Place the items into the appropriate category of being a property of basidiomycetes or ascomycetes.

Abstract

Ectomycorrhiza (ECM) is a symbiotic association of fungi with the feeder roots of higher plants in which both the partners are mutually benefited and indeed the association appears to be significant for the existence of both the partners. The majority of ECM synthesizing fungi belong to the classes Basidiomycetes and Ascomycetes that form fruiting bodies like mushrooms, puffballs, coral fungi, toadstools, truffles, etc. Though the benefits offered by ectomycorrhizal fungi are extensive, they are very sensitive. The vegetative mycelium is fragile and needs to be handled with suitable techniques and utmost care. The conventional methods of fungal isolation and cultivation are central to handling ECM fungi. The scientific community in the field of ECM research has contributed immensely toward advancement in methodologies or the techniques employed. The variations could be the simpler basic methods like the subculturing methods, the design of culture media, formulations of fungal inoculum, methods assessing mycorrhizae, to more advanced methods that range from arrays of molecular techniques like PCR, RFLP, DGGE, and temperature gradient gel electrophoresis to transcriptomic and proteomic analysis. Ectomycorrhizal fungi are not only applicable as bioinoculants but also form an important source of biological pigments, antibiotic compounds, and edible mushrooms with the known fact that they are nutritionally enriched. The methods described in this chapter provide detailed knowledge and enhanced applicability of ECM association for sustainable development.

Ectomycorrhizas

Ectomycorrhizas occur in certain families of woody gymnosperms (e.g., Pinaceae) and angiosperms (e.g., Dipterocarpaceae, Betulaceae) and are extremely important in many temperate and boreal forests. The fungal partners in ectomycorrhizal (EM) associations account for an estimated 30 percent of the microbial biomass in forest soils (Högberg and Högberg 2002). These fungi are a diverse assemblage of at least 6000 species of basidiomycetes, ascomycetes, and zygomycetes (Table 4.1; Smith and Read 1997). The oldest fossils providing clear evidence of EM associations date back 50 million years (LePage et al. 1997), yet the association is hypothesized to have evolved 130 million years ago (Smith and Read 1997).

Structurally, ectomycorrhizas are characterized by the presence of a fungal mantle that envelops host roots and a Hartig net that surrounds root epidermal and/or cortical cells and provides a large surface area for resource exchange. Hormonal interactions between plant and fungus lead to dramatically altered root architecture including the suppression of root hairs. The external component of EM associations consists of hyphae with cross walls that partition cellular components. These hyphae sometimes coalesce into macroscopic structures called rhizomorphs that attach the mycelium to sporocarps or can be morphologically similar to xylem and serve in water uptake (Smith and Read 1997). The external mycelium of EM fungi may be more extensive than that of AM fungi with as much as 200 m of hyphae per gram of dry soil (Read and Boyd 1986). Ectomycorrhizal fungi also are frequently classified using the morphology of colonized roots and their sporocarps, such as the familiar mushrooms and truffles.

4.2 Ectomycorrhizae

Ectomycorrhizal fungi (EM fungi) are phylogenetically very diverse and more than 2000 species of EM fungi worldwide have been identified, primarily from Basidiomycotina and Ascomycotina. These EM fungi form characteristic mycorrhizal associations, almost entirely with woody perennials, including Pinaceae, Betulaceae, Fagaceae, and Diperocarpaceae in tropical, subtropical, and arid environments, and are regarded as key organisms in nutrient and carbon cycles in forest ecosystems (Agerer, 2003; Becerra et al., 2005; Jakucs, Kovacs, Agerer, Romsics, & Eros-Honti, 2005). Unlike AM fungi, hyphae of EM fungi do not penetrate into the root cells but are intercellular. The hyphae penetrate into the root cortex where they form a hyphal network (“Hartig net”; see Fig. 3.2) in the intercellular space through which minerals and nutrient materials are exchanged between the fungus and the plant. The fungus forms a mantle of hyphae on the outside of the plant root that extends into the surrounding soil (Anderson & Cairney, 2007; Smith & Read, 2008). The structure of ectomycorrhizal extramatrical mycelium (extraradical mycelium) varies considerably between ectomycorrhizal species, ranging from a weft of undifferentiated mycelium around the root to a highly differentiated mycelium comprising a foraging fungal front connected to roots via rhizomorphs (Bonfante & Anca, 2009; Cairney, 2000). In angiosperm tree roots, the ectomycorrhizal hyphae penetrate the epidermal layer and spread as hyphal network (Hartig net) intercellularly (one cortical cell deep: Fig. 3.2, panels a and b), but in the case of conifers and gymnosperms the hyphal penetration reaches up to a depth of 3–4 cortical cells (Agueda, Parlade, de Miguel, & Martinez-Pena, 2006; Lupatini, Bonnassis, Steffen, Oliveira, & Antoniolli, 2008; Massicotte, Melville, & Peterson, 2005). Similar to AM fungi, ectomycorrhizae also exhibit synergistic interactions with other plant-beneficial organisms such as symbiotic N2-fixers. For example, ectomycorrhizal symbiosis enhanced the efficiency of inoculation of two Bradyrhizobium strains on the growth of legumes (Andre et al., 2005). It is also of interest that similar synergies were seen when AM fungus (Glomus mosseae), EM fungus (Pisolithus tinctorius), and Bradyrhizobium sp. were used together to inoculate Acacia nilotica, enhancement of N2 fixation, growth, and dry biomass were observed when all three organisms were present (Saravanan and Natarajan, 1996, 2000). Moreover, Bradyrhizobium sp. when co-inoculated with either the AM fungus or the EM fungus, gave enhancement of N2 fixatioin as compared to the control with Bradyrhizobium sp. only (Fig. 3.3).

