Australia: The Land Where Time Began

A biography of the Australian continent 

Mycorrhiza

A great many plants are assisted by certain fungal species to absorb water and nutrients from the soil. The roots of plants form relationships with mycorrhizal fungi which are intimately associated with the cells of the roots and fungal hyphae extend further into the surrounding soil than the unassisted roots and thereby increase the nutrients and water available to the plant. The surface area that is available for the absorption of minerals is greatly increased by this mycorrhizal association as there are many fine hyphae which make the collection of nutrients and water more efficient. This association between the plant and the fungus is mutualistic, with the plant sharing up to 25 % of the photosynthetic products with the fungus, so both mycorrhizal partners gain from their association. The plant grows much better than it would without the mycorrhizal fungus, in spite of the amount of sugar that goes to the fungus. Some mycorrhizal fungi, ‘ectomycorrhizas’, form a mat of fungal tissue around the plant root and then the fungal cells grow between the cells of the root, though they never penetrate the plant cell walls. In another mycorrhizal partnership, endomycorrhiza, the fungal hyphae enter the cells of the plant root. These penetrating hyphae form structures inside the root cells specifically for the exchange of nutrients with the cytoplasm of the plant cells. Mycorrhizas greatly improve the ability of the plant to tolerate drought as well as other extremes such as extreme temperature and soil acidity by greatly increasing the surface area of the plant root system in contact with the surrounding soil. It has been found that as many as 95 % of all plants have formed mycorrhizal associations, some indication of the importance of these fungal associations for the growth of so many plants, which includes all the crop plants and all the trees in all the forests. More than 6,000 species of fungi have been found to be capable of forming mycorrhizas. Mycorrhizal associations developed very early in the colonisation of the terrestrial environment, evidence of these early mycorrhizal associations having been found going back more than 600 Myr, evidence that these associations helped in the formation of the world as it is at the present.

There are also some animals that Moore describes as important that form mutualistic associations with fungi. Such animals are known as ruminants, such as sheep, goats, cattle, deer, and giraffes. Ruminants have a specialised stomach that is divided into 4 chambers for the digestion of their diet that is exclusively vegetarian. The chamber the food first enters is the rumen, hence they are ruminants. Ruminants spend most of their time eating grass and hay. The cell walls of plants contain cellulose but ruminants, like other animals, cannot produce the enzymes necessary for the digestion of cellulose, with the result that they need help to extract the nutrients from grass. In ruminants this is overcome by chytrids, special fungi that live in the rumen, aka rumen fungi.

Chytrids are anaerobic fungi, so don’t require oxygen to digest plant cell walls as they produce cellulases, special enzymes that break down the cell walls. The rumen acts as a fermenter as the grass is stored there while the cellulases breakdown the cellulose as well as other compounds in the plant material to produce nutrients to allow continuing production of more chytrids. Large populations of bacteria and protozoa are also supported by the nutrients that are released, which also form symbiotic relationships with chytrids, and with one another, in the gut of the ruminant. The plant material that has been processed in the rumen, known as ‘cud’, is regurgitated into the mouth of the animals where it is chewed some more to grind it down even more. When the cud is swallowed it passes through the next 3 chambers of the stomach which contain large communities of microorganisms which are then digested by the animal. Along this process the otherwise indigestible plant material is converted to microorganisms which are easy to digest. The same occurs in all ruminants.

Chytrids are found in large numbers in the dung of ruminants, though they are believed to be transferred between one animal and the next in saliva, as well as from the dung to the grass then to another ruminant in the herd. The relationship between ruminants and chytrids is mutualistic. The fungi use some of the nutrients in the ruminant’s food and live a protected life in the environment of the rumen, and they are spread among ruminants in the saliva and dung. This relationship applies to farm animals, such as cattle, sheep, etc.

Sources & Further reading

  1. Moore, David, 2013, Fungal Biology in the Origin and Emergence of Life, Cambridge University Press.

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated:  05/12/2015 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading