Most plants don’t grow alone. Beneath the soil lie vast fungal networks that associate with plant roots. This is often regarded as one of the most important partnerships found in nature.

These fungi, known as mycorrhizal fungi (“myco” = fungi, “rhiza” = root), form symbiotic relationships with over 80% of land plants, enhancing the survival of both partners. The fungi help plants absorb nutrients, particularly phosphorus, from the soil. In return, it is estimated that up to 20% of all photosynthetically fixed carbon is transferred to the fungal partner.

This relationship is ancient. It may date back over 400 million years and could have played a key role in the evolution of the first land plants. Today, that same partnership continues to shape ecosystems across the planet.

These interactions occur within the rhizosphere, the thin layer of soil surrounding plant roots that is packed with microbial life. It’s extremely biologically active, with plants and microbes constantly communicating, exchanging nutrients, and influencing each other’s survival.

How mycorrhizal fungi quietly promote plant growth

Plant roots only explore a limited volume of soil. This becomes a problem when nutrients are scarce or unevenly distributed, especially in stressful conditions like drought.

Mycorrhizal fungi solve this problem by growing fine thread-like structures, called hyphae, that extend far beyond the root system into the surrounding soil. These microscopic filaments act as an extension of the plant’s roots, allowing it to access nutrients from a much larger area (Figure 1).

As a result, plants gain access to nutrients that would otherwise be out of reach, particularly those that move slowly through the soil, such as phosphorus, zinc and copper.

A single plant can host multiple species of mycorrhizal fungi simultaneously, each contributing distinct functional benefits, highlighting the complexity of this symbiosis.

Figure 1: The symbiotic relationship between plant roots and mycorrhizal fungi increases the area accessible to plant roots, therefore improving nutrient uptake.

Evidence strongly supports how important this partnership is. In one large meta-analysis published in the Open Access Journal of Fungi, plants associated with mycorrhizal fungi showed an average growth increase of 49% under drought conditions, alongside significant increases in root and shoot biomass.

Similarly, another experiment published in the Open Access journal Agronomy demonstrates the benefits of mycorrhizal fungi on the root structure of onion and tomato plants (Figure 2). The association was also shown to increase chlorophyll content and photosynthetic activity.

This increase in photosynthesis isn’t accidental. The fungi rely on plant sugars to survive, which increases the plant’s carbon demand, effectively encouraging it to photosynthesise more.

Figure 2: The effect of different mycorrhizal fungi containing products on the root system architecture of onion and tomato plants.

These fungal networks do more than simply increase root surface area. Their ecological importance becomes especially clear in phosphorus uptake, one of the major nutritional limitations affecting modern agriculture.

Solving the phosphorus problem

Phosphorus is well known for its function in plant growth and development, yet phosphorus deficiency affects over 40% of agricultural soils globally. Even when phosphorus is present in the soil, much of it exists in forms that plants cannot easily absorb, forcing modern agriculture to rely heavily on phosphate fertilisers.

Mycorrhizal fungi help to alleviate these negative effects by extending the plant root system deeper into the soil. Their fine fungal networks can access phosphorus beyond the reach of plant roots and transport it back to the host plant in exchange for sugars produced through photosynthesis.

Research published in the Open Access journal Horticulturae found that citrus plants inoculated with mycorrhizal fungi grew faster than uninoculated plants (Figure 3). The study linked this improved growth to the activation of phosphorus transport systems within plant roots and increased phosphorus-related enzyme activity in the surrounding soil.

Figure 3: Difference in height between tangor citrus plants inoculated with different strains of fungi: Dv, Diversipora versiformis, Fm, Funneliformis mosseae, Si, Serendipita indica.

By acting as an underground extension of the root system, mycorrhizal fungi help plants access nutrients that would otherwise remain locked away in the soil. This ancient exchange has helped sustain plant life for hundreds of millions of years and may become increasingly important as agriculture looks for more sustainable ways to maintain soil fertility while reducing dependence on chemical fertilisers.

