The Secret Life of Mycorrhizal Fungi - What Is it And How Does it Work?

By Ellen Baekelmans

Fungi are very diverse and fascinating organisms. They are ubiquitous and come in all sorts of shapes, forms and specialties. Some are known for their negative impacts on crop yields¹ but fungi can also be very beneficial for multiple reasons which we will further investigate in this article. In this article, we will dive deeper into the wonderful world of a diverse group of fungi, namely mycorrhizal fungi. 

Mycorrhizal fungi – what’s that?

The terminology of mycorrhiza is derived from the old Greek words for ‘fungus’ and ‘roots’, hence, the name exactly describes where these fungi are found – in the soil, in association with the roots of plants². Mycorrhizal fungi form symbiotic relationships with approximately 80–90% of the land plants³. Symbiotic means that both plants and fungi benefit from this type of interaction. The fungi play a crucial role in nutrient cycling by providing plants with soil nutrients, and in return fungi receive carbohydrates that the plant assimilated through photosynthesis for their growth and reproduction^4. Mycorrhizal fungi develop mycelia which are extensive soil webs of long, filamentous and branching structures, so called hyphae⁴. The mycelium can actually be seen as an extension of the plant roots, enabling extra nutrient and water uptake.

Two general mycorrhizal types can be distinguished – one group that surrounds plant root tips and do not infiltrate into the plant cells, and another group that penetrates and develops inside plant root cells². Of this latter group, arbuscular mycorrhizal (AM) fungi constitute a very important group of fungi that grow inside plant cells as they are the most widespread fungi and 200,000 plant species host them^4,5. These plant species include, amongst others, herbs, grasses and many other crops⁵.

How are plant-fungi partnerships established?

Okay, so we know that mycorrhizal fungi and plants form a tight partnership. But the question remains: how do they find each other, and how do they establish these interactions?

Well, it all starts with fungal spores which are found in the soil. They start germinating into hyphae and explore the soil to search for fine plant roots². Plants excrete compounds in the proximity of the roots which can be detected by fungi, these are so called root exudates². This way, the fungus knows where the roots are. However, while the fungus explores the soil, it also releases fungal exudates which are in turn detected by the plant and trigger processes inside the root cells². As a result, the plant knows a fungus is approaching and prepares for the symbiotic interaction. 

After the chemical acquaintance, a physical encounter occurs. When the fungi finds a good location on the root, they develop a swollen hyphae, known as a hyphopodium^2. The hyphopodium then protrudes into the outer layer of the plant cell wall. New genes are expressed in the outer root cells, and result in the production of an AM-specific structure that is essential for successful penetration, the so-called prepenetration apparatus (PPA)^2,6. The fungal hyphae can grow through this structure when it penetrates the root cell⁶. Subsequently, the fungi starts branching and forms arbuscules (Figure 1), filling most of the plant cell. Arbuscules are actually the core of the symbiosis – this is where the nutrient exchange occurs².

Figure 1. Photo through microscope of a fine maize root colonised by arbuscular mycorrhizal fungi. Arbuscules are clearly visible as the “cloudy”, dark blue structures. Nutrient exchange occurs through the arbuscules. © Ellen Baekelmans (2022)

What can you do to maintain or improve AM fungal colonisation in your field?

First of all, AM fungi need living plants for their survival and reproduction as they depend on the sugars that are provided by plants. Therefore, the use of cover crops is a great solution to maintain the AM fungi in the field^8,11. Secondly, tillage practices physically disturb the fungal mycelium. To maintain the fungal networks, tillage should be reduced^8,12. Furthermore, reduction of artificial fertiliser inputs may also be beneficial for AM fungi¹³. When plants are offered an easily accessible source of nutrients – as from artificial fertilisers – they no longer see the benefit of providing sugars to the AM fungi. As a result, the fungal abundances decrease and fungal benefits and ecosystem services are lost. Lastly, avoid the use of pesticides and especially fungicides and nematicides as they kill all living fungi in the soil, including AM fungi⁹.

In summary

AM fungi contribute to sustainable agriculture by decreasing the demand of artificial fertiliser inputs, increasing soil health and increasing crop performance. You should view AM fungal networks as an extension of the plant roots, therefore one should try to maintain and enhance plant-mycorrhizal associations to improve crop performance and ecosystem services, which ultimately leads to more sustainable farming systems.

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