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Mini-review

Nutritional ecology of arbuscular mycorrhizal fungi

A. HODGE*, T. HELGASON, A.H. FITTER

Department of Biology, Area 14, PO Box 373, University of York, York YO10 5YW, UK

a r t i c l e i n f o

Article history:

Received 7 August 2009

Revision received 1 February 2010

Accepted 3 February 2010

Available online 30 March 2010

Corresponding editor: John Cairney

Keywords:

Carbon

Evolution

Glomeromycota

Mycelium

Mycorrhiza

Nitrogen

Nutrition

Phosphorus

* Corresponding author. Tel.: þ44 1904 32856E-mail address: ah29@york.ac.uk (A. Hod

1754-5048/$ – see front matter ª 2010 Elsevidoi:10.1016/j.funeco.2010.02.002

a b s t r a c t

Despite their large role in ecosystems and plant nutrition, our knowledge of the nutritional

ecology of the fungi involved in the arbuscular mycorrhizal symbiosis, the Glomeromycota,

is poor. We briefly describe the mechanisms that underlie the fluxes of the three major

elements (C, N and P) and outline a model for the interchange of these between the

partners. This model is consistent with data from physiological, ecological and taxonomic

studies and allows a new and necessary focus on the nutritional requirements of the

fungus itself, separately from its role in the symbiosis. There is an urgent need for new

studies to identify the sources of nutrients such as N and P that AM fungi (AMF) use for

their own growth and to elucidate the mechanisms that control the transfer of these to the

plant in relation to fungal demand.

ª 2010 Elsevier Ltd and The British Mycological Society. All rights reserved.

Introduction phylum Glomeromycota goes back to the Devonian as a symbi-

Traditionally the arbuscular mycorrhizal (AM) symbiosis is

viewed as a classic mutualism, an interaction in which both

partners benefit. The fungi appear to acquire their entire carbon

supply from the plant, and although colonisation of roots by AM

fungi (AMF) can confer a wide range of benefits to the plant

(Newsham et al. 1995), the most widely cited benefit is that of

enhanced phosphorus (P) acquisition. The fungal hyphae can

explore a large volume of soil and acquire P beyond the phos-

phate depletion zone that rapidly builds up around the root

surface at a much smaller carbon cost than is possible by root

growth (Harley 1989): this economy probably underlies the

evolution of the symbiosis. The fossil record of the AM fungal

2.ge).er Ltd and The British My

osis (Remy et al. 1994) and to the Ordovician as spores (Redecker

et al. 2000); they thus have a contemporaneous origin with the

land flora. The first land plants had rhizomes and rhizoids, but

no root systems. Acquisition of poorly mobile phosphate ions

was therefore a major problem and fossil evidence reveals that

these early plants had fungal structures strikingly similar to

modern AM structures of the ‘Arum-type’ in their rhizomes; it is

not a big leap to the assumption that they performed the same

function then as now, and were responsible for plant uptake of P

(Helgason & Fitter 2009). That enhanced plant P nutrition is still

a major outcome of the AM symbiosis demonstrates that while

root systems have become larger and more complex, P acqui-

sition is still a major challenge for most plants.

cological Society. All rights reserved.

268 A. Hodge et al.

The morphology of AMF colonisation can vary giving rise

to the so-called ‘Arum-’ or ‘Paris-type’ mycorrhizas. In the

Arum type inter-cellular AMF hyphae spread within the root

cortex. Short side branches form which penetrate the root

cortical cell walls and branch extensively to give the char-

acteristic ‘arbuscule’ structure. In the Paris type there is little

inter-cellular growth but, instead, extensive intracellular

coiled hyphae which spread directly from cell to cell and

from which arbuscules may develop. Much less is known

about the Paris type than the Arum type and it is the latter

that forms in the roots of most crop plant species (Smith &

Read 2008). Around two-thirds of all plants form the AM

symbiosis but, for a group with such ecological importance,

our knowledge of the biology of the Glomeromycota is

comparatively poor. Recent studies show that there is

considerable genetic and phenotypic variation among AM

fungal isolates (Koch et al. 2004; Croll et al. 2008), and

although sexual stages have never been observed, genetic

evidence suggests that recombination may occur (Gandolfi

et al. 2003; Croll & Sanders 2009). These exciting new

studies suggest that a greater understanding of AM fungal

population structure, differentiation, dispersal and persis-

tence is not far away; much nevertheless remains to be done.

Critically for the purpose of this review we do not know why

they are obligate symbionts and cannot be grown in the

absence of live plant tissue, nor the most basic details of their

physiology and especially what controls the fluxes of nutri-

ents between plant and fungus in the symbiosis. This review

examines the current knowledge of the nutritional ecology of

AM fungi.

Nutrient fluxes in the AM symbiosis

Carbon

The major fluxes in the AM symbiosis appear to be of C from

plant to fungus and of P, and possibly N, from fungus to plant.

Reverse C movement – from fungus to plant – appears only to

occur in special cases where the plant has an unusually

restricted C supply, most notably in achlorophyllous plants

(Bidartondo et al. 2002). In virtually all other cases, apparent

plant-to-plant movement of C is best explained as the AM

fungus moving C from the intra-radical mycelium in one root

system to the same mycelium within another root; the carbon

almost always remains in the roots and is retained in the

intra-radical fungal structures (Robinson & Fitter 1999; Voets

et al. 2008).

