Mycorrhiza : A boon for sustainable agriculture

Krishna Ray & Tasvina R. Borah
ICAR Nagaland Centre

 

Sustainable agriculture integrates three main goals, environmental health, economic profitability, and social and economic equity. In practical, sustainable agriculture increase the yield and quality of outputs, while decreasing inputs by reducing the use of chemical fertilisers and pesticides. Improving agricultural sustainability is particularly attractive for developing country. In wake of the green revolution these nations are experiencing increasing agricultural sustainability problems. Soil nutrient deficiencies commonly result in poor nutrient density of grain crops, which have increasingly become staple food since green revolution. Mycorrhizal fungi have the potential to improve the sustainability of commercial food grain production by improving yield and quality, consequently reduce input levels to achieve the same yield. Reducing input levels can help in addressing some of the core issues of sustainability, such as the eutrophication of water bodies caused by excessive applications of soluble P fertilisers and the depletion of non-renewable resources like rock phosphate.

In 1885 Albert Bernard Frank in his study of soil microbial-plant relationships, introduced the Greek term ‘mycorrhiza’, which literally means ‘fungus roots’. Mycorrhizal fungi form symbiotic relation-ships with plant roots in a fashion similar to that of root nodule bacteria within legumes. Of the seven types of mycorrhizae described (arbuscular, ecto, ectendo-, arbutoid, monotropoid, ericoid and orchidaceous mycorrhizae), arbuscular mycorrhizae and ectomycorrhizae are the most abundant and wide-spread.

Arbuscular mycorrhizal (AM) fungi comprise the most common mycorrhizal association and form mutualistic relationships with over 80% of all vascular plants. AM fungi are obligate mutualists belonging to the phylum Glomero-mycota and have a ubiquitous distribution in global ecosystems. They are known not to occur only in a few plants, namely members of the families Amaranthaceae, Pinaceae, Betulaceae, Cruciferae, Chenopodiaceae, Cyperaceae, Juncaceae, Proteaceae, and Polygonaceae.

Ectomycorrhizal (ECM) fungi are also widespread in their distribution but associate with only 3% of vascular plant families. These fungi are members of the phyla Ascomycota and Basidiomycota, and occur primarily in temperate forest species, although they have been reported to colonize a limited number of tropical tree species.

Ectendomycorrhizae possess characteristics of both ECM and AM fungi. As with ECM, both a hartig net and mantle structures are produced in ectendomycorrhizae, although the mantle may be reduced compared with ECM. Ectendomycorrhizae can be formed with roots of many angiosperm and gymnosperm species; fungal symbionts include members of the Basidiomycota, Ascomycota, or Zygomycota.

Similarly, arbutoid mycorrhizae possess characteristics of both ECM and AM fungi, i.e., there is a well developed mantle, a hartig net, and prolific extrametrical mycelium. These mycorrhizae are associated with members of the Ericales; namely, Arbutus and Arctostaphylos species.

Monotropoid and orchid mycorrhizae are formed between Basidiomycete fungi and achlorophyllous plant species. Monotropoid mycorrhizae are formed between plants of the Monotropaceae family and a specific subset of fungi in the Russulaceae or the Boletaceae family.

Orchid mycor-rhizae have only been found in association with Basidiomycete species. In the other mycorrhizal symbioses plants are usually generalists and associate with a wide array of fungal species.

Role of arbuscular mycorrhiza and ectomycorrhiza can be explained as – Mycorrhizal fungi are obligate symbionts and rely on the carbon provided by their plant hosts to complete their life cycle. In return, the fungus provides nutritional benefits to the plant by delivering minerals, including the biologically essential nutrients phosphorus (P) and nitrogen (N). The majority of this nutrient exchange is believed to occur within root cortical cells containing highly-branched hyphal structures termed arbuscules.

AM fungi absorb N, P, K, Ca, S, Cu, and Zn from the soil and translocate them to associated plants. However, the most prominent and consistent nutritional effect of AM fungi is in the improved uptake of immobile nutrients, particularly P, Cu, and Zn. AM fungi have biochemical capabilities for increasing the supply of available P and other immobile nutrients. These capabilities may involve increase in root phosphatase activity, excretion of chelating agents, and rhizosphere acidification.

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; histopatho-logical 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 photo-synthates; activation of defense mechanisms.

Keys to successful application of AM technology are the availability of good quality inocula and a clear understanding of the circumstances under which mycor-rhizal fungi are likely to enhance plant growth and development. AM fungi are obligate symbionts and therefore cannot be multiplied on laboratory media apart from a living host. In the most common way of producing AM inocula, the fungi are multiplied in the presence of a suitable host plant growing in some kind of matrix, which may be sand, soil, or sand-soil mixture. The supply of P and other nutrients is carefully monitored to promote maximum fungal infection and spore development. This process requires as long as 16–18 weeks. After this period, the medium is allowed to dry down to 5% moisture content or less. The plant is then cut off, and the remaining medium containing spores, pieces of fungal hyphae, and segments of infected roots serves as the crude inoculum. Such an inoculum can be stored at 22°C for as long as three years without significant loss in viability.

Cropping sequences, fertilization, and plant pathogen management practices affect both AM fungal propagules in soil and their effects on plants. In order to apply AM fungi in sus-tainable agriculture, knowledge of factors such as fertilizer inputs, pesticide use, and soil management practices which influence AM fungi is essential. In addition, efficient ino-culants should be identified and employed as biofertilizers, bioprotectants, and biostimulants for sustainable agriculture and forestry.



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