Role of Mycorrhizae in Agriculture and Forestry

Most plants require mycorrhizae for normal growth and development in natural soils. The partners in this association are members of the fungus kingdom (Zygomycetes, Ascomycetes, and Basidiomycetes) and most vascular plants (Harley and Smith, 1983; Kendrick, 1985).

Of the seven types of mycorrhizae (arbuscular, ecto, ectendo, arbutoid, monotropoid, ericoid, orchidaceous mycorrhizae), the arbuscular and ectomycorrhizae are the most abundant and widespread.

Ectomycorrhizas: –

Hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root. Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a Hartig net of hyphae surrounding the plant cells within the root cortex.

In some cases, the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectomycorrhiza.

Endomycorrhizas: –

Hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane. Hyphae enter into the plant cells, producing structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules).

The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm to facilitate the transfer of nutrients between them.

Role of Mycorrhizae in Agriculture and Forestry:

The widespread occurrence of mycorrhizal fungi of all types on crops and trees in natural ecosystems, together with effects on their mineral nutrition and growth, led to the early recognition that the different mycorrhizal symbioses might be manipulated to increase crop yields in different types of primary production systems.

Most plants used in agriculture and horticulture, as well as some forest species, form arbuscular mycorrhizas (AM), but other mycorrhizal types are important
in particular situations:

Ectomycorrhizas (ECM) for forest products and in reafforestation 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 the restoration of sites degraded by mining or by forestry operations.

ARBUSCULAR MYCORRHIZAL FUNGI AND PLANT DISEASE CONTROL

Plant diseases can be controlled by the manipulation of indigenous microbes or by introducing antagonists to reduce the disease-producing propagules (Linderman, 1992). AM fungi and their associated interactions with plants reduce the damage caused by plant pathogens (Siddiqui and Mahmood, 1995; Siddiqui et al., 1999; Harrier and Watson, 2004).

With the increasing cost of pesticides and the environmental and public health hazards associated with pesticides and pathogens resistant to chemical pesticides,

AM fungi may provide a more suitable and environmentally acceptable alternative for sustainable agriculture and forestry. The interactions between different AM fungi and plant pathogens vary with the host plant and the cultural system.

Moreover, the protective effect of AM inoculation may be both systemic and localized. Colonization of the root by AM fungi generally reduces the severity of diseases caused by plant pathogens.

Reduced damage in mycorrhizal plants may be due to changes in root growth and morphology; histopathological changes in the host root; physiological and biochemical changes within the plant; changes in host nutrition; mycorrhizosphere effects which modify microbial populations; competition for colonization sites and photosynthates; activation of defense mechanisms; and nematode parasitism by AM fungi (Siddiqui and Mahmood, 1995).

The challenges to achieving biocontrol via the use of AM fungi include the obligate nature of AM fungi, limited understanding of the mechanisms involved, and the role of environmental factors in these interactions.

Cropping sequences, fertilization, and plant-pathogen management practices
affect both AM fungal propagules in soil and their effects on plants (Linderman, 1992). In order to apply AM fungi in sustainable agriculture, knowledge of factors such as fertilizer inputs, pesticide use, and soil management practices
that influence AM fungi is essential (Allen, 1992; Linderman, 1992).

In addition, efficient inoculants should be identified and employed as bio-fertilizers, bio-protectants, and bio-stimulants for sustainable agriculture and forestry.

ARBUSCULAR MYCORRHIZAL PHOSPHATE ACQUISITION

Phosphate acquisition by the AM pathway begins with the uptake of Pi-free in the soil by fungal extra-radical hyphae. These fungal hyphae extend beyond the host root system allowing a greater soil volume to be exploited for phosphate uptake.

In addition, AM colonization promotes physiological responses in the host, such as root branching and phosphatase secretion that indirectly promote phosphate uptake (Ezawa et al., 2005).

ARBUSCULAR MYCORRHIZAL NITROGEN UPTAKE

Similarly to phosphate, nitrogen is a major limiting nutrient of plant growth, especially during the production of cereal crops. Nitrogen is available in the soil in the form of ammonium (NH4+) and nitrate (NO3).

Although the concentration of ammonium in the soil is 10–1,000 times lower than that of nitrate, ammonium is the preferential form of nitrogen absorbed when plants are subjected to nitrogen deficiency (Lee and Rudge, 1986) or grown in water-logged or acid soils.