The mechanisms of these fluxes are not yet well under-

stood. Even the location of the carbon flux is obscure, with

the best evidence – from activity of ATPases – suggesting that

it occurs in the Arum type at the inter-cellular hyphae

(Gianinazzi-Pearson et al. 1991). A model for transport of C

from intra- to extra-radical hyphae has been proposed (Bago

et al. 2003) and a hexose transporter (GpMST1) has been

identified in the fungus Geosiphon pyriforme, a non-

mycorrhizal member of the Glomeromycota (Schubler et al.

2006). The identity of the C transporters in mycorrhizal

taxa will soon become apparent as genome information is

published (Martin et al. 2008).

Phosphate

The phosphate flux is better characterised. Phosphate is taken

up by high-affinity phosphate transporters in the extra-radical

mycelium (Harrison & van Buuren 1995). Phosphate is prob-

ably transported within the fungus as polyphosphate (polyP),

and once in the intra-radical hyphae the long chains are

hydrolysed, facilitating transfer to the host plant (Harrison

1999; Bago et al. 2002; Ohtomo & Saito 2005). Fungus-to-plant

transfer appears to occur principally at the arbuscule inter-

face, although expression of P transporters around Paris type

hyphal coils has also been demonstrated (Karandashov et al.

2004). Plant ATPase activity is strongly expressed at the peri-

arbuscular membrane (Smith et al. 2009) and phosphate

accumulation as polyP strongly correlated with AM colonisa-

tion (Ohtomo & Saito 2005). Most importantly, a subfamily

(subfamily 1 under the family Pht1) of plant phosphate

transporters is now known that is expressed only in colonised

plants; the first of these was in Solanum tuberosum (StPT4;

Rausch et al. 2001), and they have subsequently been identified

in several other taxa (Javot et al. 2007). Acquisition of P via the

symbiotic pathway downregulates direct P uptake by the plant

(Smith et al. 2004, 2009).

Nitrogen

In contrast to P, fewer studies have considered a role for AMF

in N acquisition, because the greater mobility of ammonium

and especially nitrate ions in soil, compared to phosphate, led

to the assumption that little benefit was likely to plants from

enhanced N uptake. AMF can certainly transport N to roots:

AM extra-radical mycelium (ERM) exposed to 15N-labelled NO3�

or NH4þ became highly labelled and this N was subsequently

translocated to the roots (Govindarajulu et al. 2005), confirm-

ing earlier work (Tobar et al. 1994; Johansen et al. 1996; Mader

et al. 2000). N is translocated in the hyphae as arginine but

probably broken down to urea and ultimately transferred to

the plant as NH4þ with the resulting C skeletons from arginine

breakdown being re-incorporated into the fungal C pools

(Bago et al. 2001; Govindarajulu et al. 2005). A plant ammonium

transporter (AMT) has recently been identified in Lotus japo-

nicus which is mycorrhiza-specific and preferentially

expressed in arbusculated cells (Guether et al. 2009a, b), and

up-regulation of an ammonium transporter in Medicago trun-

catula has also been found (Gomez et al. 2009). Moreover, Leigh

et al. (2009) demonstrated that a fifth of plant N could be

derived from AM fungal transfer when only the fungus had

access to a compartment containing an organic N source.

However, the role played by AM fungi in N acquisition from

organic N sources, the dominant form of N in most soils,

remains controversial (but see Whiteside et al. 2009), espe-

cially when both roots and AM hyphae have access to the

same N source (Hodge et al. 2000a; Hodge 2003a).

Plant–fungus reciprocity: who drives whom?

These fluxes underlie the operation of the symbiosis, but we do

not know how the exchange is managed. Is there some

reciprocity between the C supplied by the plant and the P (or N)

Nutritional ecology of AM fungi 269

supplied by the fungus? If so, there will have been powerful

selection on the operation of this mechanism, with potentially

conflicting pressures on the two partners. There are apparently

1 000 times more species of plant involved in the AM symbiosis

than of fungi (the ratio of described species is w2� 105:2� 102).

Even if the number of fungal species is a serious under-

estimate, most fungi must be able to colonise many plant

species, a conclusion supported by the lack of specificity of all

fungi known in culture. Nevertheless there is great variation in

the effectiveness of particular pairings of plant and fungal taxa

(van der Heijden et al. 1998; Pringle & Bever 2002; Klironomos

2003; Helgason et al. 2007), suggesting that some fungi are

better partners than others. If that is the case, plants will have

been under powerful selection to discriminate among fungal

partners on the basis of symbiotic effectiveness. One possible

mechanism for that would be a set of molecular signals specific

to each fungus. The establishment of the symbiosis involves

just such a recognition process (Akiyama & Hayashi 2006), but

it appears to be a general one, that has been exploited by

symbioses that evolved subsequently (root nodules with

Rhizobium and the parasitic plant Striga). However, a recogni-

tion-based system depends on honest signals and is very

vulnerable to cheats – taxa that copy the signals but offer no

benefits to the partner. The remarkable durability of the

symbiosis – over 400 MYr – suggests that it is resistant to

invasion by such cheats, and an alternative mechanism for the

regulation of these nutrient fluxes provides a more nearly

cheat-proof model.

This model relies on the fundamental idea that the plant

will transfer C to the fungus only in direct response to the

transfer of N or P. Achieving that does not require a direct

linkage of the exchange mechanism, but relies on known

physiology and the spatial pattern of fungal colonisation and

root behaviour. Briefly, the main elements of the model (Fitter

2006; Helgason & Fitter 2009) are:

1. AM fungi colonise roots in discrete patches. If the fungus is

successfully to acquire C from the root, it must generate the

C flux at that scale.

