Draft:MycorrhizaEvolution

Mycorrhizal relationships are a form of plant-fungi mutualism between plant roots and fungal hyphae. It is estimated that somewhere between 85-90% of terrestrial plants establish mycorrhizal relationships with fungi. All major lineages of land plants excluding mosses form mycorrhizal relationships. In mycorrhizal relationships, plants transfer carbon produced through photosynthesis to fungi while fungi return mineral nutrients, water, and perhaps pathogenic resistance to the host plant. Mycorrhizal relationships were likely crucial in terrestrial plant colonization some 450-500 million years ago, suggesting that mycorrhizal relationships are coincident with the evolution of terrestrial flora. Mycorrhizal relationships have independently evolved from saprotrophic fungi a number of times, and in effect mycorrhizae have developed multiple modes of exchange between root cells and hyphae. There are three major forms of mycorrhizal relationships which have evolved independently of one another, the oldest being arbuscular mycorrhizae, followed by ectomycorrhizal relationships, and most recently ericoid mycorrhizal relationships.

Arbuscular Mycorrhizae

Arbuscular mycorrhizae are the oldest and most frequent form of mycorrhizal relationship. Arbuscular mycorrhizae establish nutrient exchange through penetrating the root cortical cells of the host plant, making the relationship endomycorrhizal (inside the cell) as opposed to the later developed ectomycorrhizae (external nutrient exchange). Arbuscular mycorrhizae leave behind arbuscules, tree-like structures formed through hyphal penetration into the cell. Arbuscular mycorrhizae take on most angiosperms, some gymnosperms, pteridophytes, and nonvascular plants as plant hosts.

Arbuscular mycorrhizas likely evolved alongside terrestrial plants approximately 450-500 million years ago when plants first began to colonize land. Some scholars suggest arbuscular mycorrhizal relationships originated between fungus-like protists and algae during this time. Paramycorrhizae, mycorrhiza-like structures, have been observed in the Rhynie Chert, a 407 million-year-old piece of fossilized earth found in Scotland, setting a lower bound for mycorrhizal relationships. The earliest root-confined arbuscular mycorrhizae observed come from a fossil where hyphae are seen colonizing the rootlet of an arborescent clubmoss, forming arbuscules.

There is a strong consensus among paleomycologists that mycorrhizal fungi served as a primitive root system for early terrestrial plants. This is because, prior to plant colonization of land, soils were nutrient sparse and plants had yet to develop root systems. Without complex root systems, early terrestrial plants would have been incapable of absorbing recalcitrant ions from mineral substrates, such as phosphate, a key nutrient for plant growth. There are a number of indicators that all land plants evolved from arbuscular mycorrhizal symbiosis. One strong indicator is that arbuscular mycorrhizae have been observed in the seedling stage of otherwise ectomycorrhizal partners, suggesting that arbuscular mycorrhizae may be able to infect almost any land plant given proper circumstances. Arbuscular mycorrhizal symbiosis occurs between plants and fungi in the division glomeromycota, which has been observed in almost every seed plant taxonomic division, or around 67% of species. As arbuscular mycorrhizae show minimal host plant specificity, and described mycorrhizae species are likely capable of forming relationships with most host plant taxa, this also suggests that terrestrial plants and arbuscular mycorrhizae evolved with one another.

Ectomycorrhizae

Ectomycorrhizae are mycorrhizal relationships formed without the hyphae of the fungi penetrating the root cells of the host plant, instead forming a sheath around the root of the symbiont for nutrient exchange. The earliest confirmed ectomycorrhizal fossil dates back to the eocene approximately 48 million years ago, However it’s believed that the first ectomycorrhizal relationships evolved in the stem group Pinaceae around the radiation of the Pinaceae crown group in the mid Jurassic, 175 million or so years ago.

Ectomycorrhizal relationships have evolved a number of times, in both plants and fungi. In angiosperms, it is believed that ectomycorrhizal partnerships have evolved independently at least 18 times, and in fungi 78-82 times. The main evolutionary driver for ectomycorrhizae is switching of nutritional modes from saprotrophs. Phylogenomic analysis of various ectomycorrhizal fungal genomes has confirmed the convergent evolution of ectomycorrhizal fungi from white and brown-rot fungi, as well as from soil saprotrophs – Ectomycorrhizal fungi likely evolved convergently from saprotrophic origins several times. Some lineages of ectomycorrhizae have likely evolved from endophytic ancestors, fungi that live within plants without damaging them, while others such as Amanitaceae evolved from saprotrophs. Some ectomycorrhizal fungi have gone through apparent evolutionary reversal back into saprotrophic ecology. This is possible because ectomycorrhizal fungi retain enzymes for breaking down lignin. Most ectomycorrhizal relationships are formed between basidiomycetes or ascomycetes and woody trees or shrubs.

Ericoid Mycorrhizae

Ericoid mycorrhizae evolved from a monophyletic origin around 140 million years ago. The earliest ericoid mycorrhizae evolved from saprotrophic ascomycetes. Ericoid mycorrhizae are only present in the Ericales order for plant hosts, and the Leotiales order of fungi. This specialization suggests that ericoid mycorrhizal partners evolved in parallel with one another in response to environmental change, rather than through reciprocal species-to-species level selection.

Ericoid mycorrhizal relationships are found in extremely nutrient poor soils in the northern and southern hemispheres. These environments of low mineral nutrient availability have led to native plants developing sclerophylly, where plants become high in lignin and low in phosphorus and nitrogen. As a result, decaying plant matter in these areas has an abnormally high carbon to nitrogen ratio, making it resistant to microbial decay. Ericoid mycorrhizae have apparently evolved to conserve minerals in nutrient deficient sclerophyllous litter by directly cycling these nutrients throughout the mycorrhiza system. Ericoid mycorrhizae also retain saprotrophic abilities, allowing them to extract nitrogen and phosphorus from unmineralized organic material, and resist negative outcomes from high concentrations of toxic cations in the acidic soil environment.