The Microscopic World of Arbuscular Mycorrhizal Fungi
Arbuscular mycorrhizal fungus in lettuce root. Image coloured by depth.
Arbuscular mycorrhizal fungi can be found in the roots of over 80% of plant species. They are not causing disease, nor are they decomposing dead plant matter, but instead they are living in partnership with the plants. Arbuscular mycorrhizal fungi grow thread-like structures, called hyphae, within the plant roots. They also grow extensive webs of hyphae out into the soil, which they use to collect nutrients and water for their plant partners. In return, the plants feed the fungi with fats and sugars. This relationship enables plants to gain more nutrients, while the fungi receive the food they need to complete their life cycle.
Not only are arbuscular mycorrhizal fungi hidden underground, but they are also microscopic - you won’t see any toadstools poking above the soil! However, by using special dyes and microscopes we can get a unique insight into what they are up to. Let us take you on a tour of the amazing microscopic world of arbuscular mycorrhizal fungi – a world that’s all around you, but you have probably never seen before:
1. Hyphae (singular = hypha)
Hyphae are the very thin strands that make up the main ‘body’ of arbuscular mycorrhizal fungi. In the soil, the hyphae grow in search of water and nutrients, branching and fusing to form networks called mycelia. There can be over 20 metres of arbuscular mycorrhizal hyphae within just one gram of soil! Hyphae absorb nutrients and water from the soil and transport them back through the network towards the plant.
In the plant, the hyphae grow in-between and/or within the plant root cells, in some cases colonising the entire root. It is from the hyphae that all of the other specialised fungal structures develop…
Fungal hyphae in a celery root (left) and a cudweed root (right)
2. Hyphopodia (singular = hyphopodium)
The first structure formed by a hypha when it encounters a plant root in the soil is a hyphopodium. The hypha attaches to the root surface and grows to produce a bulge. If the plant permits, the fungus can enter the root, growing its hypha through and between the root cells.
Hyphopodium on the surface of a lettuce root. The hypha has entered the root and developed arbuscules.
Arbuscules develop from a hypha that enters a plant root cell and branches repeatedly to give a tree-like structure. In fact, ‘arbuscule’ literally means ‘little tree’! This branching produces an enormous surface area over which the fungus can deliver nutrients to the plant cell, and the plant can feed the fungus with fats and sugars.
Arbuscules come in all sorts of different shapes and sizes depending on the plant and fungal species involved.
Top left: two arbuscules in lettuce root. Top right: arbuscule in daisy root. Bottom left: arbuscule in marigold root. Bottom right: arbuscule in scarlet pimpernel root.
Instead of arbuscules, some fungi form coils, some form both, and some form structures that are somewhere between the two. Coils can look like balls of string, paperclips, or sometimes pretzels. Like arbuscules, they are used to exchange nutrients with the plant.
Left: fungal coils in scarlet pimpernel root cells. Middle: an ‘arbusculated’ coil (somewhere between an arbuscule and a coil) in scarlet pimpernel root cell. Right: fungal coils and arbuscules in daisy root.
Arbuscular mycorrhizal fungi receive so much food from their plant-partner that they need a place to store it. For this, many fungi form large, balloon-like structures called vesicles, which serve as a kind of larder. They are packed full of lipids (fats), ready to fuel the fungus when needed.
Left and right: fungal storage vesicles in maize roots.
Once a fungus has plenty of fats and sugars from the plant, it can complete its life-cycle by producing spores – like the ‘seeds’ of a plant. These spherical structures develop on the ends of hyphae outside of the root. When the conditions are right, the spores germinate (grow new hyphae) to create the next generation of arbuscular mycorrhizal fungi, ready to partner up with more plants and produce yet more fascinating fungal forms.
Spore of arbuscular mycorrhizal fungus