Ericoid and orchid mycorrhizas tend to be host-specific and colonize only the plants in the family Ericaceae and Orchidaceae, respectively (Bergero, Perotto, Girlanda, Vidano, & Luppi, 2000; Cairney, 2000; Wilkinson, 2001). In ericoid mycorrhizas, colonization is simple. The fungus develops inside epidermal cells, forming coils (hair-like roots enmeshed in extensive weft of hyphae) that give rise to independent infection units. Normally no sheath is formed. Fungi that form ericoid mycorrhizal associations are all ascomycetes. These ericoid fungi play an important role in mobilizing the organic nutrients in the soil and making them available to the plant (Cairney, 2000; Smith & Read, 2008). In the ericoid type of mycorrhiza (in Ericaceae tribes Ericeae, Vaccinieae, Rhododendreae, and Calluneae and in related families), the fungus is endophytic. In the “arbutoid” type, found in members of Ericaceae tribe Arbuteae and subfamilies Pyroloideae and Monotropoideae, the association is ectendotrophic, i.e. the fungus grows within and also ensheaths the root tissue.

Orchid mycorrhizal associations involve partially or completely achlorophyllous plants (for some part of their life) and fungi of the basidiomycete group (Cairney, 2000; Smith & Read, 2008). Fungal symbionts of green orchids are highly effective saprophytes whereas those of achlorophyllous orchids are likely to form ectomycorrhizas on autotrophic plants. Coils produced by orchid mycorrhizae occur mostly in the inner layers of the root.

Inorganic N Sources

The ectomycorrhizal fungi are similar to other fungi in the kinds of N compounds which they can use for growth in culture. The early workers (Norkrans, 1950; Rawald, 1963; Lundeberg, 1970) found a range of relative abilities to use NH4+ and NO3−among the species and strains of mycorrhizal and saprophytic fungi that they studied (see Harley and Smith, 1983, for details). Nothing was discovered which distinguished mycorrhizal fungi as a group and more recent work has confirmed these observations (France and Reid, 1984; Genetet et al., 1984; Littke et al., 1984; Plassard et al., 1991). Most of them grow fastest on NH+4 and some can use N0−3but others not. The experimental work on N absorption is beset with difficulties concerning pH change, which can be very significant in the unbuffered media usually employed. Absorption of NH4+ results in a marked lowering of the pH, which may be followed by a sharp cessation of growth. The form of N present in the medium is strongly affected by pH. The pKa for protonation of ammonia (NH3) is 9.25, with the result that NH4+ rather than NH3 will be the predominant form in most culture media and under most growth conditions. This has implications for the membrane transport processes likely to be involved in uptake. Absorption of NO3−, as well as the release of NH4+from amides or other readily hydrolysed compounds, causes an increase in pH which is usually slower and may have a less marked effect on growth than the reductions in pH. Despite these problems, there is not much doubt about the broad results, although there is much variation between different species and strains of mycorrhizal fungi in requirements for inorganic N.