2. Roots can detect heterogeneity of nutrient supply at a fine

scale and respond by local proliferation (Drew 1975).

3. When an AM fungus transfers P (or possibly N) across the

arbuscule membranes, there will be a local increase in

phosphate (or ammonium) concentration in the root. The

plant will not be able to distinguish that from the increase

that results from enhanced epidermal uptake and will

respond by differential transport of hexoses to the site of

increased uptake.

4. Some of those hexoses will leak into the apoplast and be

acquired by the inter-cellular hyphae.

Smith et al. (2009) have suggested the Fitter (2006) model

may be an over-simplification because it does not allow for

‘cheating’ in individual fungal–plant interactions, which is

held to exist because of growth depressions of the host

observed in some AM interactions (Johnson et al. 1997;

Klironomos 2003). However, AMF are multi-functional and

the benefit to the host may not always be nutritional,

expressed in simple culture conditions or obvious (see

Newsham et al. 1995). A strict demonstration of cheating is

extremely hard to achieve, and the mere demonstration of

a growth depression is insufficient, because of potential

unmeasured responses that would be beneficial in realistic

environments. It is notable that AMF may transfer P to the

host in the absence of any growth response (Smith et al. 2009);

the same argument would describe this as the plant cheating

the fungus. The manner in which the majority of mycorrhizal

research is conducted (i.e. a single fungus and a single plant

with the plant generally trying to establish itself plus the

fungal mycelium de novo at the same time) is artificial. Under

natural conditions, the seedling would plug into an estab-

lished AM common mycelial network (CMN): the carbon

burden would therefore initially be less.

Smith et al. (2009) have also pointed out that cortical colo-

nisation is not essential for C transfer. In Paris type colonisa-

tion there is no inter-cellular phase, suggesting that transfer

must be intracellular; similarly, colonisation of the rmc

mutant of tomato occurs only in the epidermis and hypo-

dermis but can still result in the fungus producing extra-

radical mycelium and even spores (Manjarrez et al. 2008). It is

clear therefore, that C transfer cannot occur exclusively at the

cortical inter-cellular interface but instead must occur at

numerous sites. In practice, the precise location of C transfer

is not a key feature of the model: what is required is that P

(or N) release in the arbuscule stimulates a carbon flux to the

colonised areas of the root, a phenomenon that has been

experimentally demonstrated (Javot et al. 2007).

Nutrient acquisition by AM fungi

Perhaps because of their key role in plant P acquisition, AM

fungi have largely been viewed as extensions of the plant root

system. That view ignores the nutritional needs of the fungus

and, importantly, that those nutritional needs may be in

conflict with the plant’s when resource availability is low.

Unlike other mycorrhizal associations, where at least some of

the fungi involved can be grown in pure culture and we can

measure the capability of the fungus acting alone to decom-

pose organic materials and take up the products of decom-

position (Hodge et al. 1995; Read & Perez-Moreno 2003), it is

currently impossible to culture AM fungi in the absence of

a host plant. Despite pleas for a more ‘mycocentric’ approach

(Fitter et al. 2000; Alberton et al. 2005; Southworth et al. 2005;

Alberton & Kuyper 2009), this lacuna has fuelled a view that

what is good for the plant must surely benefit the fungus and

that the fungus operates in such a way as to benefit its host at

all times.

In common with plant roots (Hodge 2009), AM fungi

proliferate hyphae in nutrient-rich patches of organic matter

under both controlled and field conditions (Mosse 1959;

Nicolson 1959; St John et al. 1983; Joner & Jakobsen 1995;

Hodge et al. 2001; Cavagnaro et al. 2005), a behaviour gener-

ally viewed as a foraging response in a heterogeneous envi-

ronment. AMF, however, are not saprotrophic (Smith & Read

2008) and therefore are reliant on saprotrophic microorgan-

isms to decompose organic matter and release inorganic ions

for capture by AM hyphae (but see Whiteside et al. 2009;

Hawkins et al. 2000). Over short time scales, plant roots

compete weakly with microbes for the released resources

270 A. Hodge et al.

(Kaye & Hart 1997; Hodge et al. 2000b) and root proliferation is

more likely to affect inter-plant competition (Hodge 2009).

AMF, however, must also compete with other microbes and

their lack of saprotrophic capability should set them at

a similar disadvantage. However, the presence of AMF in an

organic patch can enhance its decomposition (Hodge et al.

2001; Atul-Nayyar et al. 2009). The mechanism for this

enhancement is unknown, but AMF hyphae may have a direct

influence on other microorganisms in the ‘hyphosphere’

(Toljander et al. 2007). Some of the carbon in the AMF hyphae

may be exuded or secreted, which would encourage bacterial

growth in a similar way to the well studied ‘rhizosphere’ effect

(Ravnskov et al. 1999).

The external AM mycelium phase is the fungal phase

which is in contact with the soil and thus responsible for

nutrient acquisition and transport to the internal mycelium

inside the root before any transfer to the plant occurs. In

return, triacylglycerides can be transported from the internal

to the external mycelium phase to support the glyoxyl cycle

for metabolic activity (Pfeffer et al. 1999; Lammers et al. 2001).

