What is Mycorrhizas?

Mycorrhizas are associations of roots with fungi, found in some $80\,\%$ of plant species and in the earliest land-plant fossils. In arbuscular mycorrhizas the fungus (a Glomeromycete) enters the root cells and branches into a tree-like arbuscule across which phosphate, nitrogen and water pass to the plant and sugars and lipids to the fungus; the fungal hyphae extend the absorbing surface of the root a hundredfold and reach phosphate the root cannot. In ectomycorrhizas, of forest trees, the fungus sheathes the root and grows between the cells. The plant genes that admit the fungus are the same common symbiosis pathway — receptor, calcium spiking, transcription factors — that legumes reuse for rhizobia: the nodule is a late invention built on a fungal partnership four hundred million years older.