While the earlier studies concentrated on the abilities of the fungi to grow on various media, more recent work has paid attention to the pathways by which inorganic N is assimilated in the fungal mycelium. Although work on inorganic N may be considered to be relatively unimportant, given the predominance of organic N in many soils colonized by ectomycorrhizal fungi, the details of these assimilatory pathways are interesting because they show what options may be available to the fungi during assimilation of all forms of N. In common with other fungi, most of the N from NH4+ enters metabolism of ectomycorrhizal fungi either as the amide group of glutamine by the glutamine synthetase (GS) pathway, or as the amino group of glutamate through the glutamate dehydrogenase (GDH) pathway (Fig. 8.1). While both of these routes, together with that involving glutamate synthase (GS-GOGAT) can be present, the relative importance of each differs according to fungal species.

Analysis of the major pathways of N metabolism in Cenococcum geophilum (Genetet et al., 1984; Martin, 1985; Martin et al., 1988a), Hebeloma cylindrosporum (Chalot et al., 1991a) and Laccaria laccata (Brun et al., 1992) indicates that the GS pathway predominates in these fungi. Using15 NH4+ as tracer, up to 40% of assimilated 15N was found in the amide group. Glutamine synthetase has been purified from L. laccata (Brun et al., 1992) and shown to have a very high affinity for NH4+, suggesting that in this fungus at least GS is the main route of NH4+ assimilation. This may also be the case in many ectomycorrhizal fungi and would be consistent with earlier work showing that in the presence of NH4+ dark fixation of14 C02 results in preferential incorporation of label into glutamine by excised Fagus mycorrhizas (Harley, 1964; Carrodus, 1967).

The role of GDH should not, however, be overlooked. In Hebeloma Crustuliniforme, for example, assimilation of NH4+ appears to be mainly via the GDH pathway (Quoreshi et al., 1995). Two forms of this enzyme are recognized: one NAD- and the other NADP-specific. Both are found in ectomycorrhizal fungi, but it appears that only the latter, which on the basis of Km values probably operates in the direction of synthesis, has high activity (Dell et al., 1989; Ahmad et al., 1990; Botton and Chalot, 1995). A considerable amount is known about this enzyme which has been isolated and purified from mycelia of C. geophilum (Martin et al., 1983a; Dell et al., 1989), L. bicolor (Ahmad and Hellebust, 1991) and L. laceata (Brun et al., 1992). Its properties are similar to those reported for NADP-GDH enzymes of Neurospora erassa and yeasts (Stewart et al., 1980). When GS is inhibited with methionine sulphoximine (MSX), glutamate, alanine and aspartate accumulate in mycelium of ectomycorrhizal fungi, confirming the presence and operation of the GDH pathway. These results suggest that NH4+ assimilation, in the three mycorrhizal fungi is achieved by parallel action of GDH and GS (see also Chalot et al., 1994a,b). The need for examination of other fungi is highlighted by the observation that in Pisolithus tinctorius GS activity is low (Ahmad et al., 1990) and that MSX blocked the synthesis of other amino acids, suggesting the operation of the GS–GOGAT pathway. An NADH-dependent GOGAT has been detected in L. bicolor (Vezina et al., 1989), but the instability of this enzyme renders characterization difficult and its status in ectomycorrhizal fungi is uncertain.

There have been relatively few studies of the role of mycorrhizal fungi in the NO3− nutrition of plants. The greater mobility of the NO3−ion in soil, together with the inhibition of nitrification often seen in acidic soils occupied by ectomycorrhizal roots may mean that, as for vesicular-arbuscular (VA) mycorrhizas (see Chapter 5) the symbiosis is relatively unimportant in acquisition of this ion. However, there are clearly some situations in which nitrification does occur in soils supporting ectomycorrhizal plants. At one site for example NH4+ and NO3− were present in the soil solution, both at 100 μM (Stewart et al., 1993). Consequently some consideration of NO3as a potential N source for fungus and plant is necessary.

Some ectomycorrhizal fungi in culture use NO3−in preference to NH 4+. Growth of a strain of Hebeloma crustuliniforme, for example, was shown to be 10 times greater on NO3−than on NH4+ supplied at the same N concentration. The nitrate reductase (NR) activity of this fungus is similar to that seen in herbaceous angiosperms. While induction of NR appears not to be dependent upon the presence of NO3−, activity of the enzyme was depressed in the presence of NH4+ (Scheromm et al., 1990a). There is, however, a striking variability both between (France and Reid, 1984; Plassard et al., 1986) and within (Ho and Trappe, 1987) fungal species with respect to their abilities to use NO3minus;. This should be borne in mind when attempting to interpret results of experiments using only one strain or species of a fungal symbiont.