However, despite the obvious importance of the external

mycelium in nutrient acquisition, few fungal transporters

have been characterised. These include a phosphate (Harrison

& van Buuren 1995), an ammonium (Lopez-Pedrosa et al. 2006)

and a putative zinc transporter (Gonzalez-Guerrero et al. 2005).

In addition, an aquaporin (water channel proteins) gene,

GintAQP1, has been discovered in the external mycelium of

Glomus intraradices which showed increased expression in

parts of the AMF mycelium not experiencing osmotic stress

compared to parts that were (Aroca et al. 2009). This suggests

communication between the different parts of the mycelium

subject to the differing external conditions, a behaviour that is

well established in roots (Hodge 2009).

In the case of ammonium a NH4þ transporter gene (Gin-

tAMT1), with high sequence similarity to NH4þ transporters

characterised in other fungi, has been identified in the extra-

radical mycelium of G. intraradices (Lopez-Pedrosa et al. 2006).

GintAMT1 was up-regulated after the addition of low NH4þ

concentrations to the media but down-regulated when higher

NH4þ concentrations were added, suggesting that it is a high-

affinity NH4þ transporter (Lopez-Pedrosa et al. 2006) and that

lower affinity NH4þ transporters have yet to be identified.

A high-affinity phosphate transporter (GvPT ) has also been

cloned from Glomus versiforme (Harrison & van Buuren 1995).

The expression and regulation of GiPT, a homolog of GvPT,

from G. intraradices was regulated in the external mycelium in

response to phosphate concentration of the external envi-

ronment. Further, the phosphate status of the mycorrhizal

root influenced both phosphate uptake and GiPT expression in

the ERM (Maldonado-Mendoza et al. 2001). This suggests the

ERM of AMF can detect and show physiological plasticity in

response to the nutrient status of their environment and host.

Morphological responses of the external mycelium of AMF

in response to nutrient status of their environment have also

been demonstrated (Bago et al. 2004; Leigh et al. 2009) as have

differing substrate colonisation strategies among AMF genera

(Cano & Bago 2005) albeit under rather artificial conditions.

Bago et al. (2004) reported that under low nutrient conditions

the ERM developmental pattern was one that allowed both

exploration and exploitation of the growth medium: runner

hyphae radially extended the fungal colony from which

branched absorbing structures developed at regular intervals

or from which spores developed in older parts. The life-spans

of these hyphal structures also appear to differ (Staddon et al.

2003); thicker hyphae probably live longer and determine the

development of the hyphal network, analogous to plant root

system development. How the external hyphae sense their

environment in order to display the substantial hyphal

proliferation in organic rich patches is unknown, but may

involve the nutrient ions acting as a direct signal as has been

demonstrated for nitrate and root proliferation (Zhang &

Forde 1998). Application of proteomic and candidate target

gene approaches specifically applied to the external mycelium

offer a way forward to further determine the functioning of

this key AM fungal phase in the environment (see Recorbet

et al. 2009; Gamper et al. 2010) but must be carried out under

ecologically meaningful conditions if the link with function is

to be established.

N capture by AM fungi was previously believed to

have little ecological relevance. An important and influential

article by Read (1991) argued that AM associations tend to

dominate in systems where nitrification is favoured and the

main form of inorganic N will consequently be nitrate. As

NO3�, unlike phosphate, is highly mobile in soils and depletion

zones around roots can be measured in centimetres rather

then millimetres, plants should not require AM fungi in order

to enhance capture of NO3�. However, it is now clear that the

external hyphae of AMF take up inorganic N as both NO3� and

NH4þ (Bago et al. 1996; Govindarajulu et al. 2005; Jin et al. 2005)

and organic N as amino acids (Hawkins et al. 2000) and

transfer some – sometimes a large fraction – to the plant. The

ecological significance of this transfer is still uncertain.

NH4þ may be the preferred fungal N source under most

circumstances (Hawkins et al. 2000; Read & Perez-Moreno

2003; but see Azcon et al. 1996). N transfer to the plant may

also be higher when NH4þ, rather than NO3

�, is supplied to the

AMF hyphae (Tanaka & Yano 2005) even though N is likely

transferred from the external to the internal AM hyphae as

arginine (Govindarajulu et al. 2005; Cruz et al. 2007) before

transfer to the plant as NH4þ (Gomez et al. 2009; Guether et al.

2009a). NH4þ uptake may be less energetically expensive for

the fungus as NO3� first has to be reduced to NH4

þ prior to

incorporation into amino acids. While these N sources must

be important for fungal nutrition, the contribution to plant

nutrition is controversial (see Read & Perez-Moreno 2003;

Leigh et al. 2009) and may depend on the relative N require-

ments of plant and fungus. In some systems, such as the

acidic organic soils frequent in many tropical areas

(Moyersoen et al. 2001), AMF NH4þ capture and subsequent

transfer to the host plant may be of considerable ecological

significance.

When mycorrhizal roots have access to patches of organic

material in soil the AMF appear not to respond (Hodge 2001,

2003b); hyphal proliferation occurs only when the AMF alone

have access to the patch (Joner & Jakobsen 1995; Hodge et al.

2001). One remarkable result in Hodge et al. (2001) was that

the fungus grew preferentially into a patch of organic matter

rather than towards a new, uncolonised host plant. If the

fungus gains all its carbon from the host, this implies that the

fungus gets an alternative benefit from the patch greater than

Nutritional ecology of AM fungi 271

that it can gain from a new carbon source. In fact there is good

evidence that AMF obtain a growth benefit from organic

matter in soil. Leigh et al. (2009) found increased hyphal

growth in the ‘plant’ compartment (i.e. that in which both

plant and fungus grew) when the fungus was allowed to

explore a second, ‘hyphal’ compartment that contained an

organic matter patch.

