Mycorrhiza
A mycorrhiza is a symbiotic association between a fungus and a plant, in which fungal hyphae and plant roots become interconnected and form an interface on the cellular level. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, the plant root system and its surroundings. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.
In a mycorrhizal association, the fungus colonizes the host plant's root tissues, either intracellularly as in arbuscular mycorrhizal fungi, or extracellularly as in [|ectomycorrhizal] fungi. The association is normally mutualistic. In particular species, or in particular circumstances, mycorrhizae may have a parasitic association with host plants.
Definition
A mycorrhiza is a symbiotic association between a green plant and a fungus. The plant makes organic molecules by photosynthesis and supplies them to the fungus in the form of sugars or lipids, while the fungus supplies the plant with water and mineral nutrients, such as phosphorus, nitrogen, or zinc, taken from the soil. Mycorrhizas are located in the roots of vascular plants, but mycorrhiza-like associations also occur in bryophytes and there is fossil evidence that early land plants that lacked roots formed arbuscular mycorrhizal associations. Most plant species form mycorrhizal associations, though some families like Brassicaceae and Chenopodiaceae cannot. Different forms for the association are detailed in the next section. The most common is the arbuscular type that is present in 70% of plant species, including many crop plants such as cereals and legumes.History of research
Mycorrhizal fungi were first formally described and named in 1885 by German botanist A.B. Frank, who coined the term "mycorrhiza".Earlier observations by others also contributed to understanding these root-fungal associations. In 1881, Franciszek Kamieński had shown that the plant Monotropa hypopitys relied on fungi from neighboring trees for nutrition, highlighting fungal dependency. Noël Bernard established in 1899 that orchids like Neottia nidus-avis require a specific fungus for seed germination.
In the 20th century, researchers expanded understanding of mycorrhizal diversity and prevalence across plant families. Paleobotanists working on the Rhynie chert fossils demonstrated that mycorrhizal-like associations existed 407 million years ago in early land plants.
Laccaria bicolor became the first ectomycorrhizal fungus to have its genome sequenced in 2008, revealing the genetic basis of symbiosis through gene duplications and specialized secreted proteins. This work opened the molecular era of mycorrhizal research.
Evolution
Emergence alongside terrestrial plants
Fossil and genetic evidence indicate that mycorrhizae emerged as early as 450-500 million years ago, potentially between fungus-like protists and algae. Arbuscular mycorrhizal relationships appeared earliest, coinciding with the terrestrialization of plants. Genetic evidence indicates that all land plants share a single common ancestor, which appears to have quickly adopted mycorrhizal symbiosis, and research suggests that proto-mycorrhizal fungi were a key factor enabling plant terrestrialization. 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.Fossil record and genomic analysis
Fossils of Glomeromycotan spores and hyphae date to 460 million years ago, but the fossils were not associated with plants. The earliest terrestrial communities were similar to modern biocrusts. Lichen-like associations between fungi and cyanobacteria were an important part of these communities. The first land plants were similar to mosses, with simple vascular systems and no leaves or roots.The earliest direct fossil evidence of early mycorrhizal symbiosis is found in the 407 million year old Rhynie chert, which contains an assemblage of "exceptionally preserved" fossil plants colonized by multiple para-mycorrhizal fungi. The Rhynie chert fossils show Glomeromycotan and Mucoromycotan fungi engaged in mycorrhiza-like associations with cells of the plants Aglaophyton major and Horneophyton lignieri. The fossils suggest a mutualistic association between the plants and the colonizing fungi, because distinctive nutrient exchange structures are preserved and the colonized cells appear to have been alive at the time of infection by the fungus.
These early associations are referred to as mycorrhiza-like or para-mycorrhizal because mycorrhiza are defined by the fungal colonization of plant roots, and early plants did not have any roots. The earliest fossils of arbuscular mycorrhizal fungi in plant roots originate from 315-303 million years ago, and show fungi belonging to Glomeromycotina in the root systems of a giant lycophyte, Lepidodendron, and an early relative of the conifers, Cordaites. Arbuscular mycorrhizal fungi were found in 240 million year old fossils of Antarcticycas schopfii.
Ectomycorrhizae developed substantially later, during the Jurassic period, while most other modern forms of mycorrhizal symbiosis, including orchid and ericoid mycorrhizae, date to the period of angiosperm radiation in the Cretaceous period. Ectomycorrhizae appear in the fossil record 48.7 million years ago, in the Eocene, with a fossil of ectomycorrhizal fungi colonizing Pinus roots. However, it is 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.
Fossils preservation of Ericoid mycorrhizae and orchid mycorrhizae is lacking. Calibrated molecular phylogeny is used to estimate when these mycorrhizal types originated. The origins of orchid mycorrhizae are unclear, though orchids themselves are thought to have originated in the Cretaceous period. Ericoid mycorrhizae are estimated to have the most recent evolutionary origins of mycorrhizal types, evolving around 118 million years ago from free-living saprotrophic ancestors. Ericoid mycorrhizal fungi evolved from multiple lineages of fungi, primarily ascomycetes from the Leotiomycetes, as well as basidiomycetes from the family Serendipitaceae.
Origins in plants
In plants, the genes for forming mycorrhizal symbiosis are highly conserved and originate from a common ancestor, meaning that the ability to form mycorrhizae is ancestral to all land plants. Non-mycorrhizal plant lineages, such as the Brassicaceae, lost the ability to form mycorrhizae at some point in their evolution. The earliest mycorrhizae were arbuscular mycorrhizae, and other forms, such as ectomycorrhizae and orchid mycorrhizae, evolved when plant hosts switched from symbiosis with Glomeromycotina to symbiosis with different fungal lineages.There is genetic evidence that the symbiosis between legumes and nitrogen-fixing bacteria is derived from mycorrhizal symbiosis. The modern distribution of mycorrhizal fungi appears to reflect an increasing complexity and competition in root morphology associated with the dominance of angiosperms in the Cenozoic Era, characterized by complex ecological dynamics between species.
Origins in fungi
In fungi, mycorrhizal symbiosis had multiple independent origins among different lineages of fungi. Arbuscular mycorrhizal fungi form their own monophyletic phylum, whereas other mycorrhizal fungi convergently evolved similar lifestyles.Arbuscular mycorrhizae
The phylum Glomeromycota, which forms the arbuscular mycorrhizal symbiosis, is the oldest mycorrhizal lineage. The arbuscular mycorrhizal symbiosis evolved only once in fungi; all arbuscular mycorrhizal fungi belong to Glomeromycota and share a common ancestor. 244 species have been identified based on differences in the appearance of their spores, but genetic studies suggest that 300-1600 species may exist in Glomeromycota. All members of Glomeromycota are obligate biotrophs, entirely dependent upon their plant hosts for survival. Arbuscular mycorrhizal fungi are considered to be generalists, with minimal host plant specificity. AM symbiosis has been observed in almost every seed plant taxonomic division, or around 67% of species. Arbuscular mycorrhizae take on most angiosperms, some gymnosperms, pteridophytes, and nonvascular plants as plant hosts. Arbuscular mycorrhizae have been observed in the seedling stage of otherwise ectomycorrhizal partners, suggesting that arbuscular mycorrhizal fungi may be able to infect almost any land plant given proper circumstances.Other forms of mycorrhizal symbiosis, such as ectomycorrhizae, orchid mycorrhizae, and ericoid mycorrhizae, emerged multiple times in different lineages of fungi through convergent evolution. Unlike arbuscular mycorrhizal fungi, some of these fungi are only facultatively symbiotic, and can live by themselves without a plant host under some conditions.