3.4.3  Functional aspects of mycorrhizas

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The association between fungus and host plant delivers nutrients via: (1) mobilisation and absorption by fungal mycelia; (2) translocation to the fungus–root interface and (3) transfer across the fungus–root interface (Cairney and Burke 1996).

(a)  Mobilisation and absorption of nutrients

In addition to hyphae in direct contact with the root surface, all mycorrhizal fungi produce mycelium (extramatrical mycelium) which grows from the infected root surface into surrounding soil. Both arbuscular mycorrhizal and ectomycorrhizal fungi produce copious extramatrical mycelium, with arbuscular mycorrhizal mycelia extending several centimetres from the infected root surface and ectomycorrhizal mycelium potentially spreading for up to several metres. In either case, the mycelium extends well beyond the nutrient depletion zone for immobile nutrients around individual roots and displays a complex architecture that renders it an efficient nutrient-collecting network. Extramatrical mycelium is the component of mycorrhiza which efficiently mines bulk soil for scarce nutrients and translocates absorbed nutrients to the fungus–root interface where transfer to the host plant is effected.


Figure 3.16 Abundant mycelium (M) of Scleroderma ramifies through soil forming a sheath (S) around roots of a eucalypt (E). Resultant ectomycorrhizas benefit the host through enhanced nutrient uptake (especially phosphorus) from surrounding soil (Photograph courtesy I. Tommerup; reproduced with permission of CSIRO Forestry and Forestry Products)

Extramatrical mycelium of many ectomycorrhizal fungi spread as a diffuse mat of individual hyphae where the leading edge progressively differentiates by hyphal aggregation behind the growing front to form complex linear multi-hyphal structures known as rhizomorphs (Figure 3.16). Hyphae up to 35 µm in diameter at the core of rhizomorphs are devoid of cell walls and play a role in transport of inorganic nutrients or photoassimilates. In arbuscular mycorrhizas, diffuse hyphae (diameter 1–5 µm) at the growing front provide a vast surface area for nutrient absorption, while larger diameter hyphae (up to 10 µm) constitute an excellent translocatory infrastructure for efficiently moving solutes from bulk soil through the rhizosphere to root surfaces (Read 1992).

Many experiments have demonstrated a relationship between arbuscular mycorrhizal infection and improved plant phosphorus status. Arbuscular mycorrhizal fungi do not appear to have access to sources of soil phosphorus that are otherwise unavailable to non-mycorrhizal roots. However, extramatrical mycelium provides a large surface area for orthophosphate absorption from bulk soil through production of up to 250 m of mycelium cm–1 of colonised root. Increased plant absorption of nitrogen and other macronutrients such as calcium and sulphur and micronutrients including zinc and copper also appear simply to reflect the increased absorptive surface of the extramatrical mycelium. Some arbuscular mycorrhizal fungi might still be shown to extract phosphate from organic forms in soil through the action of extracellular phosphatases. Absorption of orthophosphate is maximised by the action of a high-affinity transporter which is expressed only in extramatrical mycelium of arbuscular mycorrhizal fungi during symbiosis with the plant (Harrison and van Buuren 1995).

The extramatrical mycelium of ectomycorrhizal fungi increases the absorptive area of a root system several-fold. This increase is undoubtedly important in extending the volume of soil explored by the host plant and consequently the quantity of minerals available. Ectomycorrhizal fungi, however, use additional strategies to enhance nutrient acquisition. Many secrete extracellular proteinases and peptidases that effectively hydrolyse organic nitrogen sources to liberate amino acids which can be absorbed by the fungi. Like northern hemisphere forest soils, where nitrogen mineralisation through the action of these enzymes is well established, Australian forest soils have considerable organic nitrogen which can be mineralised. Even though the rate of mineralisation by ectomycorrhizal fungi is unquantified in southern hemisphere forests, ectomycorrhizal-derived enzymes are likely to be of importance in tree nutrition. Ectomycorrhizal fungi also produce extracellular phosphomonoesterases and phosphodiesterases, the latter mediating access to phosphorus sequestered within nucleic acids. Some ectomycorrhizal fungi produce hydrolytic enzymes within the cellulase, hemicellulase and lignase families that may facilitate hyphal entry to moribund plant material in soil and access to mineral nutrients sequestered therein. In these ways ectomycorrhizal fungi shortcircuit conventional nutrient cycles, releasing nutrients from soil organic matter with little or no involvement of saprotrophic organisms. Ectomycorrhizal fungi also release siderophores capable of complexing iron and oxalate to improve potassium uptake. Reducing agents released by ectomycorrhizal fungi enhance ion uptake from stable oxides (e.g. MnO2), further contributing to host plant nutrition.

