When early plants started to colonise the land, 410 to 440 million years ago, they had a problem. While the nutrients they needed to grow moved to them in the water, on land they had to move to the nutrients. Most modern-day plants have roots that twist and divide through the soil in search of new nutrients, but these early colonists were yet to evolve roots. The solution was simple – to team up with fungi.

Many forest mushrooms are the ‘fruits’ of extensive networks of ectomycorrhizal fungi associated with tree roots. (David Genney).

Most of us know fungi from the ‘fruiting’ structures that some species produce, i.e. mushrooms. But the main part of a fungus is composed of a myriad of microscopic hair-like structures called hyphae. Anyone who has seen an old slice of bread covered by mould (a fungus) will appreciate how quickly and efficiently they can forage for food and nutrients.

By penetrating the cells of those early plants, fungi were able to take on the job of seeking the nutrients from the soil, passing some of them to the host plant. This wasn’t a one-way partnership, though; in return, the plants provided the fungi with energy in the form of sugars produced through photosynthesis. This was a defining moment for the colonisation of life on land, and this symbiotic relationship is called a mycorrhiza (‘mycos’ meaning fungus and ‘rhiza’ meaning root). Some of the best preserved land plant fossils, discovered in Scotland near the village of Rhynie, Aberdeenshire, contain mycorrhizal fungi in their tissues.

Since this early association, there has been an explosion in the diversity of land plants, as well as the fungi and types of mycorrhizal associations.

A mycorrhizal eye-view of a Caledonian pinewood. The lower part of the image is a cross-section of the tight network of ectomycorrhizal root tips and their exploratory fungi (David Genney)

Most tree root tips, such as those of our mighty Scots pine and birch, are short and stumpy, and each is enveloped in a fungal ‘glove’ called an ectomycorrhiza. This type of mycorrhiza takes over the role of explorer and nutrient gatherer; after all, it takes much less effort to grow a length of microscopic fungal hypha through the soil compared to a chunky tree root. Ectomycorrhizal fungi do far more than explore; they possess an arsenal of digestive chemicals, called enzymes, which can break down everything from dead leaves and pine cones to insect skeletons to access the nutrients that are locked within them.

The importance of these mycorrhizas for our native and commercial forests is immense. Most of a tree’s nutrition is provided through the fungi rather than their own root system so without mycorrhizas there wouldn’t be enough nutrients for the trees to grow. Artificial fertilizers are sometimes applied to commercial forests, but this is not sustainable because they are expensive, will eventually run out, and their production is energy intensive.

The roots of machair plants are colonised by arbuscular mycorrhizal fungi that extend the plant root systems in search of nutrients (David Genney).

A different type of mycorrhiza, involving different fungi and called ‘arbuscular mycorrhiza’, is associated with most herbs and grasses. Here, the fungi penetrate into the plant root cells where they produce elegant tree-like structures and coils in intimate contact with the cell’s membrane. The fungi are specialist foragers, which is important for plants that normally struggle to find enough phosphorus due to its immobility in the soil. Again, despite root systems having evolved a long way since the earliest land colonists, most plants would not thrive without their fungal partners and special Scottish habitats, such as our machair grasslands, would be shadows of their current beauty.

Heather, a typical Scottish plant, grows in habitats where most of the nitrogen they need is ‘locked up’ in plant litter that is resistant to decomposition. Heather and its relatives have impressive root systems, made up of the finest ‘hair-roots’ in the plant kingdom, so soil exploration is less of a problem. Yet another group of fungi colonise heather roots to form what we call an ericoid mycorrhiza. These fungi produce a particularly potent suite of enzymes that can extract nitrogen from some of the most resistant organic matter. Research at the University of Aberdeen has also demonstrated that heather colonised by ericoid mycorrhizal fungi can supress competing upland grasses, thereby further securing the dominance of heather and ensuring the purple haze of heather-clad Scottish mountains.

Heather roots are packed full of ericoid mycorrhizal fungi. (David Genney)

This is just a quick introduction to the fascinating partnership between plants and fungi. We still have a lot to learn, but hopefully you are now convinced that mycorrhizal fungi are something we should know more about, value and take care not to damage.