physiological function, as metabolic, stored, andcycling forms.

Our knowledge of the distribution of P into metabolicpools and physical compartments comes from three typesof studies. Before 1980, information about P compoundsand their distribution within tissues was derived from theanalysis of isolated organelles or from the partitioning ofthe radioactive tracer 32P between different chemical frac-tions (Bieleski, 1973). Other information came from studieson the rate at which 32P is incorporated into or lost from

Table I. Comparison matrix of phosphate transporter polypeptides

Protein APT1a AtPT4b StPT1c AtPT2d StPT2c GvPTe Pho84f

APT2a (AtPT1) 99g 93 77 76 74 37 27APT1 93 76 76 73 37 27AtPT4 77 77 74 37 27StPT1 82 75 36 29AtPT2 72 36 28StPT2 35 28GvPT 40a Arabidopsis (Smith et al., 1997). b Arabidopsis (Lu et al., 1997). c Solanum(Leggewie et al.,

1997). d Arabidopsis (Muchhal et al., 1996). e Glomus (Harrison and van Buuren, 1995).f

Saccharomyces(Bun-Ya et al., 1991). g

% identity of aligned sequences including gaps.

Phosphorus Uptake by Plants: From Soil to Cell 449

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tissues, commonly referred to as compartmental analysis(Macklon et al., 1996). A major advance in mapping intra-cellular pools came with the application of NMR spectros-copy in plant tissues. This technique allowed analysis invivo of Pi and other important P-metabolites (Ratcliffe,1994), as well as the monitoring of time-dependent changes

in the amounts of these compounds. Figure 2 shows atypical 31P-NMR spectrum, such as is observed from sam-ples of root tips or suspension-cultured cells, and indicateswhere the observed compounds are found within the cell.

Separate signals are detectable for Pi and other soluble-Pcompounds located in the near-neutral cytoplasm or in theacidic vacuole (Fig. 2). 31P-NMR is at present the only wayto measure directly the cytoplasmic and vacuolar pools ofPi in vivo. In an NMR spectrum the intensity of the reso-nances, reflected in the peak areas, provides an immediaterepresentation of the relative amounts of the differentsoluble-P fractions present. The peak areas represent thecontent of Pi from which concentrations can be derived (see

Lee and Ratcliffe, 1993). NMR studies confirmed that asmall, rapidly turning over pool of Pi (representing 15% oftotal Pi) is located in the cytoplasm and a larger storagepool is located in the vacuole (Ratcliffe, 1994). NMR studieshave made a major contribution to our knowledge of the

behavior of the cytoplasmic and vacuolar pools of Pi withinthe plant.

REGULATION OF Pi UPTAKE

Cytoplasmic Pi is maintained at constant concentrations(510 mm), more or less independently of external Pi con-centrations, except under severe P depletion (Lee et al.,1990; Lee and Ratcliffe, 1993; Mimura, 1995). In contrast,vacuolar Pi concentrations vary widely; under conditionsof P starvation, vacuolar Pi may be almost undetectable. Pi

in the vacuole also increases more readily than other Pfractions in response to improved P status. However, itdoes not seem to increase above about 25 mm (Lee et al.,1990; Lee and Ratcliffe, 1993; Mimura, 1995).

When the supply of Pi is limited, plants grow more roots,increase the rate of uptake by roots from the soil, retranslo-cate Pi from older leaves, and deplete the vacuolar stores ofPi. In addition, mycorrhizal fungi may more extensivelycolonize the roots. Conversely, when plants have an ade-quate supply of Pi and are absorbing it at rates that exceeddemand, a number of processes act to prevent the accumu-lation of toxic Pi concentrations. These processes include theconversion of Pi into organic storage compounds (e.g. phyticacid), a reduction in the Pi uptake rate from the outsidesolution (Lee et al., 1990), and Pi loss by efflux, which can be

between 8 and 70% of the influx (Bieleski and Ferguson,1983). Any or all of these processes may be strategies for themaintenance of intracellular Pi homeostasis.

It is clear from both kinetic and molecular studies that

the capacity to transport Pi across cellular membranes in-volves several different transporters and is in some wayregulated by the external supply of Pi. Furihata et al. (1992)showed differential expression of phosphate transportersusing kinetic techniques in which the high-affinity, but notthe low-affinity, system was repressed by high concentra-tions of Pi. The expression of certain members of theputative plasma membrane or tonoplast phosphate-transporter gene family increases during periods of Pi star-vation. In Arabidopsis at least three genes encoding phos-phate transporters are expressed in roots and are up-regulated by Pi starvation. Similarly, in potato one genewas specifically induced in roots and stolons by starvingthe plants of Pi, whereas a second gene was expressed

throughout the plant under conditions of high or lowphosphate.

