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Keywords:

  • apoplast;
  • arbuscule;
  • cheating;
  • hexose transporters;
  • root proliferation

Arbuscular mycorrhizal (AM) fungi (Phylum: Glomeromycota) are obligate symbionts that obtain their carbon nutrition entirely – as far as is known – from a host plant. The key functional benefit to the plant is a supply of phosphate, a nutrient for which the dominant available forms in soil (orthophosphate ions) are very poorly mobile because of the abundance of cations such as Ca2+, Fe3+ and Al3+ (Tinker & Nye, 2000). Although other functional interactions are known, including defence against pathogens and improved water relations (Newsham et al., 1995a), these may have evolved more recently. It is likely that phosphate uptake was the original selective advantage offered, and that the symbiosis evolved contemporaneously with the land plant flora before the evolution of roots, at a time when acquisition of poorly mobile phosphate ions from soil must have been a major challenge to plants. The evolution of the mycorrhiza may have been a critical stage in the evolution of the land flora (Pirozynski & Malloch, 1975; Brundrett, 2002).

The diagnostic feature of the symbiosis is the arbuscule, a haustorium that penetrates root cortical cells and invaginates the plasma membrane, creating a large surface area for exchange. There is good evidence that the arbuscule is the site of phosphate transfer from fungus to plant, and rates of plant P uptake can be correlated with the development of arbuscules in the root system. A plant phosphate transporter (MtPt4) is expressed specifically on the peri-arbuscular membrane in Medicago truncatula (Harrison et al., 2002), and is assumed to be responsible for plant capture of phosphate from the fungus. Similar transporters have been identified in other species (Glassop et al., 2005; Nagy et al., 2005).

In contrast, the mechanism and location of the reciprocal transfer of C from plant to fungus remain unclear. It is likely that fixed C moves as hexose, probably principally glucose and fructose (Pfeffer et al., 1999). The arbuscule could be the site of both hexose and phosphate transfer, and a model for how this might work has been proposed (Blee & Anderson, 1998). However, there is as yet no evidence for a fungal hexose transporter expressed on the arbuscular membrane (Smith et al., 2001). An alternative explanation is that the intercellular fungal hyphae that grow between and among the cortical cells are the principal organs of fungal hexose uptake (Smith et al., 2001). If this is true, a simple model of phosphate and sugar movement can explain many features of the symbiosis.

Arbuscular mycorrhizal fungi typically colonize roots as defined and restricted patches, ranging from a few tenths of a millimetre to as much as a centimetre of cortex (Cox & Tinker, 1976), and surrounded by other colonization units that may belong to other mycelia. An effective mechanism for symbiotic function must therefore stimulate the plant to deliver sugars to these localized patches within the root. There are two possible mechanisms: either the plant can recognize the presence of the fungus; or it can detect the increased phosphate supply created by the fungus. The former is an evolutionarily unstable mechanism as it is open to ‘cheat’ fungi that can mimic the signals of a beneficial symbiont without delivering any benefit, whereas the latter ensures that sugar will only be exchanged for phosphate.

Recognizing locally increased P supply is something that many plants can do, as they respond to it by the local proliferation of roots (Drew, 1975; Williamson et al., 2001). Root proliferation – expressed either as an increase of initiation or elongation of laterals, or both – is a tightly regulated response, as root growth is inhibited either side of zones of stimulation (Linkohr et al., 2002). How plants detect locally enhanced phosphate acquisition is unknown, but by analogy with the response to nitrate, where the nitrate ion itself appears to be the signal molecule (Zhang & Forde, 1998), it is likely to be based on the phosphate ion.

We can therefore postulate that when an AM fungus develops an arbuscule inside a root cell and begins to move phosphate ions across the peri-arbuscular membrane, the plant will detect this increased P flux, which will stimulate increased C allocation to a localized region around the arbuscule; such fine-scale pattern in C flux may be detectable by labelling experiments. The challenge for the fungus is now to acquire the extra C, rather than it being used for new root growth. Root cortical cell membranes are leaky to sugars, which are abundant components of exudates to the rhizosphere (Jaeger et al., 1999). The increased sugar supply will therefore increase the sugar concentration in the apoplast. Although root cells do reabsorb sugars that move into the apoplast, all that is now required is that the fungus has a hexose transporter with a greater capacity for hexose acquisition than the plant, ensuring a one-way valve.