Thus, what seems to have been overlooked in the debate

over AM fungal responses to organic matter is the possibility

that it represents a major N source for the fungus itself.

Indeed, there are no studies that directly address the question

of the sources of fungal N under realistic conditions. Those

that show transfer of N by the fungus to the plant may be

revealing a consequence of over-supply of N to the fungus.

The demonstration that the supply of organic N compounds

can elicit a transcriptional response in G. intraradices

(Cappellazzo et al. 2007) adds weight to this suggestion.

Conclusions

An increased acceptance of the independent but interacting

roles of plant and fungus in nutrient transfers allows a new

and necessary focus on the nutritional needs of the fungus

itself. All fungi have a high nitrogen content and hence

potentially a high N demand. Indeed because AMF are among

the most abundant fungi on earth, their role in global N and P

cycles would repay close attention. There is abundant

evidence that AMF can acquire N (and presumably also P)

from decomposing organic material and transfer it to the

plant. The N and P transfer to the plant may, if the model

proposed above is correct, be a consequence of the fungal

demand for nutrients, with both host plant and fungus

evolving transporters to take advantage of localised increases

in nutrients.

However, the mechanisms by which these fungi actively

forage in soil for both N and P remain unclear. What is certain

is that we need to pay much more attention to the biology and

ecology of the extra-radical phase of these fungi if we are to

understand how they operate in soil and the roles that they

play in ecosystems.

r e f e r e n c e s

Alberton O, Kuyper TW, 2009. Ectomycorrhizal fungi associatedwith Pinus sylvestris seedlings respond differently to increasedcarbon and nitrogen availability: implications for ecosystemresponses to global change. Global Change Biology 15: 166–175.

Alberton O, Kuyper TW, Gorissen A, 2005. Taking mycocentrismseriously: mycorrhizal fungal and plant responses to elevatedCO2. New Phytologist 167: 859–868.

Akiyama K, Hayashi H, 2006. Strigolactones: chemical signals forfungal symbionts and parasitic weeds in plant roots. Annals ofBotany 97: 925–931.

Aroca R, Bago A, Sutka M, Paz JA, Cano C, Amodeo G, Ruiz-Lozano JM, 2009. Expression analysis of the first arbuscularmycorrhizal fungi aquaporin described reveals concerted geneexpression between salt-stressed and nonstressed mycelium.Molecular Plant Microbe Interactions 22: 1169–1178.

Atul-Nayyar A, Hamel C, Hanson K, Germida J, 2009. Thearbuscular mycorrhizal symbiosis links N mineralization toplant demand. Mycorrhiza 19: 239–246.

Azcon R, Gomez M, Tobar R, 1996. Physiological and nutritionalresponses by Lactuca sativa L to nitrogen sources andmycorrhizal fungi under drought conditions. Biology andFertility of Soils 22: 156–161.

Bago B, Cano C, Azcon-Aguilar C, Samson J, Coughlan AP, Piche Y,2004. Differential morphogenesis of the extraradicalmycelium of an arbuscular mycorrhizal fungus grownmonoxenically on spatially heterogeneous culture media.Mycologia 96: 452–462.

Bago B, Pfeffer PE, Abubaker J, Jun J, Allen JW, Brouillette J,Douds DD, Lammers PJ, Shachar-Hill Y, 2003. Carbonexport from arbuscular mycorrhizal roots involves thetranslocation of carbohydrate as well as lipid. PlantPhysiology 131: 1496–1507.

Bago B, Pfeffer PE, Shachar-Hill Y, 2001. Could the urea cycle betranslocating nitrogen in the arbuscular mycorrhizalsymbiosis? New Phytologist 149: 4–8.

Bago B, Pfeffer PE, Zipfel W, Lammers P, Shachar-Hill Y, 2002.Tracking metabolism and imaging transport in arbuscularmycorrhizal fungi. Metabolism and transport in AM fungi.Plant and Soil 244: 189–197.

Bago B, Vierheilig H, Piche Y, Azcon-Aguilar C, 1996. Nitratedepletion and pH changes induced by the extraradicalmycelium of the arbuscular mycorrhizal fungus Glomusintraradices grown in monoxenic culture. New Phytologist 133:273–280.

Bidartondo MI, Redecker D, Hijri I, Wiemken A, Bruns TD,Dominguez L, Sersic A, Leake JR, Read DJ, 2002. Epiparasiticplants specialized on arbuscular mycorrhizal fungi. Nature419: 389–392.

Cano C, Bago A, 2005. Competition and substrate colonizationstrategies of three polyxenically grown arbuscularmycorrhizal fungi. Mycologia 97: 1201–1214.

Cappellazzo G, Lanfranco L, Bonfante P, 2007. A limiting source oforganic nitrogen induces specific transcriptional responses inthe extraradical structures of the endomycorrhizal fungusGlomus intraradices. Current Genetics 51: 59–70.

Cavagnaro TR, Smith FA, Smith SE, Jakobsen I, 2005. Functionaldiversity in arbuscular mycorrhizas: exploitation of soilpatches with different phosphate enrichment differs amongfungal species. Plant, Cell and Environment 28: 642–650.