Changes in Pi-transport activity and phosphate-transporter gene expression show that plant cells respond tochanges in the Pi concentration of the external medium or inthe vacuole. However, the intracellular signals and the fac-tors that modify gene expression in the nucleus while cyto-plasmic concentrations of Pi remain relatively constant areunknown. Progress at the molecular level may eventuallyprovide insight into the processes that regulate phosphateuptake through the isolation of genes encoding proteins thatinteract and regulate phosphate-transport mechanisms.

P TRANSLOCATION IN WHOLE PLANT

Recent studies (Mimura et al., 1996; Jeschke et al., 1997)provide a picture of patterns of Pi movement in wholeplants. In P-sufficient plants most of the Pi absorbed by theroots is transported in the xylem to the younger leaves.Concentrations of Pi in the xylem range from 1 mm inPi-starved plants to 7 mm in plants grown in solutionscontaining 125 m Pi (Mimura et al., 1996). There is alsosignificant retranslocation of Pi in the phloem from olderleaves to the growing shoots and from the shoots to theroots. In Pi-deficient plants the restricted supply of Pi to theshoots from the roots via the xylem is supplemented by

increased mobilization of stored P in the older leaves and

Figure 2. 31P-NMR of carrot cells. The assignments of the labeledresonances are: 1, several P-monoesters including Glc-6-P and phos-phocholine; 2, cytoplasmic Pi; 3, vacuolar (vac) Pi; 4, -P of nucle-oside triphosphates, principally ATP; 5,-P of NTPs; 6, NDP-hexoseand NAD(P)H; 7, NDP-hexose; and 8, -P of NTPs. (Spectrum re-

drawn from Carroll et al., 1994.)


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retranslocation to both the younger leaves and growingroots. This process involves both the depletion of Pi storesand the breakdown of organic P in the older leaves. Acurious feature of P-starved plants is that approximatelyone-half of the Pi translocated from the shoots to the rootsin the phloem is then transferred to the xylem and recycled

back to the shoots (Jeschke et al., 1997). In the xylem P istransported almost solely as Pi, whereas significantamounts of organic P are found in the phloem.

A number of mutants that show altered Pi accumulationin leaves have been identified. These may help us to un-derstand the processes controlling the allocation of Piwithin the plant. One Arabidopsis mutant (pho1) was iso-lated based on reduced total phosphate concentrations inthe leaf tissue (Poirier et al., 1991) and was shown to haveroot Pi uptake rates that were the same as the wild type,

but reduced translocation rates to the shoot. In the pho1mutant, it is not known whether a gene encoding a trans-porter or regulatory molecule has been mutated; however,the phosphate-transporter genes that have been cloned donot map to the pho1 (or pho2) locus. This mutation high-lights the importance of specialized mechanisms for thetransfer of Pi to the xylem. Another Arabidopsis mutant,

pho2, accumulates P in its leaves to toxic concentrations,which is indicative of a defect in the regulation of Piconcentrations in shoots (Delhaize and Randall, 1995)and illustrates the significance of regulating intracellularconcentrations.

MYCORRHIZAE IN P UPTAKE

There is a general perception that Pi uptake by plants

occurs as a direct consequence of uptake from the soil byroot cells. However, in more than 90% of land plants,symbiotic associations are formed with mycorrhizal fungi.In these plants the fungal hyphae play an important role inthe acquisition of P for the plant (Bolan, 1991; Smith andRead, 1997). Mycorrhizae can be divided into two maincategories: ectomycorrhizae and endomycorrhizae, ofwhich vesicular arbuscular mycorrhizae are the most wide-spread in the plant kingdom (Smith and Read, 1997). Themycorrhizal symbiosis is founded on the mutualistic ex-change of C from the plant in return for P and othermineral nutrients from the fungus. Influx of P in rootscolonized by mycorrhizal fungi can be 3 to 5 times higherthan in nonmycorrhizal roots (rates of 1011 mol m1 s1;Smith and Read, 1997).

The few published studies of the kinetics of Pi uptakeindicate that mycorrhizal roots and isolated hyphae haveP-uptake systems with characteristics similar to thosefound in nonmycorrhizal roots and other fungi (Thomsonet al., 1990; Smith and Read, 1997). Germ tubes of thevesicular arbuscular mycorrhizal fungus Gigaspora marga-rita have two Pi-uptake systems (Km 23 m and 10,00011,000 m) (Thomson et al., 1990). A recent molecularstudy (Harrison and van Buuren, 1995) identified the geneGvPT, which encodes a high-affinity fungal phosphatetransporter (Km 18 m) in external hyphae that is similarin both structure and function to high-affinity transporters

in plants (Table I).

