For the past half century, evidence has been accumulating that trees are not just static plants that are left to fend for themselves. On the contrary, trees are dynamic organisms that communicate and exchange resources through a dynamic underground system that is linked to their survival. An underground communication system? Sounds a bit like science fiction — but isn’t! This post will aim to delve into how trees exchange vital nutrients and send stress signals through fungal networks.

How Trees and Fungi Thrive Together

You may have heard of symbiotic relationships in classic biology. They are, put simply, relationships between organisms that depend on each other. One organism lives off the other (receives benefits out of the relationship) whilst the other can also profit from (or at least isn’t harmed by) its presence.

A mycorrhizal network is such a symbiosis. Mycelium, a type of threaded, branch-like fungi, forms a symbiosis with trees in an ecosystem. They ‘bind’ to the roots of trees and work almost as ‘extensions’ to tree roots: they connect to other trees and flora in the ecosystem (Fig. 1). This forms the network I talked about earlier, that allows for transport of minerals and nutrients.

These fungal networks are mostly thought to be mutualistic, even though some species can engage in parasitism. Mycorrhizal networks get very complex and there are many different combinations of plant and fungus species that can further contribute to this complexity.

So, mycorrhizal networks connect trees to other organisms in the ecosystem, thus allowing for transport of water and important minerals such as nitrogen N, carbon C and phosphorus P.

But in a symbiosis the relationship doesn’t go just one way — So what is the mycelium getting out of this?

Interaction between these two species occurs with the goal of exchanging photsynthates and minerals from the soil. Something you will hear people talk about is that ‘plants make energy out of sunlight’. This is true, and as you might know this process is called photosynthesis and is described by the following chemical equation:

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

In short, plants use water and carbon dioxide (which is what we breathe out) with activation energy from the sun in form of sunlight to make sugar (glucose) which can be considered as ‘energy storage’.

You will be happy to hear that this plays a significant part in the relationship between mycelium and trees. As living things need carbohydrates to store energy and thus to survive, so does the mycelium. This thus fosters a significant relationship between tree and fungus which uses the currency of photosynthates (glucose C₆H₁₂O₆) and minerals (such as P, C, N and H₂O) from the soil.

Types of Mycorrhizal Networks

You should know now, that mycorrhizal networks connect trees to each other and are an exchange highway for sugars and minerals. But in reality, there isn’t just one type of mycelium present in this network. We divide into two major kinds of mycorrhizal fungi based on their hyphae:

Ectomycorrhizal fungi (EMF)

 Arbuscular mycorrhizal fungi (AMF)

Of which the latter is more commonly present in the root tips of trees and plant species. The two types differ mainly in their colonization strategies. This means that EMF build a ‘layer’ (a sheath) around the root cells whereas the AMF penetrate the root cell, forming vesicles and arbuscules inside of the tree root cells (Fig. 3).

But what on earth does this have to do with the chemistry/biology we’re so interested in? What kinds of biochemical interactions occur at the root tips where these magnificent networks meet?

Biochemical Interactions Between Fungi and Plants

Earlier we looked at the role photosynthesis plays in the symbiotic relationship between tree and fungus.

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

However, what happens if plants have to undergo an arid summer or a dry season, which contributes to a scarce pool of reactants and hinders the fundamental reaction that plants need to survive?

This is where the mycorrhizae come in. They extract nutrients from the soil, which are absorbed by plant root cells. Without the mycorrhiza, plant root cells’ nutrient uptake pathways are much less effective or efficient. This is partly due to the fact that mycorrhiza like ectomycorrhizal fungi can store nutrients in their sheaths. Meaning that even if the soil around them is partly depleted of nutrients, the fungus can keep on fostering the symbiosis.

Nutrients such as C, P and N are transported in the hyphae. A collection of filamentous hyphae, called extraradical mycelium (think of it simply as the hyphae deriving from the fungi: the outer environment), transport nutrients from the soil to the hartig net (Fig. 3). For the sake of simplification, let’s look exclusively at the nutrient nitrogen N in the next section:

However, nutrients typically don’t just swim around in the soil in their elementary form. As far as it goes for arbuscular mycorrhiza (AMF), it can ‘absorb’ nitrogen N from the soil either inorganically or organically: absorbing inorganic nitrate (NO₃⁻) and ammonium (NH₄⁺) or organic amino acids (composed of 1 NH₂ group), respectively. Think of these molecules as simply being ‘nitrogen carriers’. Their chemical form allows nitrogen to be picked up by the mycorrhizal fungi.

As mentioned, the extraradical mycelium is where these molecules are transported. Here, the N form that has been absorbed is converted to Arginine (an amino acid) and transported to the intraradical mycelium. This is the inner part of the fungus that is inside the root cell. Here, Arginine is broken down into  NH₄⁺ and transferred to the host plant. Fig. 4 illustrates this.

A similar process happens with phosphorus P forms. These are also taken in by the extraradical mycelium and stored as polyphosphates (polyP). A polyphosphate is a molecule with multiple phosphate PO₄³⁻ molecules linked together. Think of it as a molecule that stores phosphorus P. This polyphosphate is then broken down to arginine and phosphorus, making nutrients available to the plant.

Interestingly enough, because of the rapid intake of nutrients by the mycorrhizal fungi, a depletion zone is created. This prompts the fungus to grow further and expand its roots farther into the soil to take in nutrients. As time passes, mycorrhizal networks expand throughout the entire forest ground.

Impact on Agriculture

The world of mycorrhizal fungi is a whole universe by itself. But did you know that they have a considerable use in agriculture?

Many fertilizers use phosphate as an important nutrient for plants. Many of them are thus phosphate-based and the extraction of phosphate makes up an important part of the agricultural sector. Unfortunately, phosphate is a non-renewable resource, meaning that it cannot sustain the fertilizer industry forever. However, as we have seen, mycorrhizal fungi are extremely efficient at making phosphate available as a nutrient for plants. They thus pose a significant alternative to these fertilizers and it could be probable that they’ll soon start appearing commercially.

Final Thoughts

Now you have an overview of the dynamic workings of plants. Next time you take a walk in the forest you’ll know what is happening under your feet. I will remind you that the relationships and mycorrhizal species explored today are merely the tip of the iceberg. There is an infinitely big number of combinations between plant and fungus and far more complex nutrient transport mechanisms than what I talked about today. Our planet is infinitely complex and it takes a huge effort for us humans to understand it with our primitive tools!