Mycorrhizal Associations

Mycorrhizal associations include many taxas of fungi belonging to members of Zygomycetes, Ascomycetes and Basidiomycetes and Deutromycetes. A characteristic feature of these fungi is that they are generally widespread in soils, exhibit a strong biotrophic dependence on their host plants, and are rarely free living saprophytes. Different types of mycorrhiza are classified into seven different categories on the basis of the extent of root penetration. Among them arbuscular mycorrhizas are the most common and have gained tremendous importance in present day agriculture. The AM symbiosis is the association between fungi of the order Glomales (Zygomycetes) and the roots of terrestrial plants (Harley and Smith 1983). Conservative estimates predict that this ancient symbiosis, dating back to the early Devonian age (398 million years ago), occurs in approximately 90% of the Earth's land plants (Remy et al. 1994). The AM symbiosis is increasingly gaining recognition as an important integral component of natural ecosystems throughout the world. The AM fungus-plant association is a mutually beneficial event: the plant supplies the fungus with carbon (from its fixed photosynthates) while the fungal symbiont assists the plant in phosphate uptake and also converts some unavailable mineral nutrients to available forms for the plant. This bidirectional exchange of nutrients takes place through extensively branched haustoria, commonly called arbuscules. In addition to the nutrient uptake, mycorrhizal fungi improve the performance also of other beneficial microbes, help in resisting root pathogens, and increase the tolerance to extremes of environmental and biological stress. With increased nutritional consumption and a higher uptake of desirable nutrients due to mycorrhization, the biomass, both above and below the ground, increases. Decomposed biomass, when recycled, improves the soil fertility manifold and this is how mycorrhiza helps in restoring ecosystems.

Above-ground plant development is influenced by below-ground microbial activity. In the presence of mycorrhizal fungi, other micro-organisms such as PSB, many free-living nitrogen-fixing organisms and Rhizobium work more efficiently, improving soil fertility and plant growth. Mycorrhizas remain in the soil and form active links with growing plants and mutually benefit each other.

Papers advocating the valuable potential of mycorrhizal inoculations in plant establishments have been published since the 1960s but comprehensive information on their practical exploitation by multiple field trials has not been presented so far (Findlay and Kendle 2001). Immense potential of mycorrhiza has not been so far exploited due to its uncultivable nature unlike other biofertilizers. Mycorrhizas are conventionally propagated using pot-based methods with host trap plants. The disadvantage of this mode is the low recovery of mycorrhizal propagules, contamination by saprobes, pathogens and other mycorrhizal fungi because of improper management techniques and long gaps duration between setup and harvest. Several alternatives to this mode have been designed, but in all current methodologies of cultivating AM fungi, host plant is indispensable. Many variants of these methods have been proposed by various workers to culture glomalean endomycorrhizal fungi, with a bewildering array of claims and counterclaims. These will be described in detail in the present review. All of these involve a plant host, either intact or as root explants. Methodological differences focus mainly on differences in the cultural environment, the most dramatic being the interface between fungus, plant root, and external matrix. The various modes include pot base techniques, hydroponic films (Elmes et al. 1984) or in aeroponic mist chambers (Hung and Sylvia 1988) and the recent in vitro root organ culture (ROC) system (Becard and Fortin 1988).

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