Arbuscular Mycorrhizal Fungi (AMF) are microscopic organisms that form a partnership with most plants. Widespread in natural and agricultural settings, these fungi play a fundamental role in how plants acquire nutrients and interact with their environment. Their origins trace back hundreds of millions of years, highlighting their deep evolutionary connection with land plants.
What Are Arbuscular Mycorrhizal Fungi?
Arbuscular Mycorrhizal Fungi are endomycorrhizal fungi, meaning their hyphae penetrate the root cells of host plants. They are obligate symbionts, meaning they cannot complete their life cycle without a living plant host, relying entirely on the plant for carbon and energy. The fungi survive in the soil as spores or hyphal fragments until they encounter plant roots, which trigger germination.
The basic structure of AMF involves fine, thread-like fungal filaments called hyphae, extending both within the plant root and into the surrounding soil. Within the plant root’s cortical cells, AMF form specialized, tree-like structures known as arbuscules, which are the primary sites for nutrient exchange. They can also form globular storage organs called vesicles inside the root cells. AMF associate with an estimated 80% of all terrestrial plant species, including many common crops.
The Symbiotic Exchange
The interaction between arbuscular mycorrhizal fungi and their host plants is a mutually beneficial exchange. Fungal hyphae grow extensively into the soil, creating a vast network that significantly extends the plant’s effective root surface area, allowing access to nutrients from a much larger soil volume.
In this partnership, the fungi act as biological extensions of the plant’s root system, efficiently absorbing essential nutrients from the soil. They are particularly effective at acquiring immobile nutrients such as phosphorus, zinc, and copper, and also facilitate the uptake of nitrogen, potassium, and magnesium. In return for these minerals, the host plant provides the fungi with carbohydrates, primarily sugars and lipids, produced through photosynthesis. This exchange occurs at the arbuscules within the root cells, where carbon flows from the plant to the fungus, and nutrients flow from the fungus to the plant.
Broadening Plant and Ecosystem Benefits
Beyond direct nutrient exchange, AMF provide advantages for plant growth, resilience, and ecosystem health. AMF enhance overall plant growth and vigor, leading to increased biomass and productivity. This helps plants grow more robustly, even in nutrient-limited environments.
AMF also increase plant tolerance to various environmental stresses. They help plants withstand drought by improving water uptake and maintaining better water status. AMF enhance resistance to salinity by improving nutrient balance and activating defense mechanisms. Their presence can also strengthen plant defenses against certain diseases and pathogens.
These fungi contribute to soil health and stability. Their hyphal networks bind soil particles, forming stable soil aggregates that improve soil structure, aeration, and water retention. This aggregation reduces soil erosion and enhances water infiltration. AMF also contribute to carbon sequestration by stabilizing organic matter in the soil, partly through the production of a glycoprotein called glomalin.
Encouraging Arbuscular Mycorrhizal Fungi
Fostering healthy AMF populations involves adopting practices that support their natural growth. Minimizing soil disturbance, such as through no-till or reduced tillage farming, is important as physical disruption damages hyphal networks. Reduced tillage allows AMF to establish and maintain connections.
Avoiding excessive synthetic fertilizers, especially phosphorus, is also beneficial. High levels of readily available nutrients can suppress AMF colonization, as plants may reduce their investment in the symbiotic relationship.
Incorporating diverse plant species, including cover crops, provides a continuous living root system that AMF require to thrive. These diverse root systems support a broader range of fungal communities.
While mycorrhizal inoculants are available, creating a hospitable soil environment through these practices is generally more effective for long-term AMF proliferation. Adding organic matter, like compost or mulch, further supports AMF by improving soil structure and providing a conducive environment.