Croll D, Sanders IR, 2009. Recombination in Glomus intraradices,a supposed ancient asexual arbuscular mycorrhizal fungus.BMC Evolutionary Biology 9: 13, doi:10.1186/1471-2148-9-13.

Croll D, Wille L, Gamper HA, Mathimaran N, Lammers PJ,Corradi N, Sanders IR, 2008. Genetic diversity and host plantpreferences revealed by simple sequence repeat andmitochondrial markers in a population of the arbuscularmycorrhizal fungus Glomus intraradices. New Phytologist 178:672–687.

Cruz C, Egsgaard H, Trujillo C, Ambus P, Requena N, Martins-Loucao MA, Jakobsen I, 2007. Enzymatic evidence for the keyrole of arginine in nitrogen translocation by arbuscularmycorrhizal fungi. Plant Physiology 144: 782–792.

Drew MC, 1975. Comparison of the effects of a localized supply ofphosphate, nitrate, ammonium and potassium on the growthof the seminal root system, and the shoot, in barley. NewPhytologist 75: 479–490.

Fitter AH, 2006. What is the link between carbon and phosphorusfluxes in arbuscular mycorrhizas? A null hypothesis forsymbiotic function. New Phytologist 172: 3–6.

Fitter AH, Heinemeyer A, Staddon PL, 2000. The impact ofelevated CO2 and global climate change on arbuscularmycorrhizas: a mycocentric approach. New Phytologist 147:179–187.

272 A. Hodge et al.

Gamper HA, van der Heijden MGA, Kowalchuk GA, 2010.Molecular trait indicators: moving beyond phylogeny inarbuscular mycorrhizal ecology. New Phytologist 185: 67–82.

Gandolfi A, Sanders IR, Rossi V, Menozzi P, 2003. Evidence ofrecombination in putative ancient asexuals. Molecular Biologyand Evolution 20: 754–761.

Gianinazzi-Pearson V, Smith SE, Gianinazzi S, Smith FA, 1991.Enzymatic studies on the metabolism of vesicular-arbuscularmycorrhizas V. Is Hþ�ATPase a component of ATP-hydrolyzing enzyme-activities in plant–fungus interfaces?New Phytologist 117: 61–74.

Gomez SK, Javot H, Deewatthanawong P, Torres-Jerez I,Tang YH, Blancaflor EB, Udvardi MK, Harrison MJ, 2009.Medicago truncatula and Glomus intraradices gene expressionin cortical cells harboring arbuscules in the arbuscularmycorrhizal symbiosis. BMC Plant Biology 9, doi:10.1186/1471-2229-9-10.

Gonzalez-Guerrero M, Azcon-Aguilar C, Mooney M, Valderas A,MacDiarmid CW, Eide DJ, Ferrol N, 2005. Characterization ofa Glomus intraradices gene encoding a putative Zn transporterof the cation diffusion facilitator family. Fungal Genetics andBiology 42: 130–140.

Govindarajulu M, Pfeffer PE, Jin H, Abubaker J, Douds DD,Allen JW, Bucking H, Lammers PJ, Shachar-Hill Y, 2005.Nitrogen transfer in the arbuscular mycorrhizal symbiosis.Nature 435: 819–823.

Guether M, Neuhauser B, Balestrini R, Dynowski M, Ludewig U,Bonfante P, 2009a. A mycorrhizal-specific ammoniumtransporter from Lotus japonicus acquires nitrogen released byarbuscular mycorrhizal fungi. Plant Physiology 150: 73–83.

Guether M, Balestrini R, Hannah M, He J, Udvardi MK, Bonfante P,2009b. Genome-wide reprogramming of regulatory networks,transport, cell wall and membrane biogenesis duringarbuscular mycorrhizal symbiosis in Lotus japonicus. NewPhytologist 182: 200–212.

Harley JL, 1989. The significance of mycorrhiza. MycologicalResearch 92: 129–139.

Harrison MJ, 1999. Molecular and cellular aspects of thearbuscular mycorrhizal symbiosis. Annual Review of PlantPhysiology and Plant Molecular Biology 50: 361–389.

Harrison MJ, van Buuren ML, 1995. A phosphate transporter fromthe mycorrhizal fungus Glomus versiforme. Nature 378: 626–629.

Hawkins HJ, Johansen A, George E, 2000. Uptake and transport oforganic and inorganic nitrogen by arbuscular mycorrhizalfungi. Plant and Soil 226: 275–285.

Helgason T, Fitter AH, 2009. Natural selection and theevolutionary ecology of the arbuscular mycorrhizal fungi(Phylum Glomeromycota). Journal of Experimental Botany 60:2465–2480.

Helgason T, Merryweather JW, Young JPW, Fitter AH, 2007.Specificity and resilience in the arbuscular mycorrhizalfungi of a natural woodland community. Journal of Ecology95: 623–630.

Hodge A, 2001. Arbuscular mycorrhizal fungi influencedecomposition of, but not plant nutrient capture from, glycinepatches in soil. New Phytologist 151: 725–734.

Hodge A, 2003a. N capture by Plantago lanceolata and Brassica napusfrom organic material – the influence of spatial dispersion,plant competition and an arbuscular mycorrhizal fungus.Journal of Experimental Botany 54: 2331–2342.

Hodge A, 2003b. Plant nitrogen capture from organic matter asaffected by spatial dispersion, interspecific competition andmycorrhizal colonisation. New Phytologist 157: 303–314.