Conclusions

The ease with which many ECM fungi can be grown in axenic culture has enabled extensive screening of their abilities to use different forms of N. Most species readily use ammonium, nitrate and some simple organic-N compounds, although there are differences at both the inter- and intraspecific levels. Much new information has been gained concerning the biochemistry of N assimilation, in particular in relation to the enzymes involved in assimilation of ammonium which, in many ECM fungi, appears to be the preferred source of inorganic N. These studies have been extended to allow analysis of N assimilation by the fungi when grown in symbiosis with plants and, importantly, to investigate the influence of the plants themselves upon the pattern of events. Progress has been made towards characterization of the processes of nitrate and ammonium uptake and transport at the molecular levels. This has enabled a better understanding of the mechanisms and pathways whereby uptake of mineral N sources takes place in ECM symbioses.

Increasingly, it has been recognized that much of the N contained in the superficial layer of soil occupied by ECM roots is in organic form and that some ECM fungi have access to these more complex N sources. Emphasis has turned on the one hand to characterization of the primary sources of organic N in forest soils, with consideration of the extent to which these are accessible to ECM fungi and, on the other, to consideration of the molecular and biochemical events involved in the uptake, assimilation and transport of organic N compounds.

The molecular studies, mostly using Amanita muscaria (Nehls et al., 1999) or Hebeloma spp. (Javelle et al., 2001; Wipf et al., 2002; Selle et al., 2005; Wright et al., 2005; Benjdia et al., 2006), by revealing N dependent expression profiles of N importer genes, have enabled us to envisage how N uptake by the fungi and transfer to the plant may occur in nature (Figure 9.11). Expression of these genes is likely to be regulated by the internal N status of the hyphae. This will be relatively reduced when they are exposed to the essentially low ambient N concentrations of the soil solution, but higher in the N-accumulating mantle tissues surrounding the root. Under these circumstances strong expression of N importers in the soil compartment can be expected. Conversely, their activities will be repressed in the hyphae of the Hartig net adjacent to the root. This repression would inhibit reabsorption of N compounds by the fungus at the fungus-plant interface and help to facilitate the export of N either in the form of amino acid or ammonia, to the plant.

It is important to recognize that the pathways and processes involved in the import and transfer of N across the ECM interfaces are intrinsically linked to those by which carbon is transferred in the reverse direction (Figure 9.11 and see Chapter 8). In addition, the ability to capture and assimilate N will be influenced by the availability of the other macronutrients, phosphorus (P) and potassium (K). The processes whereby these elements are acquired and transported are considered in the following chapter.

Diversification of Ectomycorrhizal Fungi

In contrast to the endomycorrhizal fungi, ectomycorrhizal fungi are characterized by lack of intracellular hyphae, coupled with the formation of a hyphal mantle on the root surface and a network of intercellular hyphae, referred to as the Hartig net, within the root. Highly diverse, ectomycorrhizal associations are estimated to have arisen independently at least 80 times in the Ascomycota and the Basidiomycota (Tedersoo and Smith, 2013). Molecular evidence suggests ectomycorrhizal fungi typically have arisen from saprotrophs, but there is some evidence suggesting that some ectomycorrhizal fungi have evolved from plant endophytes. Convergent evolution has led to morphological and physiological similarities among these lineages of ectomycorrhizal fungi, with some shared suites of gene changes related to decay function and mycorrhizal formation, but there also is evidence that independent origins of the symbiosis are associated with unique physiological changes in ectomycorrhizal fungi from a given lineage. The repeated independent origins of ectomycorrhizal fungi, coupled with these unique physiological adaptations and the combined effects of coevolution with host plants and evolution in response to environmental factors, likely play a key role in the high diversity of ectomycorrhizal fungal species.

