Mycorrhiza (EcM)

200-145 mya Jurassic

Palaeosols Macro-aggregation Plants Earthworms Insect s Higher Oribatids

EcM

We saw, over 400mya, the vital symbiotic role between fungi and plant roots - the mycorrhizal fungi. The first mycorrhiza were a particular form of fungi called Endomycorrhiza (AMF – arbuscular mycorrhiza), meaning they lived IN the roots. In this period, we see the arrival of a different sort of mycorrhiza – living AROUND the roots, called Ectomycorrhiza (EcM).

Ectomycorrhiza (EcM) have evolved many times (Strullu-Derrien et al 2018). "Ectomycorrhizal (EcM) symbiosis in Pinaceae probably originated in the continent of Laurasia during this period" (Rothwell et al., 2012). ‘

The most familiar EcM to us are ‘truffles’, the famous delicacy.

The mechanism of this symbiosis involves many steps "Both roots and fungus release an array of various metabolites (morphogens and signalling molecules) that establish a molecular cross-talk between symbionts". (Felton et al 2011).

Truffles & Springtails

"Some Collembola groups may be attracted by the fungal metabolites produced by Tuber aestivum (truffle), while other Collembola and other microarthropods (Symphyla & Pauropoda who have been around for 100+my) may find an unfavourable environment in the soil of the brûlé." Menta et al 2014. Brûlé eh?

Why Now?

What was it about this period that gave an evolutionary advantage of ectomycorrhiza over endomycorrhiza, that have been around for 200 mys?

EcM symbioses appear to have evolved in boreal forests that are relatively productive but in which nutrient cycling is still limiting.

Both are more effective than tree roots alone, but it seems EcM nutrient handling is more effective.

EcM have the capacity to mine soils more. They can go where the roots cannot

"Hyphal tip growth is principally driven by internal pressure and the growing tip may thus exert considerable pressure at the micrometre scale upon interaction with surfaces" (Rosling et al 2009), which has been modelled (Goriely & Tabor 2008)

EcMs are known to occur in deep tree roots (> 2 metres), some 4 meters. (Robin et al 2019)

Mycelia

EcM enable the roots to go deeper, and so help plants build these underground structures which also help aerate the soil and hold water round the roots, enabling the trees to grow better.

"The extensive growth of their mycelia enables the fungi to colonize and exploit nutrient-rich substrates, and to assimilate and translocate (reallocate) nutrients and carbon in soils " (Rosling et al 2009), particularly Nitrogen (Zui et al 2023) and Phosphorus. "Solubilisation of inorganic P (Pi) and hydrolysis of organic P by EcM fungi in soil occurs largely at the growing mycelial front, where Pi absorption is facilitated by high affinity transporters" (Cairney 2011)

This mixture would have been enough to prove an evolutionary advantage for these fungi at this period, especially if mineral soils were getting deeper.

EcM symbiotic with woody roots are ubiquitous in boreal (permanently cold) forest soils, where Podzols may have been be common

A comparison of the sporocarps (left) and ectomycorrhizal root tips (right) formed by several common ectomycorrhizal fungal species:
(A) Paxillus involutus; (B) Scleroderma citrinum;
(C) Suillus luteus; (D) Thelephora terrestris;
(E) Xerocomellus pruinatus.
Scale bar = 1 mm.
Photos courtesy of the Institute of Dendrology, Polish Academy of Sciences.

Hyphae

Ectomycorrhizas are differentiated from other mycorrhizas by the formation of a dense hyphal sheath, known as the mantle, surrounding the root surface.

Unlike AM fungi, hyphae of EM fungi do not penetrate into the root cells but are intercellular. The hyphae penetrate into the root cortex where they form a hyphal network (“Hartig net” right) in the intercellular space through which minerals and nutrient materials are exchanged between the fungus and the plant.

Hartig

“The ECM, is distinguished on the basis of a sheath of fungal hyphae enveloping the root and an intercellular penetration pattern where the hyphae form a network between cortical cells called a Hartig net. Many species of Ascomycota, Basidiomycota and a few members of the genus Endogone (Mucoromycotina) form ECMs. The associated plants are mostly shrubs and trees from temperate, boreal and Mediterranean regions, but there are also some ecologically important tropical families." (Strullu-Derrien et al 2018)

Origins

EcMs and the fungal genomic data allow us to understand the iterative nature of their evolution. They probably evolved from saprophytic fungi that had been around for several hundred million years. EcM plants and fungi exhibit a wide taxonomic distribution across all continents (apart from Antarctica). Pines are the oldest existing plant family in which symbiosis with EcM fungi occurs. Fossils from this family date back to 156 mya EcM are associated mainly with trees and perennial plants – now more commonplace than the original mycorrhizal (AM) association. What were the circumstances which led to the development of this other form of mycorrhiza and why did it wait so long?

