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"Arbuscular mycorrhizae" is the scientific way of referring to a specific type of plant fungi, of the Glomeromycota phylum (one of six fungal phyla), which 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 absorb water and nutrients by vastly increasing the surface area of the roots with its hyphae — long, branching filamentous cells. In return, the fungus gets valuable carbon and other essential biochemicals. The singular form of "arbuscular mycorrhizae" is arbuscular mycorrhiza.
What separates arbuscular mycorrhizae from other forms of fungi that live within plants is the hyphae of the mycorrhizae pierce the cell walls of the host plant. Within the plant, branching structures called arbuscules serve as the interface of nutrient exchange with the plants. These go in and out of cells. The symbiosis is so close that the expression of DNA of the plant changes in cells where arbuscules are present, the cell's cytoskeletons form around the arbuscules, and the cell's vacuoles shrink to make room for the structures.
Arbuscular mycorrhizae is an ancient symbiotic strategy, dating back to at least the middle Ordovician, about 460 million years ago. This is only about 15 million years after the appearance of the first land plants in the fossil record, and it is plausible that the symbiosis is so ancient that the common ancestor of all land plants engaged in it. The widespread presence of this fungus in so many modern genera suggests strongly it was present in the land plants from which all extant land plants evolved. Another symbiosis between fungi and photosynthetic organisms are lichen, which are made of closely interlocking fungi and algae. Lichen may have been among the first life on land.
A challenge to the scientific study is that the fungus cannot be grown in culture — it is so interdependent on plants to survive, it dies without them. This makes arbuscular mycorrhizae obligate symbionts. Recently, scientists found a species of arbuscular mycorrhizae-like fungus, Piriformospora indica, that can be grown in culture. This strain of fungus is an important symbiont of the medicinal plant Adhatoda vasica.