Mycorrhiza (meaning fungus from the Greek 'mycos' and meaning root from 'rhiza') are symbitotical associations between plant roots and fungi. In the type of association ectomycorrhiza, which is predominant on trees in temperate forests, the fungal organism remains outside of plant cells. In endomycorrhiza, including ericoid, orchid and arbuscular mycorrhizal (AM), a part of the fungal hyphae is inside the plant cells (Parniske, 2008). Arbuscular mycorrhizal fungi (AMF) are the most common mycorrhizal types and they are formed in an enormously wide variety of host plants (Smith and Read, 2008).
Three important components are identified in AM: the plant root itself, the fungal structures within and between the cells of the root and an extraradical mycelium in the soil (hyphal network) (Smith and Read, 2008). The hyphal network is specialized for nutrient (predominantly phosphate) and water uptake. In return AMF obtain carbohydrates from plants. Up to 20% of the photosynthesis products of terrestrial plants are estimated to be consumed by AMF (approximately 5 billion tonnes of carbon per year). The beneficial effects of AM fungi are most evident under conditions of limited nutrient availability. Although the regulatory mechanisms are not clearly understood, the root colonization typically decreases when nutrients are in abundance (Parniske, 2008).
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In brief, AMF are recognized as obligate biotrohps symbionts of a very wide range of plant species. The symbioses interactions are based largely on bidirectional nutrient transfer between the symbionts, sometimes supplemented by other benefits such as stress and disease tolerance.
Morphology and Biology
It has been possible to define two simple morpholigical types of AM fungi, the Arum-type and the Paris-type. These types were considered after the plants in which they were first described Arum maculatum and Paris quadrifoli, respectively. In the Arum-type, fungi form intercellular hyphae between the cortical cells and intracellular arbuscules within them. The Paris-type is characterized by extensive intracellular hyphal coils and arbusculate coils in the root cortex. Studies of different fungal and plant combinations suggest that AM morphological type is largely dependent on the plant species and also influenced by the fungal identity (Cavagnaro et al., 2001). Smith and Smith (1997) concluded that 41 families formed the Paris-type, 30 families form the Arum-type and 21 families form an intermediate morphology or have members with both Paris- and Arum-type.
The hyphal network of Arum-type and the Paris-type is usually coenocytic (Multiple nuclei inside the same cell), and aseptate (not containing septae) and with hundreds of nuclei into same cytoplasm. Also individual spores contain hundreds of nuclei. As an obligate biotroph, AM fungi depend on a living photoautotrophic organism to complete their life cycle; however their spores can germinate in absence of host plants (Parniske, 2008). The fungus hyphal growth starts with the spore germination, subsequently the root surface and cortex are penetrated and colonized by the appressoria. Then the hyphae penetrate the cell walls and develop within the cortex cells tree-like structures, called arbuscules, by repeated dichotomous branching. The most important feature of AMs is the arbuscule, responsible for nutrient exchange (Strack et al., 2003). Each fungal branch within a plant cell (Figure 2) is surrounded by a plant-derived periarbuscular membrane (PAM). The apoplastic interface between the fungal plasma membrane and the plant-derived PAM is called the periarbuscular space (PAS) (Parniske, 2008).
Figure 2. Schematic drawing of an arbuscule, the symbiotic structure and AM (Parniske, 2008).
Development and Ecology
During the germination of the spores and the hyphae growth the stimulatory effect of plant root exudates plays a key role. The strogolactones are kind of phytohormones that are responsible for the spore germination and hyphae branching. The strilogactone perception by the fungus induces the presymbiotic stage, which is characterized by continued hyphal growth, high physiological activity and the hyphae branching. Then fungi produce mycorrhiza factors that induce changes in the calcium concentration in the epidermal root cells and activate plant symbiosis-related genes. AMF form special types of appressoria (developed from mature hyphae) that reaches the plant roots. Plant cells produce a prepenatration apparatus (PPA) and subsequently fungal hyphae enters the PPA, which guides the fungus until it reaches the cortex. Then the fungus leaves the plant cells and reaches the apoplast, where it branches and grows along the root axis. Finally new spores are typically synthesized outside the plant root (Parniske, 2008).
