Endomycorrhizal fungi are microscopic organisms that form a symbiotic, or mutually beneficial, relationship with the roots of most plants on Earth. The name “endomycorrhiza” describes its function, as “endo” means within and “mycorrhiza” translates to “fungus-root.” This intimate connection has existed for hundreds of millions of years and was a foundational element that allowed plants to first colonize land.
The Two Major Types of Mycorrhizal Fungi
To understand endomycorrhizal fungi, it is useful to compare them to their counterparts, ectomycorrhizal fungi. The primary difference lies in how they interact with plant roots. Endomycorrhizal fungi, specifically a group called arbuscular mycorrhizal fungi (AMF), penetrate the root’s cortical cells. Inside these cells, they form intricate structures that facilitate resource exchange. This association is the most widespread, occurring in approximately 85% of all plant families, including most agricultural crops, grasses, and flowers.
Ectomycorrhizal fungi, on the other hand, do not penetrate the root cells. Their hyphae, or fungal filaments, form a dense sheath around the exterior of the root tips. They also grow between the root cells, creating a network known as the Hartig net, but remain outside the cell walls. This relationship is common among woody plants and trees, such as pines, oaks, and birches, accounting for about 10% of plant families.
How the Symbiotic Relationship Works
The symbiotic relationship is a highly coordinated exchange of resources. The process begins when plant roots release chemical signals, called strigolactones, into the soil. These signals stimulate the germination of fungal spores and guide the growth of their hyphae toward the root. Once the hyphae make contact, they penetrate the root’s outer layers and enter the cortical cells, a process managed by complex signaling between the two organisms.
Inside the root cells, the fungus forms highly branched, tree-like structures known as arbuscules. These arbuscules are the central hub of the symbiotic trade, dramatically increasing the surface area of contact between the fungus and the plant cell’s membrane. This allows for an efficient transfer of nutrients. The extensive network of hyphae extending from the root into the soil acts as a vast extension of the plant’s own root system, reaching far beyond what the roots could alone.
Through this hyphal network, the fungus absorbs water and nutrients from the soil, particularly immobile nutrients like phosphorus, zinc, and copper. These resources are then transported and delivered to the plant through the arbuscules. In return for these hard-to-reach nutrients, the plant provides the fungus with sugars, such as carbohydrates, produced through photosynthesis. This two-way exchange sustains both organisms, allowing them to thrive.
Influence on Plant Health and Soil Ecosystems
The endomycorrhizal symbiosis extends beyond simple nutrient exchange, influencing both individual plant health and the broader soil environment. For the plant, enhanced nutrient uptake translates to more robust growth, improved flowering, and increased yields. The hyphal network is also effective at absorbing water, improving a plant’s tolerance to drought conditions by accessing moisture from a larger volume of soil. This network can also act as a physical barrier, protecting roots from soil-borne pathogens.
These fungi are also engineers of the soil ecosystem. The hyphae excrete a sticky glycoprotein called glomalin, which binds soil particles together to form stable aggregates. This process improves soil structure, creating better aeration and increasing water infiltration and retention. This in turn reduces soil erosion and makes the soil more resilient to environmental stresses.
The hyphal networks also create an underground web that connects different plants, sometimes of different species, allowing for the transfer of nutrients between them. This enhances the stability and diversity of the plant community. By drawing carbon from plants and storing it in the soil through their networks and the production of glomalin, these fungi also contribute to carbon sequestration.
Practical Use in Agriculture and Gardening
Harnessing the benefits of endomycorrhizal fungi is common in both large-scale agriculture and home gardening. One method is through the application of commercial mycorrhizal inoculants. These products contain fungal propagules—spores, colonized root fragments, and hyphae—that establish the symbiotic relationship when introduced to a plant’s root zone. For best results, inoculants should be applied to make direct contact with the roots, such as during transplanting or seeding.
Beyond adding inoculants, certain management practices can foster and protect native populations of these fungi. Minimizing soil disturbance is a primary consideration, as tillage breaks up the delicate hyphal networks essential for their function and survival. Adopting no-till or low-till farming and gardening methods helps preserve this underground structure, allowing the fungal communities to thrive.
The use of chemicals can also impact mycorrhizal populations. High-phosphorus chemical fertilizers can reduce a plant’s reliance on its fungal partner, causing the symbiotic relationship to diminish. Certain fungicides can be directly harmful to mycorrhizal fungi. Reducing these inputs and focusing on building soil health with organic matter, like compost, creates an environment where these fungi can flourish naturally.