Sunday, March 22, 2009

Magic Fungi: Mycorrhiza.

Mycorrhizae are highly specialised organisms classified among the order Glomales, and are found closely associated with the root systems of around 95% of all plants. The term mycorrhiza derives from the Greek for fungus roots. The fungus may colonize the roots of a host plant either intracellularly or extracellularly and is an essential part of living soil. The relationship is a symbiotic one, in which both organisms derive benefit. Because the fungus cannot perform photosynthesis, to fix its own carbon, it receives some of the carbohydrates (sugars such as glucose and sucrose) which the plant passes down to its roots. In return, the plant receives essential mineral nutrients and water too, from the fungus via its very extensive mycelium which reaches out much more extensively further than the plant roots, and effectively forms a secondary root system.
e.g. On their own, plant roots may be ineffective at imbibing immobile phosphate anions, for example if they are present in alkaline soils (pH above 7). However, the mycorrhizal fungus can access these phosphorus sources via its mycelium, and pass them on to the plants they have colonised. Mycorrhizal mycelia have far narrower diameters than even the smallest root, and can hence penetrate more of the soil, so allowing absorption over a greater surface area.
Mycorrhizal plants are often more resistant to diseases caused by microbial soil-borne pathogens, and more readily survive under drought conditions because they can access water more easily. Tilling soil damages the mycelium and so they work best with no-till methods such as permaculture, where they are very useful in transporting nutrients and water throughout the growth medium.
The two principal forms of mycorrhizae are Ectomycorrhizae and Endomycorrhizae. The Ectomycorrhizal Fungi have a thick network of cells which form a sheath around the root hairs of the associated plant and do not penetrate into the cells of the plant, hence the prefix 'ecto' meaning outside (in contrast 'endo' means inside). The Endomycorrhizal Fungi are a more primitive form and have hyphae which do penetrate the root cell walls and on into the cell membrane. They do not, however, enter the protoplasm. Inside the root cells the fungal structure may be tree-like, with fine hair-like hyphae that can access plant nutrients and water through an extensive secondary root system.

One well-known Ectomycorrhizal fungus is the truffle, readily sniffed-out by a pig on a lead. There are several species of truffle, the best known being the Black Truffle T. melanosporum which grows exclusively with oak trees. It is found that when they are grown in a sterile medium plants often do not thrive without a beneficial fungal comrade. To allow new plants to become established more quickly or to get a better growth of existing plants, fungal spores can be added to the soil.
Mycorrhizal symbiosis was discovered around 100 years ago, and since then there has been much speculation as to its role in nitrogen fixation by plants. While there are numerous reports of the fixation of atmospheric nitrogen by mycorrhizal fungi in the earlier literature, it is now thought that only procaryotic organisms can fix atmospheric nitrogen and that both ecto- and endomycorrhizal fungi lack this capacity. It is important to note that many vascular plants possess both mycorrhizae and nitrogen-fixing symbiotic organs, e.g. legumes with rhizobial nodules and non-legumes with actinorrhizal nodules, with mycorrhizae that are either ectotrophic or endotrophic, or both. Nitrogen fixation in forests and other natural ecosystems has recently been attributed mainly to associative-symbiotic bacteria, i.e. bacteria living in the rhizosphere or close proximity of plant roots. Since the roots, in fact, are usually also infected by mycorrhizal fungi, a new concept of mycorrhizosphere has been introduced.

The exact nature of the relationships between mycorrhizal fungi and nitrogen-fixing bacteria within the mycorrhizosphere are as yet not well understood. Nitrogen-fixing bacteria have been found even inside the fungal mantle of ectomycorrhizae, and so the circumstantial evidence is overwhelming that an interplay occurs between the two organisms, presumably to their mutual benefit. Permaculture systems are thought of as nature acting on a series of overlapping layers, where nutrients that are captured at one level are passed down to another, or may form part of the symbiotic mechanism of an individual layer. The forest garden principle which is a series of clearings cut into forest is the supreme example of this action of biodiversity, where each layer and each organism feeds another, throughout forming a balanced ecosystem.

Related Reading.
[1] P.U. Mikola, Relationship between nitrogen fixation and mycorrhiza, World Journal of Microbiology, 1986, 175-282.

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