Mycorrhiza

Realm Soil Ecology

In the Begining

Fungi clearly existing before. But they were free standing or on stems. Now some were developing relations with roots, which we now call mycor (fungal) rhizas (roots). The first ones lived inside (endo) roots and called Arbuscular mycorrhiza fungi - AMF. The fungi penetrate the cells of the roots of a vascular plant forming structures within the cells called arbuscules, meaning ‘shrublike’ hence AMF. In other words the structures look a bit like trees themselves.This was to a monumental step in soil evolution. But how did it come about and why?

Mycorrhiza can take in water 10X that which roots alone could do, because of water tensions. AMF maintains the 'hydraulic continuity' between root and soil, especially when soils dry out; this is increasingly seen as making them ever more important.

Rhizomes

The Early Devonian (around 400mya) saw the development of terrestrial flora. Evidence of small plants have been found in the Rhynie chert – named after the village of Rhynie in Scotland, where there is a sedimentary deposit with fabulous fossils- from around 400 mya. It seems that Rhynie is based on peat which implies that plants were growing and dying but not decomposing.

Also found there are structures resembling vesicles and spores of the fungus Glomus species – still found today. Colonized fossil roots have been observed in spore bearing Aglaophyton and Rhynia – small plants less than 20cm high. These ancient plants possess characteristics of vascular plants and mosses and liverworts and have rhizomes – which are essentially underground modified stems. Fungi commonly colonize rhizomes of vascular plants and the ribbons attached to substratum (thalli) of early-diverging land plants, likely liverworts and hornworts. So mycorrhiza preceded true roots. These early fungal associations are sometimes called mycorrhiza-like or ‘para’ mycorrhizas because the early plants had no true roots, but the general consensus is now that they are the first mycorrhiza

Four sorts

The first to evolve - around 400 mya is the Endo mycorrhiza, or Arbsucular Mycorrhiza (AM). The fungi enter into the plant roots.

A couple of hundred million years later, a form that surrounds the root evolved - called Ectomycorrhiza. ECM

Other later mycorrhiza are for heather and orchids. The latter are well studied because the plant is v valuable, and the fungi seem to start in the seed.

Glomalin

The root/fungal relationship produces the compound 'Glomalin' an important feedstuff for many soil creatures, particularly collembola. While feeding, the collembola accidently pass on fungal spores which otherwise would find hard to distribute in the soil.

Note: The roots do not produce glomalin as a 'root exudate', but need the fungi to do that.

This process is aerobic, ie needs oxygen to work.

Collembola poo out 'Glomalin Related Soil Protein'. Since its discovery in mid 1990s there has been increasing confusion between Glomalin and GRSPs. Perhapd the collembola can explain it.

Soil evolution

This diagram, by Royal Society shows the rise of mycorrhiza. It is also about the only diagram ever which drew 'developed soil', and giving a rough estimate of what happened when.
Only another 300 million years to go. With all the creatures too. Which is what my book 'Story of Soil' will attempt to do.

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