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Arbuscular mycorrhizae are a type of plant fungi that have a symbiotic relationship with at least 80% of vascular plants, helping them absorb water and nutrients. The fungus pierces the cell walls of the host plant and forms branching structures called arbuscles for nutrient exchange. This ancient symbiotic strategy dates back to at least 460 million years ago and cannot be grown in culture. Piriformospora indica is a mycorrhiza-like species of tree fungus that can be grown in culture and is an important symbiont of the medicinal plant Adhatoda vasica.
“Arbuscular mycorrhizae” is the scientific way of referring to a specific type of plant fungi, of the phylum Glomeromycota (one of six fungal phyla), that has had a symbiotic relationship with at least 80% of vascular plants. Arbuscular mycorrhizae may be the most abundant type of fungus on Earth. The fungus helps the plant to absorb water and nutrients by significantly increasing the surface of the roots with its hyphae, long and branched filamentous cells. In return, the fungus gets valuable carbon and other essential biochemicals. The singular form of “arbuscular mycorrhiza” is arbuscular mycorrhiza.
What separates arbuscular mycorrhizae from other forms of fungi that live inside plants is that the hyphae of the mycorrhizae pierce the cell walls of the host plant. Within the plant, branching structures called arbuscles serve as an interface for the exchange of nutrients with plants. These go in and out of cells. The symbiosis is so close that the expression of plant DNA changes in the cells in which the shrubs are present, the cytoskeletons of the cell form around the shrubs, and the vacuoles of the cell constrict to make room for the structures.
Arbuscular mycorrhizae are an ancient symbiotic strategy, dating at least to the Middle Ordovician, about 460 million years ago. This is only about 15 million years after the first land plants appeared in the fossil record, and it is plausible that symbiosis is so ancient that the common ancestor of all land plants engaged in it. The widespread occurrence of this fungus in so many modern genera strongly suggests that it was present in the land plants from which all extant land plants evolved. Another symbiosis of fungi and photosynthetic organisms are lichens, formed by closely related fungi and algae. The lichen may have been among the first living things on earth.
One challenge to scientific study is that the fungus can’t be grown in culture—it’s so interdependent on plants for survival, it dies without them. This makes arbuscular mycorrhizae obligate symbionts. Recently, scientists discovered a mycorrhiza-like species of tree fungus, Piriformospora indica, that can be grown in culture. This mushroom strain is an important symbiont of the medicinal plant Adhatoda vasica.
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