Beneath The Soil: Plant + Mycorrhizal Fungi Interactions

By Dr. Michael Schwartz

Plants are arguably the most badass kingdom on the planet, but their badassary wasn’t achieved alone. Most plants evolved to form relationships with fungi and this contributed to their colonization of the land. While some relationships with fungi are toxic, contributing to 16% of all crop losses, many fungi play a role in plant health. The overall relationship between plant and fungi consists of the fungi receiving sugars (the product of photosynthesis) for water and micronutrients (potassium, nitrogen, etc). Since there is so much bad in the word, let’s focus on the good for this blog post. For now we will ignore, though not diminish their importance, the parasitic relationship between plants and fungi and focus on the positive relationship plants form with fungi. 

One of the major groups of beneficial fungi are mycorrhizal fungi. From the Greek words for “root fungus” it’s estimated that 6,000 species of mycorrhizal fungi exist and form a symbiotic, or mutually beneficial, relationship with plants.  The relationship begins when filamentous fungal cells, called hyphae (pronounced hy-fee; the same way SCWEEZ and Mistah Fab say it), come in contact with a host plant’s roots. The hyphae infect the roots, either around the outside and connect with the epidermis (outer layer of the root) or penetrate into the root, of the plant. 

Some mycorrhizal fungi are arbuscular mycorrhizal (AM) fungi. AM fungi form vesicles, or arbuscules, in an internal layer of root cells called the cortex cells. The fungi do not penetrate into the cell. Instead the hyphae folds into the cell membrane to establish a connection with each cell. The formation of the arbuscule increases the surface area that connects the plant cell to the fungal hyphae which maximized nutrient exchange. Shown here are examples of the diversity among AM fungi:

In contrast, ectomycorrhizal don’t fully penetrate into the root. Instead, they form a structure called a mantle around the outside of the root. The mantle supports a Hartig net which are hyphae that penetrate into the root to allow for nutrient exchange. These are more common in species of orchids and members of the heath (Ericacea) family. Hartig nets can be seen in these images towards the outside of the root, while the network of hyphae extends internally. 

The number of species which benefit from mycorrhizal fungi is extensive. They have even been used as an application in commercial settings ranging from orchards to nurseries and even home gardens. This resilient soil amendment as mycorrhizal fungi can protect plants from both abiotic and biotic stress in addition to supplying the plant with nutrients. A non-exhaustive, though quite extensive, list of plants which benefit from mycorrhizal fungi can be found here. 

All images were borrowed from Mark Brundrett (http://mycorrhizas.info/index.html#intro)