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Function of Ectomycorrhizas

Introduction

In contrast to arbuscular mycorrhizas, ectomycorrhizal fungi are actively associated with enzymic digestion of organic materials thereby releasing minerals. These organic forms of N and P are then absorbed by hyphae and delivered via the mycelium to the root. Like arbuscular mycorrhizas, the minerals also move about the mycelium interconnecting different plants. Ectomycorrhizas also have a storage function. The extensive mycelium and ectomycorrhizal tissue can function as a store of organic and mineral nutrients. The nutrients can be mobilised for further growth of fungi or plant. By forming a sheath around the root, ectomycorrhizal fungi regulate movement of minerals taken into the mycorrhizal root. The potential for control of nutrient influx has ramifications for both symbionts. Finally, the ectomycorrhizal fungus gains most of its organic energy from the host. Reliance of the fungus on its host(s) for energy detemines survival of the fungus, especially under conditions where the host may have reduced rates of photosynthesis.

Mineral Uptake

In axenic culture, ectomycorrhizal fungi have been found to access a wide variety of organic and inorganic forms of N and P. Most ectomycorrhizas are located in the organic layer above the mineral soil in forests. The form of N and P most commonly found in these layers is organic. As the mineral soil is likely to be depleted in both N and P, the most important source of N and P is the litter layer.

Many isolates of ectomycorrhizal fungi excrete enzymes capable of digesting organic matter. While many saprotrophic fungi have cellulolytic activity in culture, ectomycorrhizal fungi can compete effectively with saprotrophs because they are not reliant on exogenous sources of energy for initiation of their metabolic activity. Ectomycorrhizal fungi also have extensive and organised mycelia that effectively detect and degrade concentrated deposits of energy and minerals, and transport the resources to storage sites in the mycelium or ectomycorrhizal roots. In other words, by having an energy supply, ectomycorrhizal fungi rapidly degrade resources, transporting the products to storage,thus effectively competing with saprotrophic fungi and other microbes.

Many experiments have effectively demonstrated organic resources added to non-sterile soil are rapidly colonised by ectomycorrhizal fungi. Enzymes involved with the degradation process probably include proteinases, responsible for release organic N usually as amino acids or small polypeptides from protein. Phosphate becomes available following the degradation of litter components by fungal phosphatases. Phytate and various other forms of organic P are degraded and phosphates absorbed. Not all organic sources of N and P are available to all ectomycorrhizal fungi. For instance fungi appear unable to access nitrogen contained within lignin/polyphenolic complexes. The degree of mobilisation of minerals in litter remains to be clarified. Further, the succession of fungi colonising a root system over time will influence the nature of nutrient mobilisation from organic resources, and our understanding of succession remains largely speculative. The dynamics of nutrient uptake from organic sources are complex in space and time.

Mineral N and P can also be taken up by mycorrhizal fungi. Most experimental examinations of these phenomena have resulted in variable data. Some P, for instance, may become available following the acidification of the rhizosphere or mycorrhizosphere releasing P from aluminium and iron complexes. Some fungi can increase plant uptake of mineral N and P, and the effect on plant growth may be marked. However, different isolates of one species may give contradictory results. The variation is associated with differences in the host, fungus and environment.

Fungi differ in their requirement for ammonium and nitrate. In part, the preference is associated with soil pH. Ammonium is more common in acid soils, and fungi found in acid soils tend to take up ammonium preferentially. Similarly, nitrate is the preferred substrate of fungi from soils with a neutral to alkaline pH.

Finally, mineral uptake by fungi takes place in a complex microbial community. The microbes are fed by various sources of energy, and each provides a different suite of activities to the complex environment. Ectomycorrhizal fungi are only one part of the complex.

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Control of Mineral Uptake

ecto
TS of an ectomycorrhiza.

The mantle of an ectomycorrhiza is differentiated. The outer layer tends to be more open than the layer immediately above the root surface. Hyphae excrete materials that fill the interstitial spaces. The nature of the materials is still unclear, but probably includes hydrophobins. LINK Hydrophobins can function as agents that bind adjacent hyphae. They also repel water from the hyphal surface. If the mantle is present as a continuous layer surrounding the root, then hydrophobins would explain the observation of an apoplasmic barrier to movement of solutes through an ectomycorrhiza to the root interior.

The barrier directs solutes into the symplast of the fungi. While the mantle is not sealed, the movement of solutes into the ectomycorrhiza has a step controlled by the fungus. As most plant roots also have an endodermis with differentiated passage cells, the cortex becomes an exchange region. In the exchange region, the fungus regulates inward movement of water and minerals, and the plant regulates the loss of organic compounds.

The barrier also forces movement of organic molecules through the fungus from the root. Passage through the fungus would enable the fungus to extract from the stream of nutrients all that might be used, converted or stored by the fungus. While the concept of plant and fungal control of solute movement, called exchange region, is controversial, the concept explains why the fungal partner can influence nutrient exchange with the host plant.

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Energy

Like all fungi, ectomycorrhizal fungi require an exogenous supply of energy for maintenance and growth. The fungi constitute up to 40% of the dry weight of roots. In addition, up to an estimated 80 cm hyphae per cm ectomycorrhiza is found in soil. The host plant supplies the vast majority of the energy required for this mass.

In broad terms, sucrose is delivered to the plant-fungal interface, where an invertase converts it to hexose. Glucose, actively taken up by the fungus, is immediately converted to trehalose, mannitol and glycogen within the fungus thus maintaining the density gradient.