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Since AM improve plant nutrition and growth and they are in symbiosis with the roots of the majority of land plants, few would doubt about their ecological importance. The composition of the AMF community may be strongly influenced by the host species through differential effects on hyphal growth and sporulation. In return, the plant community structure may be strongly influenced by the specific composition of the associated AMF and the effectiveness of each of the fungal species in promoting growth of each host (Burrows and Pfleger, 2002).
Host range and specificity
The AM fungi have extremely wide range of potential plant. It develops in most families of angiosperm and gymnosperms, together with sporophytes of ferns and lycopods. Additionally, the free living gametophytes of pteridophytes, as well as those of some hepatic are often colonized by AM fungi. In a compiled incidence of all types of mycorrhizas within the angiosperm, class Dicotyledonae has been observed to have more incidences than class Monocotyledonae (Table 1) (Smith and Read, 2008).
Table1. Numbers and percentages of species of subclasses and classes of angiospermae examined for mycorrhiza formation and percentage of examined species by type of mycorrhiza (Smith and Read, 2008).
AM: Arbuscular mycorrhiza; Other: Mycorrhizas formed by ascomycetes and basidiomycetes; NM: non-mycorrhizal (non-host)
Most herbaceous species that have been studied for occurrence of arbuscular mycorrhizas activity showed positive incidences. Smith and Read (1997), suggested that as many as 75% of species of the family Dipterocarpaceae once were thought to be exclusively ectomycorrhizal, may form arbuscular mycorrhizal (Wang and Qiu, 2006). This finding confirms the prevalence of AM symbioses in taxonomically diverse tropical forests, as well as in some temperate forest systems. AM are believed to be ecologically the most important type of mycorrhizal in New Zealand forest and Northern temperate broadleaf forest. AM are characteristic of valuable trees such as Acer, Araucaria, Podocarpus and Agathis as well as all the family Cupressaceae, Taxodiaceae, Taxaceae, Cephalotaxaceae and majority of tropical hardwoods. Some economically important tree genus that have been used for experimental work on arbuscular mycorrhiza include Malus (apple), Citrus, Salix, Populus, Persea (avocado), Coffea, Araucaria, Khaya, Anaacardium (cashew), Liquidambar and many others. Among the land plants as a whole, they found that 80% of species and 92% of families potentially form at least one type of mycorrhiza (Table2) (Smith and Read, 2008).
Table 2. Mycorrhiza status of four groups of land plants, indicating numbers and percentages of the families and species that have always (obligate), sometimes (facultative) and never (non-mycorrhizal) been observed to form mycorrhizas (Smith and Read, 2008).
Some plant species have been recorded as occurring in both mycorrhizal and non mycorrhizal states and members of some plant families typically form mycorrhizas of types other than AM or, indeed, more than one type of mycorrhiza (Wang and Qiu, 2006). The factors that cause failure of a potentially mycorrhizal species to become colonized are lack of inoculum of an appropriate fungus at the site, environmental conditions such as high nutrients, cold or waterlogging and seasonal variation in the development of the fungi in roots. Species which are not always colonized are often referred to as 'facultative mycorrhizal', to distinguish them from those 'obligately mycorrhizal' species that are consistently colonized.
It is considered that arbuscular mycorrhizal has no absolute specificity, because a given AM species can colonize a range of plant species and a given plant species can be colonized by several different AM species. There are about 150 species of arbuscular mycorrhizal fungi which have mutualistic symbiotic associations with the roots of about 80-90 % (approximately 200,000 plants) of terrestrial plant species. This indicates a low fungus to host species ratio, and because of this, it is believed that the AMF are not host-specific. It means that each fungal species theoretically must have many hosts. Many recent studies conducted continue to support this view. In the earlier investigation carried out by Gerdemann (1955), he concluded that there is no absolute specify between taxa of AM fungi and taxa of potential host plant. Practical observation in the field indicates that single plant root systems can contain many AM fungi and that different plant species at the same site often contain the same fungi. Several experiments of plant and fungal species revealed that an AM fungus isolated from one species of host plant can be expected to colonize and other species which has been shown to be capable of forming arbuscular mycorrhizas (Smith and Read, 2008).
