Mycorrhizal fungi establish a mutually beneficial partnership with plant roots. This ancient association, dating back 400 to 500 million years, forms extensive underground networks that significantly influence plant health and the broader ecosystem.
The Fungal-Plant Alliance
The relationship between mycorrhizal fungi and plants is a classic example of symbiosis, where both organisms receive advantages. Plants, through photosynthesis, produce sugars, which they transfer to the fungi. This carbohydrate supply is a direct energy source for the fungi, supporting their growth and development.
In return, the fungi provide plants with enhanced access to water and mineral nutrients from the soil. They do this by developing a vast network of thread-like structures called hyphae. These hyphae extend far beyond the plant’s own roots, increasing the plant’s absorption surface area by 10 to 100 times.
This extended network allows the fungi to “mine” the soil for nutrients. These include phosphorus, nitrogen, and micronutrients such as zinc, manganese, and copper. The absorbed water and nutrients are then transported through the fungal hyphae back to the plant roots, completing the exchange.
Key Types of Mycorrhizal Fungi
Mycorrhizal fungi are categorized into types based on how their hyphae interact with plant root cells. The two main categories are arbuscular mycorrhizal fungi (AMF) and ectomycorrhizae. While both form beneficial associations, their structural integration with the plant root differs.
Arbuscular mycorrhizal fungi (AMF), also known as endomycorrhizae, are the most common type, associating with approximately 80% of vascular plant families, including many common crops. These fungi penetrate the cortical cells of the plant root, forming tree-like structures called arbuscules. Arbuscules are the primary sites for nutrient exchange. AMF can also form storage structures called vesicles, which store lipids and spores.
Ectomycorrhizae primarily associate with woody plants, such as pines, oaks, birches, and beeches, accounting for about 10% of plant families. Unlike AMF, ectomycorrhizal fungi do not penetrate the plant’s root cells. Instead, they form a mantle around the root tips. From this mantle, a network of hyphae, known as the Hartig net, grows into the spaces between the root cells. This intercellular network is where nutrient exchange occurs.
How Mycorrhizal Fungi Transform Soil and Plant Health
Mycorrhizal fungi enhance plant health and contribute to a more stable soil ecosystem. Their extensive hyphal networks increase the surface area for nutrient absorption, particularly phosphorus. These fungi access phosphorus bound in insoluble forms by secreting organic acids and phosphatases, which release orthophosphate for plant uptake. Over half of a plant’s phosphorus uptake in low-bioavailability soils can be attributed to AMF.
Mycorrhizal fungi also play a role in nitrogen uptake. They acquire various forms of nitrogen, including ammonium, nitrate, and organic nitrogen compounds like amino acids, from the soil. The fungal hyphae then transfer this nitrogen to the plant. This contribution to nitrogen nutrition is beneficial in low-nitrogen environments and can influence plant productivity.
Mycorrhizal associations also improve plant resilience to drought. The fungal hyphae extend into smaller soil pores, enhancing water absorption and increasing the root’s capacity to acquire water from a greater distance. Mycorrhizal plants exhibit improved stomatal conductance and water use efficiency, helping them maintain hydration during dry periods. These fungi can also trigger biochemical responses in plants, such as osmolytes accumulation and antioxidant defense activation, aiding drought tolerance.
Mycorrhizal fungi improve soil structure, a process known as soil aggregation. Their hyphae physically entangle soil particles, forming macroaggregates. These fungi also produce glomalin, a glycoprotein that acts as a “biological glue.” Glomalin binds soil particles, enhancing aggregate stability and improving soil moisture retention and aeration. This improved structure leads to better water infiltration, reduced erosion, and a more favorable environment for root growth and soil microbial activity.
Cultivating a Favorable Environment for Mycorrhizal Fungi
Supporting naturally occurring mycorrhizal fungi leads to healthier plants and more resilient ecosystems. One strategy is to minimize soil disturbance. Practices like tilling and plowing disrupt hyphal networks, breaking fungal connections and pulverizing soil structure. Adopting no-till or minimal-till methods helps preserve these networks, allowing them to flourish.
Reducing reliance on synthetic fertilizers, especially those rich in phosphorus, is beneficial. When plants have abundant soluble phosphorus, they may become less dependent on mycorrhizal fungi, decreasing root colonization. Excessive use of other synthetic fertilizers and pesticides can also harm mycorrhizal fungi. Opting for organic amendments supports a thriving fungal community.
Incorporating organic matter like compost, leaf litter, and mulch provides a food source for mycorrhizal fungi and enhances soil health. This material improves soil structure, moisture retention, and nutrient availability, creating an ideal environment for fungal growth. Decomposition of organic matter also releases compounds that stimulate hyphal growth.
Planting a diverse range of species encourages a robust mycorrhizal community. Different plant species attract and support different types of mycorrhizal fungi, leading to a more varied and resilient fungal population. This diversity improves nutrient partitioning among plants and contributes to ecosystem functioning. By implementing these practices, gardeners and farmers can foster these natural partnerships, promoting long-term soil and plant health.