A threat to the fungus would occur if the plant were to initiate new lateral roots at the sites of colonization: these would use sugars and disrupt fungal growth in the cortex. Mycorrhizal colonization can suppress lateral development (Fitter, 1977), consistent with this model. The recent report (Olah et al., 2005) that a diffusible signal from the extraradical mycelium of an AM fungus stimulates lateral formation in M. truncatula is not inconsistent with the idea that the intraradical mycelium will suppress lateral growth, as new colonization may be favoured by the development of new laterals, even if growth of existing colonization units is not.

The mechanism proposed here is that C–P exchange in the arbuscular mycorrhiza is regulated by the transport of phosphate across the arbuscule interface, directly stimulating the supply of C by the plant to a spatially defined location in the root, with the fungus capturing hexoses that consequently leak into the apoplast. This mechanism would be resistant to invasion by cheat fungi: if they fail to supply phosphate through the development of arbuscules, they will not stimulate an increased sugar supply. Such fungi could still colonize roots, but would have to scavenge for sugars at the normal and typically low concentrations in the apoplast. In contrast, a mechanism based on recognition processes between plant and fungus within the root (for example at the peri-arbuscular membrane) would be highly susceptible to invasion by cheats, as it would always be possible for a fungus to possess the recognition signals but to offer no benefit. There will have been strong selection pressure for such a recognition mechanism to be active before colonization of the root (Gianinazzi-Pearson & Brechenmacher, 2004; Akiyama et al., 2005).

Numerous plant mutants (and indeed species) are known in which the fungus fails to develop arbuscules and therefore fails to deliver phosphate to the root. This outcome could be explained by the plant regulating fungal growth where there is no benefit, but is as likely to be caused by the fungus aborting the development of colonization where there is no reciprocal response by the plant in the form of an increased sugar supply. Some of these mutants may therefore have a restricted root-proliferation response to phosphate.

A mycorrhizal symbiosis can supply the entire P uptake of a colonized plant, even when there is no growth stimulation and when root development and P availability suggest that the plant would be capable of acquiring substantial amounts of phosphate directly across root cell membranes (Smith et al., 2003). This finding supports the model proposed here, as a successful fungus must continue to provide the plant with P through the arbuscules, in order to maintain a reciprocal C flux. However, the variation in P inflow to plants colonized by different AM fungi, shown by Smith et al. (2003) and by Jakobsen's group (e.g. Munkvold et al., 2004), raises an important question: if C acquisition is determined by P supply, why would fungi vary in the amount of P they supply? The question cannot be answered definitively, but a likely explanation is that fungi (like all organisms) vary in growth strategy and therefore in their demand for C.

This model explains a number of other features of AM symbioses, notably the controversy over C transport from plant to plant via fungi. Although it has been suggested that C may move in the reverse of the normal direction (that is, from fungus to plant), the only firm evidence for this phenomenon in the AM symbiosis is in achlorophyllous plants for which the fungus is the sole source of C (Leake, 1994; Bidartondo, 2005). An explanation for the reversal of C flux in these systems might be that the plant cells have such low sugar concentrations that the flux of hexose from cortex to apoplast is less than from fungus to apoplast, and that the plants have evolved a transport system with an even greater hexose acquisition capacity than the fungus. There are a few other cases where C movement has been postulated from fungus to a green plant, usually where the plant was in deep shade or experiencing intense C demand. In only one case involving an AM fungus has a substantial flux been quantified (Lerat et al., 2002); the plants were tree seedlings rapidly expanding their leaves in spring, and a bulbous plant with a large C store in the bulb. Typically, although C can move from root system to root system through the mycelium, it remains in fungal tissues and is not transferred to the plant (Fitter et al., 1998; Pfeffer et al., 2004).

