A survey of vesicular arbuscular mycorrhizae (VAM) with special reference to taxonomy, physiology and economical importance

Abstract

Vesicular-arbuscular mycorrhizae (V AM) are the most common type of mycorrhiza found in nature. They are ubiquitously found in soils throughout the world where they form symbiotic associations with most plants as well as crops. VAMF are classified as zygomycetes fungi and can be divided into six genera: Glomus, Gigaspora, Sclerocystis, Acaulospora, Scutellospora and Entrophospora.Vesicular-arbuscular mycorrhizal fungi (V AMF) form complex extramatrical and intramatrical mycelia within the roots of host plants. When hyphae of VAMF get into contact with host roots, appressoria are formed on the surface of roots. The penetrating hyphae branch to form coils, intercellular and intracellular hyphae throughout the cortex of host roots. The intramatrical hyphae form typical arbuscules within the cortex cells as well as vesicles which can be formed inter- or intracellular. Arbuscules are considered as the main centre of nutrient exchange between the host plant and the fungus. V AMF also form extensive mycelia outside host roots where they are able to explore large volumes of soil for nutrients, beyond the depletion zone of plant roots. During the symbiotic association of V AMF and host plants the fungi benefit the host plants mainly by absorbing nutrients from nutrient deficient soils and transferring it to the host. The fungi benefit from hosts by getting carbon compounds and protection. This association is influenced by environmental and climatic factors such as soil fertility, Ph and moisture content of the soil, temperature and light as well as the presence of other soil organisms. V AMF usually have several beneficial effects to host plants. The host plant benefit by enhanced growth and crop yield, better absorption of nutrients, especially phosphorus, from low fertility soils and increased resistance against disease, increased tolerance to water stress. These beneficial effects of VAMF can, with careful investigation and selection of most suitable V AMF, be manipulated for agriculture and horticulture uses to enhance crop production. V AMF cannot be considered as substitutes for fertilizer application but can be implemented to utilize applied fertilizer more efficiently, thus reducing the rate and amount of fertilizer application. The application of pesticides can also be reduced on V AMF infected plants. This can lead to more economical agriculture, horticulture and afforestation practices. To enhance crop yield, it is necessary to produce VAMF inoculum a large-scale basis. Unfortunately, the production of pure pathogen-free inocula has not been achieved as the growing of VAMF axenically is not yet possible. However, there are several other methods to produce viable inocula but the contamination with unwanted organisms is still at risk. There is still a need for further research to test the viability of inoculum production to the economy and only then will V AMF be successfully utilized for horticultural, agricultural and afforestation enterprises.