Mycorrhizae represent a widespread symbiotic relationship where a fungus forms an association with a plant’s root system. This partnership, literally meaning “fungus root,” is ancient, having been crucial for plants to colonize land approximately 450 million years ago. The plant provides the fungus with sugars produced through photosynthesis, and in return, the fungus extends its hyphal network, acting as an extension of the root system to enhance the plant’s uptake of water and nutrients, particularly phosphorus. Establishing this biological connection is a process, not an instantaneous event, and the time it takes depends on a series of microscopic steps and environmental conditions.
The General Colonization Timeline
The timeline for mycorrhizal establishment varies greatly, but researchers can generally distinguish between the initial infection and a fully functional network. Initial contact and penetration of the root can begin quickly, often within a few days of the fungus encountering a compatible host root. Under ideal laboratory conditions, the earliest evidence of colonization can be detected in as little as 72 hours. Visible or measurable colonization, where the fungal structures are extensive enough to be observed in the root cortex, typically requires a longer period, usually between one and four weeks. Factors like high ambient temperatures, around 25°C to 32°C, can accelerate this process. Full establishment, where the fungal network significantly benefits the plant, is a continuous process that can take a month or more.
Biological Steps of Establishment
The colonization process begins with a signal exchange between the plant root and the fungal propagule, which can be a spore, a colonized root fragment, or hyphae. Growing root tips release chemical signals, known as exudates, into the soil which stimulate the dormant fungal spores to germinate and grow hyphae toward the root. This directed growth toward the host plant is called chemotropism. Upon reaching the root surface, the fungus forms a specialized structure called an appressorium, which adheres to the root epidermis. From this structure, the fungus penetrates the outer layer of the root, growing through the epidermal cells and into the root cortex. Once inside the cortex, the fungus begins to colonize the root cells, forming the specialized internal structures responsible for nutrient exchange, such as arbuscules.
Environmental and Host Factors Influencing Speed
The speed of colonization is highly sensitive to external conditions, which explains the wide range in reported timelines.
Soil Temperature
Soil temperature is a major factor. Most arbuscular mycorrhizal fungi (AMF) prefer a moderate range, often between 20°C and 25°C, for optimal development. Colonization can be significantly slowed or even inhibited when soil temperatures drop below 10°C, though the fungi themselves may not be killed.
Soil Moisture
Soil moisture is also a determining factor, as adequate water is necessary for both plant root growth and the extension of the fungal hyphae through the soil. Colonization rates are positively correlated with soil moisture, and seasonal variations often show the highest colonization during rainy periods. Conversely, soil that is either too dry or completely saturated can impede the fungus’s ability to grow and infect the root.
Phosphorus Availability
The most influential factor is the availability of soluble phosphorus (P) in the soil. The symbiotic relationship evolved to help plants acquire phosphorus from nutrient-poor environments. When soil P levels are high, typically above 10 ppm, the plant’s incentive to signal the fungus is reduced, significantly inhibiting or preventing colonization.
Inoculum and Host Species
The quality and application method of the inoculum also affect the initial timeline. Using active hyphae or colonized root fragments leads to faster colonization than relying solely on spore germination, which can be slower due to spore dormancy. The host plant species itself plays a role, as some plants are more responsive to the fungal signals and are considered obligate hosts, leading to a more rapid and extensive establishment.
Differences Between Ecto and Endomycorrhizae
Mycorrhizae are broadly categorized into endomycorrhizae, which are much more common and include Arbuscular Mycorrhizal Fungi (AMF), and ectomycorrhizae. The difference in their colonization structure is directly related to their establishment speed. Endomycorrhizae penetrate the plant’s root cell walls to form highly branched structures called arbuscules inside the cortical cells, enabling direct internal nutrient exchange. Ectomycorrhizae, in contrast, do not penetrate the host cell walls. Instead, they form a dense, external fungal sheath, or mantle, around the root tip. They also form a structure called the Hartig net, which grows between the cortical cells. This structural approach means that AMF (endomycorrhizae) can establish their internal exchange structures more quickly than ectomycorrhizae can build their dense external mantle and Hartig net.