Multiple evolution

Ectomycorrhizas evolved independently numerous times, not only in plants (18 times in angiosperms, but also in fungi (78–82 times) and also over an extended geological period. It may even be that some current lineages of fungi are shifting (or able to shift) to the ECM. Multiple origins of ECM indicate the action of important evolutionary drivers. Switching of nutritional mode is considered to be a key driver" (Strullu-Derrien et al 2018)

Associations

“During the mid‐Jurassic, the supercontinent Pangaea started to rift, giving rise to modern continents and ocean basins. Geochemical models indicate a high CO2 atmosphere through the period with a maximum of four to seven times the present‐day level. Ectomycorrhizal (ECM) symbiosis in Pinaceae probably originated in the continent of Laurasia during this period”. (Strullu-Derrien et al 2018)

Trees

Pinaceae is the oldest existing plant family in which symbiosis with EcM fungi occurs, and fossils from this family date back to 156 million years ago (Lepage et al 1997)

It has been proposed that habitat type and the distinct functions of different mycorrhizas help determine which type of symbiosis is predominant in a given area. Ectomycorrhizas are intermediate in their ability to take up nutrients, being more efficient than arbuscular mycorrhizas, making them useful in an intermediate nutrient situation.

EcM trees

Some examples of trees commonly associated with ectomycorrhizal fungi (ECM):

Conifers such as pines (Pinus spp.), spruces (Picea spp.), and firs (Abies spp.).
Broadleaf trees like oaks (Quercus spp.), beeches (Fagus spp.), and birches (Betula spp.).
Some tropical trees, such as certain species of Dipterocarpaceae in tropical rainforests
Oaks (Quercus spp.): Fossil evidence suggests that the genus Quercus first appeared in the Paleogene (66-23mya) period, with various species diversifying throughout the Cenozoic (66-0mya) era. Some suggest appear around 35 to 40 mya.
Beeches (Fagus spp.): Fagus likely originated in the late Paleocene (66-56mya) whenn fossilsl found to early Eocene period (50 to 55 mya).
Birches (Betula spp.): Betula is believed to have originated in the same Paleogene period. Fossil have been found from 50 mya.

Weathering

EcMs diversified out of the tropics by various fungi switching plant hosts from tropical to temperate over the next hundred million years. EcM fungi may have contributed to the long‐term cooling trend later. Plant roots and their associated mycorrhizal fungi enhance the weathering of calcium‐magnesium silicates, which draws CO2 out of the atmosphere and thus would have contributed to episodes of global climate cooling observed over the next 120 million years. EcM would have helped build soils in this period, sequestering carbon dioxide.

Geomycology

"The term geomycology, by definition an interdisciplinary research topic, has been coined for the study of the role fungi play in biogeochemical processes including mineral weathering in terrestrial ecosystems...The extramatrical mycelia of ectomycorrhizal fungi may thus provide a direct link between photosynthetically-assimilated carbon from trees and weathering of soil minerals...models can be developed to encompass the functional diversity " (Rosling et al 2009)

Necroglue?

The early soils would have held together thanks to GRSPs from AMF, whereas EcM can effectively burrow deeper into mineral soils like nothing else providing a more extensive rhizosphere. But they do not seem to produce glomalin.

We saw - about 200 million years back - that endomycorrhiza (AMF) provide the bulk of glomalin (& GRSPs) in soil. It has proved impossible to find any link between ectomycorrhiza and glomalin. "AMF predominates in well studied agricultural systems". (Irving et al 2021).

EcM & Soil

This is about the extent EcM affects soil

EcM fungal species differ in their effects on root hydraulic conductivity (and causing different amounts of soil to adhere to roots)

EcM richness influences root biomass

EcM species differ in mycelium architecture (e.g. cord, fan formation)
(Rillig & Mummey 2006)

The authors suggest an area of study would be "The addition of AMF necromass compared with necromass from ectomycorrhizal fungi, saprotrophic fungi or bacteria would show if the Glomeromycotina are uniquely beneficial for soil, or if ‘glomalin’ might better be described as a generalised ‘necroglue’". (Added to Glomalin Research Gaps) My money is on Glomus fungi

Endomycorrhiza 400-360mya

Mycorrhiza (ErM & OM)

N-fixing nodules

There are still two other forms of mycorrhiza to evolve. With so many variations of plants and fungi, it is not that surprising that particular forms for heather and orchids were produced later.

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