Before plants had roots, fungi did the job for them
When plants first crept onto land, they had no roots and the soil held little they could reach. Fungi threaded into their cells and fetched minerals such as phosphate in exchange for sugar. That trade, the mycorrhiza, is still running beneath most of the world's plants today.
A mycorrhiza, from the Greek words for fungus and root, is a partnership in which fungal threads and plant tissue meet at the level of individual cells. The plant hands over sugars or fats made by photosynthesis; the fungus returns water and nutrients such as phosphorus, nitrogen and zinc. The arrangement is usually mutual, although in some species or conditions it tips into parasitism. The German botanist A. B. Frank coined the term in 1885, after Franciszek Kamieński had shown in 1881 that the plant Monotropa hypopitys fed via fungi linked to nearby trees.
The partnership is extraordinarily old. Genetic and fossil clues place its origin 450 to 500 million years ago, and specialists broadly agree that fungi served as a primitive root system. The Rhynie chert, a 407-million-year-old deposit, preserves living plant cells of Aglaophyton major colonised by fungi with nutrient-swapping structures intact. Because those plants had no roots, researchers call such links para-mycorrhizal. True arbuscular mycorrhizae inside roots appear by 315 to 303 million years ago, in the giant clubmoss Lepidodendron.
Later forms arose repeatedly. Ectomycorrhizae, which sheath roots rather than entering cells, probably began with the pine family around 175 million years ago; ectomycorrhizal fungi have evolved separately around 80 times, often from wood-rotting ancestors that lost decay genes. Ericoid mycorrhizae, the youngest at about 118 million years, help heathland plants recycle minerals from tough, nitrogen-poor litter. Even the bond between legumes and nitrogen-fixing bacteria seems to descend from mycorrhizal machinery.
Arbuscular partners are the most widespread, found in roughly two-thirds to 70% of plant species, cereals and legumes among them, while families such as the cabbage family have lost the ability altogether. A 2008 genome of the fungus Laccaria bicolor opened the molecular era. Newer experiments add nuance: one greenhouse test of nine herbs found fungal inoculation barely changed how precisely roots sought out nutrient patches.
Source: Mycorrhiza