Hodge A, 2009. Root decisions. Plant, Cell and Environment 32:628–640.

Hodge A, Alexander IJ, Gooday GW, 1995. Chitinolytic enzymesof pathogenic and ectomycorrhizal fungi. Mycological Research99: 935–941.

Hodge A, Campbell CD, Fitter AH, 2001. An arbuscularmycorrhizal fungus accelerates decomposition and acquiresnitrogen directly from organic material. Nature 413: 297–299.

Hodge A, Robinson D, Fitter AH, 2000a. An arbuscular mycorrhizalinoculum enhances root proliferation in, but not nitrogencapture from, nutrient-rich patches in soil. New Phytologist145: 575–584.

Hodge A, Robinson D, Fitter AH, 2000b. Are microorganisms moreeffective than plants at competing for nitrogen? Trends in PlantScience 5: 304–308.

Javot H, Penmetsa RV, Terzaghi N, Cook DR, Harrison MJ, 2007.A Medicago truncatula phosphate transporter indispensablefor the arbuscular mycorrhizal symbiosis. Proceedings ofthe National Academy of Sciences of the United States of America104: 1720–1725.

Jin H, Pfeffer PE, Douds DD, Piotrowski E, Lammers PJ, Shachar-Hill Y, 2005. The uptake, metabolism, transport and transfer ofnitrogen in an arbuscular mycorrhizal symbiosis. NewPhytologist 168: 687–696.

Johansen A, Finlay RD, Olsson PA, 1996. Nitrogen metabolism ofexternal hyphae of the arbuscular mycorrhizal fungus Glomusintraradices. New Phytologist 133: 705–712.

Johnson NC, Graham JH, Smith FA, 1997. Functioning ofmycorrhizal associations along the mutualism–parasitismcontinuum. New Phytologist 135: 575–586.

Joner EJ, Jakobsen I, 1995. Growth and extracellular phosphataseactivity of arbuscular mycorrhizal hyphae as influenced bysoil organic matter. Soil Biology and Biochemistry 27: 1153–1159.

Karandashov V, Nagy R, Wegmuller S, Amrhein N, Bucher M,2004. Evolutionary conservation of a phosphate transporter inthe arbuscular mycorrhizal symbiosis. Proceedings of theNational Academy of Sciences of the United States of America 101:6285–6290.

Kaye JP, Hart SC, 1997. Competition for nitrogen between plantsand soil microorganisms. Trends in Ecology and Evolution 12:139–143.

Klironomos JN, 2003. Variation in plant response to native andexotic arbuscular mycorrhizal fungi. Ecology 84: 2292–2301.

Koch AM, Kuhn G, Fontanillas P, Fumagalli L, Goudet J,Sanders IR, 2004. High genetic variability and low localdiversity in a population of arbuscular mycorrhizal fungi.Proceedings of the National Academy of Sciences of the United Statesof America 101: 2369–2374.

Lammers PJ, Jun J, Abubaker J, Arreola R, Gopalan A, Bago B,Hernandez-Sebastia C, Allen JW, Douds DD, Pfeffer PE,Shachar-Hill Y, 2001. The glyoxylate cycle in an arbuscularmycorrhizal fungus. Carbon flux and gene expression. PlantPhysiology 127: 1287–1298.

Leigh J, Hodge A, Fitter AH, 2009. Arbuscular mycorrhizal fungican transfer substantial amounts of nitrogen to their hostplant from organic material. New Phytologist 181: 199–207.

Lopez-Pedrosa A, Gonzalez-Guerrero M, Valderas A, Azcon-Aguilar C, Ferrol N, 2006. GintAMT1 encodes a functional high-affinity ammonium transporter that is expressed in theextraradical mycelium of Glomus intraradices. Fungal Geneticsand Biology 43: 102–110.

Mader P, Vierheilig H, Streitwolf-Engel R, Boller T, Frey B,Christie P, Wiemken A, 2000. Transport of 15N from a soilcompartment separated by a polytetrafluoroethylenemembrane to plant roots via the hyphae of arbuscularmycorrhizal fungi. New Phytologist 146: 155–161.

Maldonado-Mendoza IE, Dewbre GR, Harrison MJ, 2001. Aphosphate transporter gene from the extra-radical myceliumof an arbuscular mycorrhizal fungus Glomus intraradices isregulated in response to phosphate in the environment.Molecular Plant-Microbe Interactions 14: 1140–1148.

Manjarrez M, Smith FA, Marschner P, Smith SE, 2008. Is corticalroot colonization required for carbon transfer to arbuscular

Nutritional ecology of AM fungi 273

mycorrhizal fungi? Evidence from colonization phenotypesand spore production in the reduced mycorrhizal colonization(rmc) mutant of tomato. Botany 86: 1009–1019.

Martin F, Gianinazzi-Pearson V, Hijri M, Lammers P, Requena N,Sanders IR, Shachar-Hill Y, Shapiro H, Tuskan GA, Young JPW,2008. The long hard road to a completed Glomus intraradicesgenome. New Phytologist 180: 747–750.

Mosse B, 1959. Observations on the extramatrical mycelium ofa vesicular-arbuscular endophyte. Transactions of the BritishMycological Society 42: 439–448.

Moyersoen B, Becker P, Alexander IJ, 2001. Are ectomycorrhizasmore abundant than arbuscular mycorrhizas in tropical heathforests? New Phytologist 150: 591–599.