The extra-radical mycelium of mycorrhizal fungi can absorb ammonium nitrate (Johansen et al., 1996) and amino acids (Hodge et al., 2001), and the role of mycorrhizal nitrogen delivery is becoming increasingly recognized. The majority of nitrogen is thought to be taken up in the form of ammonium via the action of a fungal partner.

AM FUNGI AND ALLEVIATION OF SOIL HEAVY METAL STRESS

Some heavy metal elements such as Cu, Fe, Mn, Ni, and Zn are essential for the normal growth and development of plants. These metals are required in numerous enzyme-catalyzed or redox reactions, in electron transfer, and have structural functions in nucleic acid metabolism (Gohre 2006).

In contrast, metals like Cd, Pb, Hg, and As are not essential (Mertz, 1981) and may be toxic to plants at very low concentrations in soils. Essential heavy metals are transferred into the root by specific uptake systems, but at high concentrations, they also enter the cell via nonspecific transporters.

At high concentrations heavy metals interfere with essential enzymatic activities by modifying protein structure or by replacing an essential element, resulting in deficiency symptoms. As a consequence, toxicity symptoms such as chlorosis, growth retardation, browning of roots, effects on both photosystems, cell cycle arrest, and others are observed.

AM fungi are significant in the remediation of contaminated soil as accumulation (Jamal et al., 2002). The external mycelium of AM fungi allows for wider exploration of soil volumes by spreading beyond the root exploration zone (Khan et al., 2000), thus providing access to greater quantities of heavy metals present in the rhizosphere.

Higher concentrations of metals are also stored in mycorrhizal structures in the root and in fungal spores.

AM fungi occur in the soil of most ecosystems, including polluted soils. By acquiring phosphate, micro-nutrients, and water and delivering a proportion to their hosts they enhance the host’s nutritional status.

Similarly, heavy metals are taken up via the fungal hyphae and can be transported to the plant. The plant-fungus–the heavy metal combination is influenced by soil chemical and physical conditions.

In many cases, AM fungi serve as a filtration barrier against the transfer of heavy metal ions from roots to shoots. The protection and enhanced capability of mineral uptake result in greater biomass production, a prerequisite for a successful remediation.

AM isolates existing naturally in heavy metal-polluted soils are more metal-tolerant than isolates from non-polluted soils and are reported to efficiently colonize plant roots in heavy metal-stressed environments.

AM FUNGAL COMMUNITIES AND GRAIN PRODUCTION

Growth of grain and oil seed crops such as barley, corn, soybean, and wheat have been an important part of the agricultural economy for years and the continuous increases in demand and prices have led farmers to apply highly intensive agricultural management practices, with the aim of increasing crop productivity.

Fertilizer use represents a common agricultural management practice, but they have negative impacts on ecosystems from their use.

Arbuscular mycorrhizal symbioses have been shown to benefit the growth of many field crops in large part due to the extensive hyphal network development in soil, more efficient exploitation of nutrients, and enhanced plant uptake (Smith and Read, 1997).

AM symbiosis also increases resistance to biotic and abiotic stresses and reduces disease incidence, representing a key component of sustainable agriculture. The efficient use of AM fungi may allow for the attainment of acceptable yield levels with minimum fertilizer dose while also reducing costs and environmental pollution risk (Covacevich et al., 2007).

This is a promising approach for obtaining high yields with low fertilizer inputs in order to support sustainable agricultural systems.

ECTOMYCORRHIZAE AND FOREST ECO-SYSTEMS

Forest trees are in general completely dependent upon a symbiotic association of their roots with ectomycorrhizal fungi. These fungi mobilize minerals from the soil and transfer them to the plant. In exchange, the trees deliver assimilated C to the fungi.

Ectomycorrhizal fungi have a limited capability to enzymatically degrade the complex carbohydrates of most organic detritus and, instead, rely upon their tree hosts for their energy needs.

In return, they take up P, N, sulfur, and zinc from soil and translocate them to their host and greatly extend the functional root system of the host (Allen, 1991). An ectomycorrhizal fungus can connect to and integrate the roots of several trees, such that fungi and roots grow as one intact unit.

Ectomycorrhizal associations are widespread, particularly in temperate regions, and involve many of the ecologically important tree species such as Pseudotsuga, Picea, Pinus, Abies, Salix, Quercus, Betula, and Fagus.