Up to 30% of photosynthates may be transferred to the fungal symbiont. The loss of energy has several potential effects on the host. Initially, the rate of photosynthesis is stimulated, increasing the pool of energy available for distribution. Increased photosynthesis is possible in a light saturated tissue. Below the compensation point, the drain of energy to the fungus may reduce the rate of growth of the plant, and the plant may reduce allocation of energy to the fungal partner. The latter response results in reduced colonisation of roots. This scenario is complicated by the capacity of ectomycorrhizal fungi to store energy. Energy stores may be used during periods of low PAR.

Seedlings growing in deep shade may require ectomycorrhiza for uptake of minerals. However, they have insufficient colonisation to obtain adequate supply of minerals. Despite these difficulties, seedlings of some forest trees appear to continue to survive and grow in deep shade. The concept of a pool of energy in the interconnecting mycelium is explored below.

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Movement of Nutrients in the Fungus

Following acquisition of carbon from the host, carbohydrates move to the mantle. Some carbon is then immediately transferred to the mycelium. Several patterns appear in the research of carbon movement. Young ectomycorrhizas appear to be more active in their acquisition and respiration of carbon than older structures. In addition, movement of carbon from the ectomycorrhiza differs with season.

In forests of the northern hemisphere, more carbon is allocated below ground in autumn than spring. This allocation coincides with formation of fungal fruit bodies and a seasonal peak in plant uptake of phosphate from soil.

Though field research for Australian conditions is unavailable, export of carbon from the plant to support formation of fruiting bodies would be expected. As fruiting patterns are similar in southern Australia, the carbon cost to the host is likely to be maximised in autumn.

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Transfer of Nutrients Between Plants

One consequence of the extensive mycelium surrounding mycorrhizal plants is that more than one plant and plant species is likely to be associated with the mycelium of an individual fungus. Thus adjacent plants may be connected below ground. Isotope studies have concluded that carbon, nitrogen and phosphorus can move through these connections. However, in only a few cases have the calculations indicated a net transfer and even fewer demonstrate a significant growth effect on one or more of the interconnected plants.

Organic N may enter the mycelium from diverse sources. Curiously, N fixing plants appear to remove more organic N from the mycelium than non-fixing plants.

Tranfer of minerals and energy through the mycelium has been suggested to have importance for the survival of seedlings and achlorophyllous plants. Seedlings have a small demand for carbon that could readily be met from an extensive mycelium. Their growth in shade would benefit from the supply of carbon to the shared mycelium from plants with the leaves in full light. However, laboratory experiments indicate the seedlings would also have a small connection to the network because of the physiological response of the plant to carbon “demand” of the symbiont. The importance of interconnections remains to be explored thoroughly in the field.

mycohet
Carbon moves from tree to underground plant connected by fungi.

Survival of achlorophyllous plants depends on the supply of carbon from the shared mycelium. Heterotrophic plants in many different families, especially orchids, are attached to ectomycorrhizal fungi, which in turn are attached to photosynthetic hosts. While only a few associations have been synthesised and studied in controlled conditions, it is clear significant quantities of carbon are transferred to the heterotrophic plant from the mycelium. The mechamisms that enable the parasitic plant to remove energy from the fungal mycelium remain unclear.

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Conclusion

The view developed in this section is that the fungi in ectomycorrhiza are far from being passive extensions of the root system. Many fungi are found on the roots of one plant. Some of these fungi mobilise nitrogen and phosphorus from organic sources and transport the mineral ions to the host plant. Plants and fungi also take up inorganic nitrogen and phosphate from available soil pools that the fungi may influence. Further, ectomycorrhiza regulate the delivery of solutes to the plant as a consequence of the exchange region, and due to the extensive interconnections with a diversity of hosts. These characteristics indicate a very tight integration of function between the symbionts, and also the possibility of sharing resources within a plant fungus community. The details of this integration are still unclear, even in the most closely studied plant/fungus combinations. Presumably, the diverse fungi of ectomycorrhiza will show a wide variety of interactions, but these variations remain to be confirmed.

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References

Allen M.F. (Ed) (1992) Mycorrhizal Functioning. Chapman Hall, New York.

Lindahl BD et al (2005) in Dighton J, White JF & Oudemans P (Eds) The Fungal Community 3rd Edition, Taylor and Francis.

Smith SE & Read D.J. (2008) Mycorrhizal Symbiosis. Academic press, London.

Pine in Australia

Plantations of the softwood tree, Pinus radiata, have been established in much of southern Australia. The fast growing tree is particularly suited to the conditions and is widely planted, even though it has a severely restricted distribution in its native habitat in California. Indeed, Pinus radiata is becoming a weed and requires very careful management outside plantations.
It has not always been easy to establish. Initial attempts in both Africa and Australia failed: establishment required an understanding of the interactions with ectomycorrhizal fungi normally associated with the plant. Examine the roots of plantation or nursery seedlings and you will find ectomycorrhiza. Plant seeds in areas where the tree has not established, and likely as not, the seedlings will fail: examine their roots and if ectomycorrhiza have established, only a few root tips will be colonised, and the fungi will not be the usual occupants. The question of plant spread concerns the presenc of appropriate fungal associates.

pine
Pine spreading in recovering woodland.

It is the ectomycorrhizal association that enables and determines the spread of the tree. Seed can disperse in wind hundreds of metres from the source tree. However, only if the ectomycorrhizal fungi normally associated with the tree also disperse will emergent seedlings thrive. If you examine the figure to the right, you will see that the original planting (ectomycorrhizal seedlings) has led to the establishment of a pocket of mixed age trees on the right, and a few single younger trees scattered down the slope. This is a typical pattern. All naturally germinated seedlings of significant size are found with ectomycorrhiza.
Does this information help with controlling the weedy species? If you remove all pine from native bushland then you will not have local fungal inoculum. If you remove adult trees, then the ectomycorrhizal fungi will also quickly die out.

 

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