Newsham KK, Fitter AH, Watkinson AR, 1995. Multi-functionalityand biodiversity in arbuscular mycorrhizas. Trends in Ecologyand Evolution 10: 407–411.

Nicolson TH, 1959. Mycorrhiza in the Gramineae. I. Vesicular-arbuscular endophytes, with special reference to the externalphase. Transactions of the British Mycological Society 42: 421–438.

Ohtomo R, Saito M, 2005. Polyphosphate dynamics in mycorrhizalroots during colonization of an arbuscular mycorrhizalfungus. New Phytologist 167: 571–578.

Pfeffer PE, Douds DD, Becard G, Shachar-Hill Y, 1999. Carbonuptake and the metabolism and transport of lipids in anarbuscular mycorrhiza. Plant Physiology 120: 587–598.

Pringle A, Bever JD, 2002. Divergent phenologies may facilitate thecoexistence of arbuscular mycorrhizal fungi in a NorthCarolina grassland. American Journal of Botany 89: 1439–1446.

Rausch C, Daram P, Brunner S, Jansa J, Laloi M, Leggewie G,Amrhein N, Bucher M, 2001. A phosphate transporterexpressed in arbuscule-containing cells in potato. Nature 414:462–466.

Ravnskov S, Larsen J, Olsson PA, Jakobsen I, 1999. Effects ofvarious organic compounds on growth and phosphorusuptake of an arbuscular mycorrhizal fungus. New Phytologist141: 517–524.

Read DJ, 1991. Mycorrhizas in ecosystems. Experientia 47: 376–391.Read DJ, Perez-Moreno J, 2003. Mycorrhizas and nutrient cycling

in ecosystems – a journey towards relevance? New Phytologist157: 475–492.

Recorbet G, Rogniaux H, Gianinazzi-Pearson V, Dumas-Gaudot E,2009. Fungal proteins in the extra-radical phase of arbuscularmycorrhiza: a shotgun proteomic picture. New Phytologist 181:248–260.

Redecker D, Kodner R, Graham LE, 2000. Glomalean fungi fromthe Ordovician. Science 289: 1920–1921.

Remy W, Taylor TN, Haas H, Kerp H, 1994. Four hundred-million-year-old vesicular-arbuscular mycorrhizae. Proceedings of theNational Academy of Sciences of the United States of America 91:11841–11843.

Robinson D, Fitter A, 1999. The magnitude and control of carbontransfer between plants linked by a common mycorrhizalnetwork. Journal of Experimental Botany 50: 9–13.

Schubler A, Martin H, Cohen D, Fitz M, Wipf D, 2006.Characterization of a carbohydrate transporter fromsymbiotic glomeromycotan fungi. Nature 444: 933–936.

Smith SE, Read DJ, 2008. Mycorrhizal Symbiosis, 3rd edn. AcademicPress Ltd, London, UK.

Smith SE, Smith FA, Jakobsen I, 2004. Functional diversity inarbuscular mycorrhizal (AM) symbioses: the contribution ofthe mycorrhizal P uptake pathway is not correlated withmycorrhizal responses in growth or total P uptake. NewPhytologist 162: 511–524.

Smith FA, Grace EJ, Smith SE, 2009. More than a carbon economy:nutrient trade and ecological sustainability in facultativearbuscular mycorrhizal symbioses. New Phytologist 182: 347–358.

Southworth D, He XH, Swenson W, Bledsoe CS, Horwath WR,2005. Application of network theory to potential mycorrhizalnetworks. Mycorrhiza 8: 589–595.

Staddon PL, Ramsey CB, Ostle N, Ineson P, Fitter AH, 2003. Rapidturnover of hyphae of mycorrhizal fungi determined by AMSanalysis of 14C. Science 300: 1138–1140.

St John TV, Coleman DC, Reid CPP, 1983. Association of vesicular-arbuscular mycorrhizal hyphae with soil organic particles.Ecology 64: 957–959.

Tanaka Y, Yano K, 2005. Nitrogen delivery to maize viamycorrhizal hyphae depends on the form of N supplied. Plant,Cell and Environment 28: 1247–1254.

Tobar RM, Azcon R, Barea JM, 1994. The improvement of plant Nacquisition from an ammonium-treated, drought stressed soilby the fungal symbiont in arbuscular mycorrhizae. Mycorrhiza4: 105–108.

Toljander JF, Lindahl BD, Paul LR, Elfstrand M, Finlay RD, 2007.Influence of arbuscular mycorrhizal mycelial exudates on soilbacterial growth and community structure. FEMS MicrobiologyEcology 61: 295–304.

van der Heijden MGA, Klironomos JN, Ursic M, Moutoglis P,Streitwolf-Engel R, Boller T, Wiemken A, Sanders IR, 1998.Mycorrhizal fungal diversity determines plant biodiversity,ecosystem variability and productivity. Nature 396: 69–72.

Voets L, Goubau I, Olsson PA, Merckx R, Declerck S, 2008. Absenceof carbon transfer between Medicago truncatula plants linkedby a mycorrhizal network, demonstrated in an experimentalmicrocosm. FEMS Microbiology Ecology 65: 350–360.

Whiteside MD, Treseder KK, Atsatt P, 2009. The brighter side ofsoils: quantum dots track organic nitrogen through fungi andplants. Ecology 90: 100–108.

Zhang HM, Forde BG, 1998. An Arabidopsis MADS box gene thatcontrols nutrient-induced changes in root architecture. Science279: 407–409.