Fungal Biotechnology [Part 3]

June 28, 2012
By Krishiworld

USE OF FUNGI TO CONTROL WEEDS:

Weeds, in the widest sense of the world, include algae, ferns, annual & perennial plants, and also herbaceous or woody species. Fungi that are potential mycoherbicides can be divided into facultative parasites, which can be grown in the laboratory, and obligate parasites which at present cannot.

Two approaches to used control using fungi have been developed (i) the inundative or mycoherbicide approach, and (ii) the classical biocontrol method. In the former approach, massive inoculations of fungi are applied at the optimum time for infection. The biocontrol agent then persists & controls the weed population.

The classical method of biocontrol is achieved over a longer time period & depends upon the gradual increase in disease, the long term development of the biocontrol agent means that significant weed control may take months or even years.

USE OF FUNGI TO PRODUCE CHEMICAL PESTICIDES & BIO-FERTILIZERS -

Biologically active secondary metabolitises produced by fungi are being evaluated as potential pesticides, particularly for use in controlling plant growth. These compounds have the advantage over convential pesticides in being effective at very low concentrations while proving essentially non-persistant & harmless to the environment.

USE OF FUNGAL INOCULANTS TO IMPROVE CROP GROWTH -

Many of soil bacteria & fungi can liberate plant- available phosphorus from insoluble phosphates. This ability is usually associated with the production of organic acids & chelating agents, including citric acid & 2 – cetogluconic acid . Insoluble phosphates are found in abundance throughout the world, mainly as rock phosphate. Since rapidly available phosphate is desirable, the release of phosphorous, from these materials by natural weathering is far too slow to be a general benefit in agriculture. The rate of such phosphate release can, however, be stimulated by inoculating a microbial phosphate solubiliser into the soil. This could be a heterophilic bacterium, a member of the genus, Thiobacillus, or a phosphate solubilising fungus. Since fungi are heterotrophs, a nutrient source has to be added together with the inoculant & insoluble phosphate. Suitable carbon sources include molasses & sugarbeet bagasse.
Mycorrhizas are symbiotic associations between soil fungi & most higher plants. It was soon recongnized that mycorrhizal infection can often greatly increase the rate of uptake of nutrients such as nitrogen & phosphorus from nutrient-deficient soils. This has led to the view that the inoculation of mycorrhizal fungi into soils should lead to an increase in the uptake of these essential plant nutrients.MYCORRHIZAL FUNGI AS INOCULANTS FOR USE IN IMPROVING CROP GROWTH -

Two types of mycorrhiza have been recongnized, the endotropic or vesicular -arbuscular mycorrhiza (VAM) , & the ectotrophic type. In VAM the fungal partner is restricted to the cells of the plant cortex where it grows within & without the cells, invaginating the host cells at intervals to form a dichotomously branched structure called the arbuscle, thought to be the site of nutrient exchange between plant & fungus. The fungal partner appears to have no independent existence is soil. Neither is the interactioin specific, since a single species of fungus can infect a wide range of plant, including most crop species. It is for this reason that VAMS are economically the most important type of mycorrhizal association.

In , the fungal partner forms a tight sheeth around the plant root & from this sheath hyphae grow into the outer cortex to form a network called the Hartig net. Ectotrophic mycorrhizas, unlike VAMS , tend to be non-specific. The fungal partner is readily grown in culture, and forms symbioses with trees in temperate forests.

Vesicular-arbuscular mycorrhizas can directly enhance the uptake by plants of essential nutrients such as phosphorus, copper & iron on the other hand, Zinc & manganese uptake may be reduced, so that the mycorrhizal association many protect some plants from the toxic effects of the larger amounts of these elements. Ectotrophic mycorrhizas also show enhanced uptake of phosphorus, and by mineralising organic nitrogen make nitrogen available for plant use. They may also protect their plant hosts from heavy metals & attack by pathogens, and they may also help increase the uptake of water from soil to plants.

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