Another striking feature of AM symbioses is the common existence of multiple colonization of a single root system by a number of different fungi, often closely co-located in the root (Abbott, 1982; Clapp et al., 1995; Merryweather & Fitter, 1995). Multiple colonization might occur frequently if the recognition systems for colonization were generic, and if fungal success within the root varied with their ability to supply phosphate to the plant. Although there could potentially be competition for colonization sites within the roots, selection pressures would act principally on the external mycelium that exists in the highly variable environment of soil. We can predict, therefore, that AM fungi will show substantial interspecific variation in their response to soil factors. We already know that they vary in response to pH (Wang et al., 1993) and disturbance (Helgason et al., 1998); it is likely that many more ecological differences of this sort await discovery.

This model suggests that cheats, fungi that gain C from their host without supplying benefits in return, will be rare. Although some plants – notably achlorophyllous mycoheterotrophs – apparently cheat their fungal partner (Bidartondo, 2005), the evidence for fungal cheats is less clear. Depression of plant growth in experimentally synthesized AM symbioses, especially when the partners do form mycorrhizas in nature (Klironomos, 2003), is often taken to suggest the existence of fungal cheats. However, no experimental design can explore all the possible conditions under which the symbiosis might be mutualistic. Newsham et al. (1995b) found that the Vulpia ciliata ssp. ambigua /Glomus BEG6 mycorrhiza did not promote P acquisition by the plant. Instead, protection from a pathogen was the real plant benefit, but to demonstrate that required an explicit test in the presence of a specific pathogen. The number of potential experimental conditions required to eliminate the possibility that there is some benefit, under some set of environmental conditions, at some stage in the life cycle, is legion.

Cheat fungi might still persist under this model. As already noted, colonization units of distinct fungi may be closely co-located in roots, so stimulation of C supply by one fungus to a region of the root might allow another fungus to benefit from improved sugar supply. We can predict, therefore, that colonization of the root by one fungus may stimulate colonization by another. In a model where the symbiosis is controlled internally by a recognition process between plant and fungus, such behaviour would be hard to explain.

Finally, Mosse (1973) was the first of many to demonstrate that plants can reject new colonization by AM fungi if they have plentiful phosphate, but that they do not eliminate existing colonization units. Similarly, existing colonization can suppress new colonization in the other half of a split-root system (Catford et al., 2003). That outcome is what would be predicted from the current model: as long as the fungus continues to supply phosphate, even if in excess of plant need, it should stimulate sugar supply.

In this simple model for symbiotic function in arbuscular mycorrhizas, the fungus supplies phosphate to the host across the arbuscular interface, and in so doing stimulates a response from the plant by which sugars are transported to the region around the arbuscule. The mechanism involves no unknown biology; explains many patterns of behaviour in the symbiosis; and is evolutionarily stable in being resistant to invasion by cheat fungi. In addition, it offers a number of clear predictions about the biology of the symbiosis, including that:

  • • 
    all AM fungi have the capacity to transport P across the arbuscular interface;
  • • 
    C uptake occurs from the cortical apoplast;
  • • 
    plants respond to colonization by localized C transport, as they do to local nutrient enrichment;
  • • 
    AM fungi have hexose transporters with a greater capacity to acquire sugars from the apoplast than those on the plant cell membranes;
  • • 
    colonization of the root by one fungus should locally promote colonization by others;
  • • 
    late stage nonmycorrhizal mutants may be unable to proliferate roots in response to a localized P supply;
  • • 
    there will be greater differentiation among AM fungi in traits of the extraradical mycelium than of the mycelium inside the root.

Acknowledgements

  1. Top of page
  2. Acknowledgements
  3. References

I am grateful to the organizers of, and several participants in, the 2006 Monte Verita Conference on Mycorrhizas, where I was able to test these ideas on various members of a captive audience, and to Maria Harrison for valuable information and insight. Three referees offered helpful comments.

References

  1. Top of page
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