The fungus also repels parasites, nematodes, and soil pathogens. Indeed, most forest trees are highly dependent on their fungal partners, and areas of poor soil quality could possibly not exist in their absence. Thus, in optimal forest husbandry, a lack of management of mycorrhizal fungi could result in damage to trees and forest crops.

The importance of ectomycorrhiza in forest plantations received much attention when it was observed that trees often fail to establish at new sites if the ectomycorrhizal symbiont is absent. This effect has been observed in exotic pine transplantation in different parts of the world.

In Western Australia, Pinus radiata and P. pinaster failed to establish in nursery beds in the absence of mycorrhizal fungi (Lakhanpal, 2000). Even the addition of fertilizer had no effect on the establishment of seedlings on such sites.

The addition of forest soil produced normal and healthy seedlings, however, because the forest soil contained propagules of mycorrhizal fungi. High ectomycorrhizal diversity is important in the healthy functioning of woodlands.

Different fungi appear to occupy different niches. Some may be more proficient at supporting the tree in taking up particular nutrients, others may be specialized at protecting against pathogens, and others may assist in enzyme production.

Pine wilt disease (PWD) is a globally serious forest disease and demonstrates the importance of tree-ectomycorrhizal relationships. PWD killed pines planted on a mountain slope of Japan (Yamaguchi Prefecture), but some trees managed to survive on top of the slope, where mycorrhizal relationships had developed better than on lower slopes.

The abundant mycorrhizae found in the upper slope enhanced water uptake by the pines, mitigated drought stress, and thereby decreased the mortality from pine wilt disease (Akema and Futai, 2005). Moreover, under laboratory conditions, inoculation of pine seedlings with ECM fungi confirmed their enhanced resistance to pine wood nematode infection.

Pine seedlings are also known to tolerate environmental stresses such as acid mist when infected with ECM fungi (Asai and Futai, 2001).

ECM fungi also make a significant contribution to forest ecosystems by increasing their network among trees through which nutrients may be transported. In addition, ECM fungi improve the growth of host plants at the seedling stage.

This association has been successfully applied to reforestation in tropical forests by inoculating mycorrhizae on nursery seedlings (Lakhanpal, 2000).

More than 90% of land plants are associated with mycorrhizal fungi, and two-thirds of them are arbuscular mycorrhizae. But tree species predominant in temperate forests are ectomycorrhizal.

MYCORRHIZAL BIOTECHNOLOGY IN THE RESTORATION OF

DISTURBED ECOSYSTEMS

Surface mining activities generate huge areas of disturbed land in many parts of the world and there is an urgent need for soil reconstruction and restoration of productive and functional soil-plant systems on these degraded lands.

Of the various types of mycorrhizae, four main types, i.e.,

• Arbuscular mycorrhizae (AM)
• Ectomycorrhizae (ECM),
• Ectendomycorrhizae, and
• Ericoid mycorrhizae (ERM) Are important for returning disturbed sites to productive agricultural and forested lands.

The ECM, which is generally associated with forest plants has the ability to provide buffering capacity to plant species against various environmental stresses (Malajczuk et al., 1994).

The majority of vascular plants form arbuscular mycorrhizae. These symbiotic fungi promote plant growth (Klironomos, 2003) and enhance a number of essential ecosystem processes such as plant productivity, plant diversity, and soil structure, and act as bio ameliorators of stressed soil conditions.

The ectendo-mycorrhizae exhibit some structural similarities to both ecto and endo-mycorrhizae. They are frequently found on the roots of plants growing on disturbed lands.

The ericoid mycorrhizae (ERM) is associated with plants belonging to the order Ericales. The ERM is associated with Ericaceous plants that have very fine root systems and typically grow in acid, peaty, and infertile soils. The fungus enables access to recalcitrant sources of minerals and provides protection from adverse soil conditions.

Ericoid plants withstand extremely difficult environments and can become established on various eroded lands. Ericoid mycorrhizae have demonstrated a special role in the mineralization of soil organic N (Read et al.,1989).

Successful establishment of forest tree seedlings at reforestation sites is often dependent on mycorrhizal association and on the ability of seedlings to acquire site resources early in the plantation establishment period (Amaranthus and Perry, 1987).

Source of Information

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• Sally E. Smith, David J. Mycorrhizal Symbiosis 2008. (book)
• Sally E. Smith. Mycorrhizal-symbiosis-second-edition 1997. (book)
• Ajit Varma. Mycorrhiza – Nutrient Uptake, Biocontrol, Ecorestoration. (book)