A mycorrhiza is a symbiotic association formed between a fungus and the roots of a plant. This partnership, which has existed for over 450 million years, is widespread, occurring in the majority of plant species on Earth. Mycorrhizal fungi act as an extension of the plant’s root system, fundamentally changing how plants interact with the soil environment. This collaboration profoundly influences plant survival and nutrient cycling in nearly every terrestrial ecosystem.
Defining Mycorrhizal Mutualism
The relationship between a plant and its associated fungus is classified as a symbiosis. More specifically, this arrangement is a mutualism, meaning both the fungus and the plant receive benefits from the partnership. This exchange is a biological trade of resources that neither partner can obtain as efficiently on its own.
The plant supplies the fungus with carbohydrates, typically sugars or lipids, which are products of photosynthesis. Since fungi cannot perform photosynthesis, they rely on the plant for a constant supply of carbon necessary for growth and metabolism. In return for this energy, the fungus enhances the plant’s ability to absorb vital soil resources.
This mutualistic exchange allows both organisms to thrive, especially in resource-poor environments. The plant receives an augmented supply of water and mineral nutrients, while the fungus secures a predictable and direct source of organic carbon. This balanced give-and-take illustrates how the fungal partner has become an integrated part of the plant’s survival strategy.
Essential Functions in Plant Nutrient Acquisition
The primary role of mycorrhizae is increasing the plant’s reach into the soil for immobile nutrients. The fungal body consists of a network of fine threads called hyphae, collectively known as the extraradical mycelium, which spreads far beyond the plant’s own root hairs. These hyphae are much thinner than plant roots, allowing them to penetrate smaller, inaccessible soil pores.
This vast network effectively expands the root surface area for absorption by up to a thousand-fold. The increased surface area is particularly important for acquiring phosphorus (P), a nutrient that moves slowly in the soil and quickly becomes depleted near the root surface, forming a “depletion zone.” The fungal hyphae extend past this zone to scavenge new sources of P, which is then transported directly back to the plant root.
Mycorrhizae also aid in the acquisition of nitrogen (N) and other elements, such as zinc and copper. The fungi absorb various forms of nitrogen, including ammonium and nitrate, and transfer them to the host plant. Furthermore, the hyphae enhance the uptake of water from the soil, helping plants maintain hydration and increasing resistance to drought conditions.
Beyond nutrient transfer, the fungal network provides secondary benefits that support plant health and resilience. The colonization of the root system can physically protect against soil-borne pathogens by creating a barrier around the root cells. Some fungi also induce biochemical changes in the host plant that activate defense responses, leading to increased disease resistance. The fungi also produce a sticky protein called glomalin, which binds soil particles together, improving soil structure, water retention, and aeration.
Distinguishing Between Major Mycorrhizal Types
Mycorrhizal fungi are broadly categorized into two main structural types based on how their hyphae interact with the plant root cells. The most common type is Arbuscular Mycorrhizae (AM), also known as Endomycorrhizae, which are found in approximately 80% of all land plant species, including most agricultural crops. AM fungi are characterized by their penetration into the cells of the root cortex.
Once inside the cell, AM fungi form highly branched, microscopic structures called arbuscules, which resemble tiny trees. These arbuscules are the primary sites where the nutrient-for-carbon exchange occurs between the fungus and the host plant. They may also form swellings called vesicles, which serve as storage organs for lipids and other compounds within the root tissue.
Ectomycorrhizae (EM) represent the second major type, typically associated with woody plants, such as conifers, oaks, and beeches. EM fungi do not penetrate the interior of the plant’s root cells. Instead, the fungal hyphae form a dense, visible sheath, known as a mantle, that encapsulates the root tip.
From this mantle, the hyphae grow inward and form a network called the Hartig net, which spreads between the outermost cells of the root cortex. The nutrient exchange takes place across the cell walls within the intercellular spaces of the Hartig net, never breaching the plant cell membrane.