The improvement of nutrient uptake by AMF
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Evidences from pot experiments conducted with AM responsive plants in the controlled conditions of glass houses or growth chamber and some field studies have confirmed that AM improves nutrient uptake of host plants. It is well established that AM roots are often more efficient in nutrient acquisition, per unit length than non-colonized roots (Merryweather and Fitter, 1998). Phosphorous (P) is believed to be the major nutrient that is being absorbed by AM. Since P is poorly mobile in soil and occurs in very low concentrations in the soil solution, being rapidly fixed as iron, aluminium or calcium phosphate or immobilized in the microbial biomass, it is poorly absorbed by plants. It has also been found that AM increased the uptake of zinc, which is also poorly mobile and is deficient in some soils, and copper. Recently, the attention has turned to nitrogen which, in addition to its organic forms, occurs as either poorly mobile ammonium or nitrate; the later is highly mobile in moist soil, but not in dry soil. It is becoming clear that AM fungi have the potential to play a considerable part in plant N nutrition and that uptake of both ammonium and nitrate can be increased in mycorrhizal plants. Mechanisms of AM for increased up-take of nutrients are as follow:
Exploration of a larger soil volume
Increased area of up taking surface
Faster movement of P into tissue due to higher affinity to P-ions and lower threshold concentration for P-uptake
Uptake of poorly soluble P-sources (e. g. iron, aluminum, or rock-phosphate)
Solubilization of substrate-P by excretion of organic acids (e. g. oxalate) or phosphatases
AM plants have two potential pathways of nutrient uptake, directly from the soil or via an AM fungal symbiont. The AM pathway depends on three essential processes (Figure 3): uptake of the nutrients by the fungal mycelium in the soil; translocation for some distance within the hyphae to the intraradical fungal structures (hyphae, arbuscules and coils) within the roots; and transfer to the plant cells across the complex interface between the symbionts.
The mycelium of AM fungal in soil can absorb nutrients beyond the zone depleted through uptake by the roots themselves, thereby increasing the effectiveness with which the soil volume is exploited (Smith and Read, 2008).
Figure 3. Diagrammatic representation of potential pathways of nutrient acquisition from soil in an arbuscular mycorrhizal root. The mycorrhizal pathway involves nutrient uptake by the external mycelium of an AM fungus, translocation through the hyphae to fungal structures in roots and transfer across symbiotic interfaces to the plant root cells. The direct pathway involves uptake by root hairs and epidermis. Depletion of relatively immobile nutrients in soil, such as P, following rapid uptake via either pathway is also indicated (Smith and Read, 2008).
The potential of AMF to improve plant resistance or tolerance to pathogens or pest
The potential of AMF to improve plant resistance or tolerance to pathogens or pest
The effect of arbuscular mycorrhizal fungi on pathogens and insect herbivores (Smith and Read, 2008) are in most of the cases indirect, and result from altered physiology plant and increase nutrition of the host. AM fungi may increase the host tolerance (ability of the plant to sustain the performance despite the infection caused by pathogens or pest) by improving root growth and function, even AM plants may persist a greater attack by pathogens and grow better than no arbuscular mycorrhiza plants. AMF also may enhance host resistance (decrease pathogen performance) by altering root exudations used by pathogens or stimulating a defence reply (Borowicz, 2001). Limited studies have or emerged that AMF compete directly with other biotrophic organisms for photosynthate for the same cortical space and resources, suppressing the growth of pathogen (Graham, 2001). Other mechanisms involves are frequently cited as structural and biochemical resistances induced by the AM fungus in the root cortex, or as attributable to alterations in the mycorrhizosphere microflora (Lindermann, 1994).
The response of plant growth to inoculation at low nutrient supply soon motivated interest in AMF as stress-reducing agents and biocontrol, specially after initial studies the tolerance of roots to infection by the root knot nematode, Meloidogyne incognita and fungal pathogens such as Thielaviopsis basicola were shown (Graham , 2001).
Azcon-Aguilar and Barea (1997) reported that AM inoculation can be effective even against soil-borne pathogens, which are not easy to control by physical and chemical treatments. In contrast, formation of the AMF mainly leads to higher susceptibility to shoot pathogens, aphids and viruses. This AM side-effect of enhanced shoot pathogens is studied in detail because individual plants in some cases may be able to compensate negative influences but cultivars or species may demonstrate high variability. Gernns et al, 2001 showed that AM barley plants were more susceptible to shoot pathogen Erysiphe graminis, nevertheless suffered less than no AM plants in terms of ear yield, grain number and weigh; reporting that the symbiosis neutralised the positive relationship between yield loss and disease severity.