Compared with arbuscular and orchid mycorrhizal fungi, ectomycorrhizal reflect a middle ground in the roles of environmental factors and host specialization in driving speciation. Host specialization in ectomycorrhizal fungi runs the gamut from cosmopolitan species that form associations with most, if not all, of the plants species that host ectomycorrhizal fungi, to fungal species that are only known to form associations with a narrow range of host species. Further, within the more generalist fungi, some genotypes may exhibit greater host specialization than others. While the typical ectomycorrhizal association is with autotrophic plants, some ectomycorrhizal fungi form associations with both mycoheterotrophic and autotrophic plants, though the effects of these interactions on fungal diversity and distributions are unclear. Ectomycorrhizal fungi typically have more restricted geographic distributions than arbuscular mycorrhizal fungi (Allen et al., 1995). While this pattern reflects in part effects of host specificity, a wide range of abiotic factors, including soil characteristics, such as nitrogen and phosphorus concentration, and successional dynamics, also affects ectomycorrhizal fungal distributions. Taken together, diversification in response to both host specialization and environmental factors has resulted in a high degree of niche differentiation and specialization among ectomycorrhizal fungal species.

Introduction

Mycorrhizas with many of the characteristics of ectomycorrhizas, but also exhibiting a high degree of intracellular penetration, have been described at various times in the last century in various species of tree. These ectendomycorrhizal structures appear to be quite distinct from ectomycorrhizas where a few cells only are penetrated by the fungus, or where the senescent cortex becomes fully colonized by hyphae in the late Hartig net zone (see Chapter 6). Ectendomycorrhizas are also distinct from ‘pseudomycorrhizas’ described in Pinus by Melin (1917, and later) as forms of colonization by septate fungi which did not form sheath and Hartig net.

The term ‘ectendomycorrhiza’ should be used as a purely descriptive name for tho se mycorrhizal roots which exhibit some of the structural characteristics of both ectomycorrhizas and endomycorrhizas, and it implies no functional significance. The symbioses described here, occurring mainly in conifers, are distinct from ageing ectomycorrhizas and from mycorrhizas in some members of the Ericales in which a considerable degree or a specialized kind of intracellular penetration

The Use of Ectomycorrhizal Inoculation Programmes to Produce Edible Fungi

While emphasis in applied research on ectomycorrhizas has so far concentrated on improvement of tree production, there is an increasing awareness of the potential to exploit the commercial value of the fruit bodies produced by ectomycorrhizal fungi. At present, a small number of mycorrhizal species (Table 17.2) are prized for their gastronomic quality and are hence of high value. They are collected mostly from natural stands and constitute only a small fraction of the total global production of edible fungi (Fig. 17.1), most of which are saprophytes grown under controlled conditions. In order to increase supply of mycorrhizal fruit bodies, the current demands for which far outstrip supply, numerous commercial organizations are involved in planting trees which have been pre-colonized by inoculation with appropriate fungi. Particular emphasis has been placed on truffles (Tuber spp.) because of their extremely high economic value, the most important of these being the black truffle, T. melanosporum. Techniques for the germination of the ascospores of this fungus and for the aseptic production of mycorrhizas by a number of Tuber spp. were pioneered in France (Grente et al., 1972; Chevalier and Desmas, 1975; Chevalier and Grente, 1978) and Italy (Palenzona, 1969; Fontana and Bonfante-Fasolo, 1971). T. melanosporum has a broad host range and can be successfully grown on calcareous soils with the hardwood genera Corylus, Quercus, Carpinus and Castanea, as well as softwoods such as Pinus. Commercial production of colonized seedlings, particularly of Quercus and Corylus, now takes place in a number of centres in both the northern and southern hemispheres. In France alone about 160000 plants colonized by T. melanosporum are produced annually, mostly by Agri-Truffe of St Maixant; some are exported to the USA (Hall et al., 1994). On a smaller scale, colonized plants are being produced in New Zealand, by the New Zealand Institute for Crop and Food Research, in a programme pioneered by Hall, involving the introduction of both fungus and plant as exotic species. Truffieres have been established on both North and South Islands, usually as mixed plantings of Quercus and Corylus on potentially favourable sites, at some distance from any other ectomycorrhizal communities to reduce competition between pre-existing and introduced fungi. Truffles were collected from the introduced, inoculated plants within 5 years of establishment of a truffiere at Gisbourne, New Zealand (Hall et al., 1994). Since in other parts of the world (e.g. Europe and California) the first truffles are usually produced only after 7–10 years, the prospects for production of truffles in New Zealand and development of an export industry appear bright.