The rhizosphere as a defence system

Beyond improving growth and nutrient uptake, the rhizosphere also plays an important role in protecting plants from disease.

A balanced and diverse microbial community within the rhizosphere can supress pathogens in several ways as highlighted in a review article published in the Open Access journal Diversity. Some microbes physically occupy the space on the root surface, limiting opportunities for harmful organisms to establish themselves. This direct competition for space and nutrients starves pathogens and protects the plant from infection.

Certain species of soil microbe can trigger a plant’s immune system through a process known as induced system resistance. Others produce antimicrobial compounds that directly inhibit pathogen growth.

Together, these interactions reveal that the rhizosphere functions as an important site for nutrient exchange and a critical line of defence for plant health. To achieve such an efficient partnership requires complex chemical signalling between plants and microbes.

A chemical conversation underground

Plants and fungi belong to entirely different kingdoms of life. They diverged roughly one billion years ago and differ in how they grow, reproduce, and obtain nutrients.

Yet despite these differences, they engage in an extraordinarily sophisticated chemical dialogue.

One important group, known as arbuscular mycorrhizal fungi, forms specialised branching structures called arbuscles inside plant root cells (Figure 4). These structures are surrounded by a plant-derived membrane that carefully regulates nutrient exchange between the two, preventing any overexploitation by the fungal partner.

Figure 4: Simplified diagram depicting nutrient exchange between plants and arbuscular mycorrhizal fungi. P – Phosphorus, N – Nitrogen, Mg – Magnesium.

A review article published in the Open Access journal Genes highlights the stepwise chemical dialogue required to establish this symbiosis:

• Plants release hormones called strigolactones into the rhizosphere, stimulating fungal growth toward the root.
• In response, fungi release signalling molecules that are recognised by receptors in the plant.
• These signals trigger genetic changes within the plant that prepare the roots for colonisation.
• The fungi then enter the roots and form specialised nutrient-exchange structures.
• From there, fungi deliver nutrients and water while plants provide carbon-rich sugars.

Strigolactones also play a key role in plant stress response. Under nutrient-poor conditions, plants increase production of these hormones, recruiting fungi partners to improve water and nutrient uptake.

The wood-wide web

While these interactions begin in the rhizosphere, they can extend far beyond a single plant.

Many researchers describe interconnected fungal systems as the “wood-wide web”, underground fungal networks linking multiple plants together. There is strong evidence that these physical connections exist (Figure 5). Studies in Douglas-fir forests, for example, have shown that individual fungi can link dozens of trees into shared underground networks.

Figure 5: A top-down of Douglas-fir trees (green shapes) where each fungal network is shown with different coloured lines.

The ecological function of these specialised networks is yet to be determined. Some studies suggest that nutrients and carbon can move between plants through fungal networks. Others argue that the scale and ecological importance of this transfer may be overstated.

Rather than a cooperative network with intent, many scientists view these systems as a byproduct of fungi optimising their own growth and resource acquisition.

Despite uncertainty, the ecological importance of mycorrhizal fungi is undeniable. Research published in the Open Access journal Agronomy has shown that mycorrhizal fungi can accelerate recovery in degraded landscapes, such as former mining sites, by improving plant establishment and supporting ecosystem regeneration.

From roots to ecosystems

The relationship between plant roots and mycorrhizal fungi reveals an extraordinarily interconnected underground world. These fungal partnerships enhance nutrient uptake, strengthen plant defences, improve resilience to environmental stress, and influence entire ecosystems aboveground.

Despite decades of research, scientists are still uncovering the complexity of these interactions. Understanding them better could play an important role in developing sustainable agriculture, restoring damaged ecosystems, and improving food security in a changing climate.

More insights into different aspects of soil science can be found in Exploring the Field: Soil Science.

Studies on soil health and its applications in agriculture can be found across the portfolio of MDPI journals covering Environmental and Earth Sciences.

Alternatively, you can access the full MDPI journal list here.