Mycorrhiza

A mycorrhiza (from Ancient Greek μύκης (múkēs) 'fungus' and ῥίζα (rhíza) 'root'; pl. mycorrhizae, mycorrhiza, or mycorrhizas) is a symbiotic association between a fungus and a plant. 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.

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, 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.

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.

The 400 million year old Rhynie chert contains an assemblage of fossil plants preserved in sufficient detail that arbuscular mycorrhizae have been observed in the stems of Aglaophyton major, giving a lower bound for how recently arbuscular mycorrhizal symbiosis may have developed. The earliest root-confined arbuscular mycorrhizae observed come from a fossil where hyphae are seen colonizing the rootlet of an arborescent clubmoss, forming arbuscules.

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. The earliest confirmed ectomycorrhizal fossil dates back to the Eocene approximately 48 million years ago, 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.

Ericoid mycorrhizae 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.

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.