Table 17.2. Some high-priced mycorrhizal truffle and edible mushrooms

Despite advances in science and technology which provide the prospect of large-scale production of a number of edible mycorrhizal fungi, the commercial success of any venture is not assured. To a large extent the value of the commodities (especially of truffles) is based upon its limited availability, so that prices will certainly drop if large-scale production is achieved. However, especially in crops that can be used for timber, harvesting of edible fruit bodies could be an additional source of revenue or food, especially in developing countries. The large-scale establishment of eucalypt plantations in China, using planting stock preinoculated with edible fungi, has considerable potential to provide an important dietary supplement (Dell and Malajczuk, personal communication).

Introduction

Mycorrhizas with many of the characteristics of ectomycorrhizas (ECM), but also exhibiting a high degree of intracellular penetration, have been described on numerous occasions in the last century and in various species of tree and shrub. Two categories of this kind of structure are recognized here. The first, termed ectendomycorrhiza (Mikola, 1965; Laiho, 1965; Egger and Fortin, 1990; Yu et al., 2001a), occurs primarily on Pinus and Larix and is distinguished by the fact that, in addition to a usually thin fungal mantle and well-developed Hartig net of the ECM type, the epidermal and cortical cells are occupied by intracellular hyphae. The fungal symbionts forming ectendomycorrhizas are also taxonomically distinctive. Originally described by Mikola (1965) as ‘E-strain’ fungi, it is now known that most are members of the pezizalean ascomycete genus Wilcoxina (Egger, 1996). Since most ECM fungi, as well as some that are weakly pathogenic, are capable of producing intracellular penetrations, especially in senescent parts of fine roots, there has been confusion concerning the status of root symbioses of these intracellular kinds. Melin (1917) recognized fungal symbionts, many of which had dark septate (DS) hyphae that had deleterious effects upon plant performance. Among these he described Mycelium radicis atrovirens (MRA) α and β, that formed intracellular penetrations that were disadvantageous to their hosts. He referred to these as ‘pseudomycorrhizal’ fungi. The application of molecular methods has enabled a clearer understanding of the taxonomic position and status of these fungi. Like Wilcoxina, they are shown to be Ascomycetes, but are representatives of genera including Phialocephala and Phialophora which occupy distinctive taxonomic positions within the Ascomycota (Gams, 1963; Ahlich and Sieber, 1996; Menkis et al., 2004; see also Chapter 11). These fungi are described in this chapter. It is emphasized that while DS fungi do not form ectendomycorrhizas as defined above, many of them do penetrate cells of the root. They occupy that portion of the mutualism-parasitism continuum in which the colonization of the roots can have negative impacts on plant performance.

The second category of ectendo-type mycorrhiza, referred to as arbutoid, is found in the ericaceous genera Arbutus and Arctostaphylos and in several genera of the ericaceous subfamily Pyrolae. It is distinguished from the first category by the restriction of intracellular penetration to the epidermal layers of the root and by the involvement of a distinct suite of largely basidiomycetous fungi more normally found as ECM symbionts of trees. On the basis of similarities of structure, some (e.g. Zak, 1974) have considered structures of the arbutoid kind to be simply variants of ectendomycorrhizas. However, since there are consistent differences between the two types in terms both of structure and the fungal taxa involved in their formation, we here follow Peterson et al. (2004) in considering them as separate categories of mycorrhiza.

Publisher Summary

This chapter reviews the application of arbuscular and ectomycorrhizas in managed environments and discusses possible avenues for future work. Most plants used in agriculture and horticulture, as well as some forest species, form arbuscular mycorrhizas (AM), however other mycorrhizal types are also important, for example, ectomycorrhizas (ECM) for forest production and in reforestation programs , ericoid mycorrhizas (ERM) for fruit crops such as blueberries, and orchid mycorrhizas for enhanced propagation particularly for conservation. As components of the soil biota, all mycorrhizal types are potentially important in restoration of sites degraded by mining or by forestry operations. In consequence, the effects of such disturbance on communities of mycorrhizal fungi and outcomes for productivity are receiving increasing attention. Furthermore, the multifaceted roles of mycorrhizas in soil aggregation and stabilization, in disease tolerance, and in mobilizing forms of nutrients that are not directly available to roots have attracted attention in the areas of biological farming and sustainable management of production systems. Enormous efforts have been made to harness the potential benefits of mycorrhizal symbioses in commercial production systems, whether in horticulture, agriculture, or forestry. Research directed towards understanding the activities of mycorrhizal fungi in production systems is valuable both in determining appropriate management strategies and as a background against which effective inoculation techniques may be developed in future. There remains enormous scope for investigation of both fundamental and applied aspects of mycorrhizal symbioses.