Molecular probes provide special opportunity for comparative studies of expression of plant tolerance in AMF versus pathogen interactions. So far, the evidence that AM colonization systemic resistance to pathogens, such as through up-regulation of pathogenesis-related proteins, remains polemic (Blee and Anderson, 2000). The expression of genes apparently encoding plant defences is weak when arbuscular mycorrhiza fungi colonize roots; colonization even suppresses host-plant defence-related genes. It is not unreasonable to expect suppression of general plant defences, given the low specificity of arbuscular mycorrhiza fungi (Graham, 2001). Systemic priming of root tissues to form biochemical and structural barriers is usually dependent on an elevated level of root colonization and cannot always be verified as mycorrhiza specific because the phosphorus status of the host plant before pathogen challenge is not known (Cordier et al., 1998).
Some studies support that AMF increase plant resistance, but in others AM fungi had no resistance effect, or even enhance the pathogen infection, or insect attack. Nevertheless, it stays unanswered whether it exist a general model of mycorrhizal benefit (Borowicz, 2001).
Aspects of AM application
The number of new companies producing AMF inoculates around the world has been increasing, in the last few years (Gianinazzi and Vosátka, 2004), and there are several reasons for the development of this agricultural biotechnology industry such as: (a) arbuscular mycorrhizal fungi are considered as a natural plant health insurance, and their impact on plant performance and development, and phytoremediation, remain increasing (Leyval et al., 2002); (b) it exist a higher awareness of biodiversity, and acceptance of these natural technologies as alternatives to chemical inputs (Barea 2000); (c) there is a demand for more sustainable ways of production.
The formulation of the AM inoculum procedure involves putting fungal propagules like fragments of fungal, root fragments colonized with AMF, and spores, in a carrier like perlite, inorganic clay, peat, and vermiculite, for a certain application. Because biological inoculates belong to different taxonomic groups, they have different environmental and nutritional requirements, therefore, the final configuration of the formulation will be determined by the way of producing inoculums (Table 3), the microbe involved, and the target inoculum application like seeds, bare-root plants or cuttings. The fungi should be chosen to be compatible with the target environment (Vosátka and Dodd 2002). The mass production and fungal propagules ought to be formulated in such a way that they can be distributed and stored without losing viability, under a wide range of temperatures. Additionally they should be easy to transport and to apply.
Table 3.Inoculum produced with commercial applications, in 4 different ways with its advantages and disadvantages (developed from information reported by Gianinazzi and Vosátka, 2004).
There is no universal method of inoculum application, and some plantings need a specific mode of application. A relevant issue is to optimize the introduction of AMF as early as possible in the plant growth, by layering inoculums below seeds or integration inoculum into the growth substrate for containerized plants. The micropropagated plants can be inoculated post vitro at the transplantation phase, while bare-root plants can be dipped into gel formulations of AM inocula before transplantation stage, or dry formulations of inoculum can be spread into the planting. For larger-scale application machinery like sowing machines or mixing tanks for substrates and field inoculations are needed. Ecological aspects such as amount of fertilizer input, potential of fungal populations, soil properties, should be taken into account for the introduction of inoculum into the field (Vosátka and Dodd 2002). One marketing branch of AM products includes the production of biotized plants; and this is used for medicinal plants (Germany), endomycorrhizal trees for recultivation (Czech Republic), and for micropropagated plants (France).
Due to regulation the use of microbial products for plant protection in the EU, the AM fungal inoculum should not be regarded as a biocontrol agent; nevertheless AM fungi do not produce harmful substances, and AM do no attack other organisms, so they should be declared as a natural part of the plant, and the part concerning of the regulation "risk assessment" criteria is inappropriate to AMF therefore, the European network on AMF is in discussions within the EU on the need for a registration procedure for AMF.
There are still several cases of conventional farmers who prefer chemical inputs and use plenty of NPK fertilizer for the soil, instead of worrying about fungi that are not seen with the naked eye. For that reason it should be an objective of industrial and scientists partners to promote AM inoculations as biotechnology tool, with potential implementations in the majority of the places of sustainable plant production.