In contrast, ericoid mycorrhizal fungi produce little extramatrical mycelium and infection does not significantly increase the absorptive surface of the host root system (Figure 3.14). Hair root systems of plants in the Ericales form extremely dense mats with a potentially large absorptive area in heathland soils, reducing the need for extensive extramatrical mycelium. A major contributor to nutrient acquisition in the Ericales is production of a complex array of extracellular enzymes that can release nitrogen and, to a lesser extent, phosphorus from simple organic compounds and plant litter. Ericoid mycorrhizal fungi are important sources of these enzymes. This is of particular importance in high-rainfall, low-temperature environments where the activities of decomposer organisms are extremely limited and organic matter accumulation is large.

(b)  Movement of carbon and nutrients across the fungus–root interface


Figure 3.17 A simplified view of the symbiotic interface showing fungal (FPM) and plant (PPM) plasma membranes through which fluxes of solutes occur. Carbohydrates efflux into the interfacial apoplasm where they become available for influx to the fungus. Mineral nutrients extracted from soil by the fungus are effluxed to the same space, making them available for the host plant. Fungal cell wall (FCW) occupies the apoplasm in all mycorrhizal associations but plant cell wall (PCW) is only present in the interface of ectomycorrhizas, where fungal hyphae are intercellular. In arbuscular and ericoid mycorrhizal associations, hyphae penetrate PCW and therefore only FCW occupies the interface (Based on Smith and Read 1997).

Regardless of the mycorrhizal type, nutrients arrive at the fungus–root interface within the symplasm of the fungus (Figure 3.17). Transfer to the host plant involves efflux across the fungal plasma membrane and subsequent absorption from the apoplasm of the interface across the plasma membrane of the host root cells. Escape of substrates from the interface is minimised by elaborate fungal structures. Impermeable extracellular material deposited between hyphae of the mantle in some ectomycorrhizas and at the points of hyphal entry into cells in arbuscular mycorrhizas and ectomycorrhizas create a defined apoplasmic compartment. Not only does this prevent leakage from the interface apoplasm but it also means that local chemical and physical conditions can be controlled by the activities of both partners.

Ectomycorrhizal fungi derive carbon for growth and metabolism from host roots, largely as photoassimilate. Sucrose is thought to be hydrolysed in root cell walls and glucose is then absorbed by hyphae from the interface apoplasm.

Identifying control steps in phosphate transport across a fungus–root interface has proved difficult because fungi store phosphate as polyphosphates, making it difficult to estimate the concentration gradient of free orthophosphate across the fungal plasma membrane. Indeed, rates of phosphate release from these polyphosphate reserves might determine phosphate efflux rate to the apoplasm. Transport proteins in fungal and host plasma membranes must also play a central role in phosphate uptake by mycorrhizal roots and considerable effort is being made to discover the combination of phosphate transporters and channel proteins coordinating this flux. Cloning of a high-affinity transporter from arbuscular mycorrhizal fungi is a start in this search. Absorption of phosphate across the host plasma membrane is believed to be mediated by a 2H+/orthophosphate symporter energised by an H+-ATPase. A quantitative picture of how mycorrhizal associations transport phosphate will require more knowledge of transport kinetics, fungal phosphate metabolism, channel gating factors (Section 4.1) and interaction between fungal and host genomes.