A number of factors may contribute to the increased rateof Pi uptake measured in mycorrhizal plants (Smith andRead, 1997). An extensive network of hyphae extends fromthe root, enabling the plant to explore a greater volume ofsoil, thereby overcoming limitations imposed by the slowdiffusion of Pi in the soil. Several studies have shown thatthe depletion zone around plant roots, which is caused byplant uptake and the immobile nature of Pi, is larger inmycorrhizal than in nonmycorrhizal plants (Bolan, 1991).Mycorrhizal fungi may also be able to scavenge Pi from thesoil solution more effectively than other soil fungi because C(which may be limiting in the soil) is provided to the fungus

by the plant. The plant/fungus association could thereforeenable the plant to compete more effectively with soil mi-croorganisms for the limited amount of available soil Pi.Mycorrhizal fungi may also be able to acquire P from or-ganic sources that are not available directly to the plant (e.g.phytic acid and nucleic acids) (Jayachandran et al., 1992).

Little is known about the transport of P compoundswithin mycorrhizae or the mechanism of P efflux from thefungus. Pi and organic P (such as polyphosphate) could becarried within the fungus by cytoplasmic streaming or by

bulk flow to the plant root from external hyphae located inthe soil. The current view is that Pi is the major formeffluxed by the fungus across the interfacial membranes.However, there is also evidence in higher plants that phos-phocholine can be broken down outside cells to release Pi.It is possible that phosphocholine is also effluxed by thefungus to the plant; Pi would then be taken up by the plantvia an H cotransporter, as in nonmycorrhizal roots. Sinceit is known that the phosphate transporter cloned fromGlomus versiforme (GvPT) is not expressed in fungal struc-tures inside the plant, it cannot be a candidate for thefungal P efflux mechanism. Efflux of P must depend on adifferent transporter of unknown structure.

The role of P in the regulation of symbiosis is still poorlyunderstood, in part because of conflicting experimental re-sults. In mycorrhizal roots demand for P by the plant mayregulate the activity of P transporters in the fungus, withefflux from the fungus being the limiting step. However,NMR studies of ectomycorrhizal roots of Pinus resinosa(MacFall et al., 1992) showed that although there was anincrease in polyphosphate P in mycorrhizal roots, the vacu-olar Pi content of mycorrhizal and nonmycorrhizal rootswas similar. The mycorrhizal plants did not accumulate Pi in

the vacuoles, which suggests that the fungus (Hebelomaarenosa) may be able to limit the efflux of P to the plant.

Mycorrhizal roots are able to take up Pi from solutionscontaining up to 100 mm Pi (Smith and Read, 1997), con-centrations far above that likely to be encountered in thesoil. High external Pi concentrations (up to 16 mm) hadlittle adverse effect on germination and growth of germtubes in the vesicular arbuscular mycorrhizal fungus G.margarita(Tawaraya et al., 1996). These results suggest thatthe low levels of colonization seen in plants growing insoils with high P status may not be the result of directregulation of the activity of the fungus by soil Pi, but,rather, that specific signals from the plant regulate the

activity of the fungus.


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CONCLUSIONS

Considering that P is an essential and often limitingnutrient for plant growth, it is surprising that many aspectsof P uptake and transport in plants are not thoroughlyunderstood. 31P-NMR studies have provided a picture of

where Pi is distributed in a living cell, kinetic studies haveelucidated the general functional characteristics of plasmamembrane and tonoplast Pi transporters, and molecularstudies have confirmed the presence of multiple genesencoding phosphate transporters that are differentially ex-pressed. Perhaps the next important leap in our conceptualunderstanding in this area will come from the integrationof these techniques to provide a comprehensive picture ofthe function of phosphate transporters and how the controlof their spatial and temporal expression allows the plant tocope with changing environmental conditions.

A final issue to raise is that the soil Pi concentration hasoften been ignored by plant physiologists. It is common to

find experiments in which plants were grown in 1 mm

Pi,which may be 100-fold higher than the Pi concentrationsplants encounter in agricultural or natural ecosystems. Tofully understand how plants acquire Pi from soils andregulate internal Pi concentrations, future studies on Piuptake by plants must more closely mimic soil conditions,in which the concentration of Pi is always low and soilmicroflora influence both acquisition and mobilization.

ACKNOWLEDGMENTS

We thank Professors F.A. and S.E. Smith for their critical com-ments and discussions. We apologize to the colleagues whosepapers were not directly cited because of space limitations.

Received July 23, 1997; accepted October 9, 1997.Copyright Clearance Center: 00320889/98/116/0447/07.

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