VAM (Vesicular-Arbuscular Mycorrhizae), now more commonly referred to as Arbuscular Mycorrhizae (AM), and Ectomycorrhizae (EM) are two distinct types of symbiotic relationships between fungi and plant roots, each playing a crucial role in nutrient uptake and plant health. The primary differences lie in the fungal partners involved, their host plant specificity, and the structural interactions they form with plant roots.
While both enhance a plant's ability to absorb water and nutrients from the soil, particularly phosphorus and nitrogen, their evolutionary history, prevalence, and mechanisms of interaction vary significantly.
Key Distinctions Between AM and Ectomycorrhizae
The following table summarizes the main differences between these two vital mycorrhizal associations:
| Feature | Arbuscular Mycorrhizae (AM/VAM) | Ectomycorrhizae (EM) |
|---|---|---|
| Fungal Group | Glomeromycota (formerly Zygomycetes) | Basidiomycetes (mainly mushroom-forming), some Ascomycetes |
| Spore Formation | Form microscopic underground spores | Fruiting bodies (mushrooms, puffballs, truffles) often visible above ground; also form spores. |
| Host Plants | Majority of plant species (approx. 80-90%), including many trees, agricultural crops, grasses, shrubs, and tropical trees. | Almost exclusively limited to woody species, primarily temperate and boreal forest trees (e.g., pines, oaks, birches, eucalyptus). |
| Root Interaction | Endomycorrhizal: Fungi penetrate into root cortical cells, forming arbuscules and vesicles inside the cells. No visible fungal mantle on the root surface. | Ectomycorrhizal: Fungi form a dense mantle (sheath) around the root tips and a Hartig net that grows between root cortical cells. |
| Visible Structure | Not visible externally; requires microscopy to observe internal structures. | Often visible as thickened, dichotomously branched root tips with a distinct fungal sheath. |
| Nutrient Exchange | Primarily facilitates the uptake of phosphorus, nitrogen, and water. | Facilitates uptake of nitrogen, phosphorus, water, and can offer protection against heavy metals and pathogens. |
| Evolutionary Age | More ancient, thought to have co-evolved with early land plants. | Relatively more recent in evolutionary terms. |
Detailed Examination of Differences
Let's delve deeper into the specific characteristics that set AM and Ectomycorrhizae apart:
H3.1. Fungal Partners and Their Life Cycles
The fungi responsible for forming these associations belong to different taxonomic groups:
- Arbuscular Mycorrhizae (AM/VAM): These fungi are members of the phylum Glomeromycota. They are obligate symbionts, meaning they cannot complete their life cycle without a host plant. They reproduce by forming microscopic underground spores which germinate and infect root systems. These fungi do not form large, visible fruiting bodies like mushrooms.
- Ectomycorrhizae (EM): The majority of EM fungi are mushroom-forming Basidiomycetes, though some Ascomycetes also form this type of association. These fungi are often saprophytic or parasitic in other contexts but form a beneficial symbiosis with specific trees. Their life cycle often includes the production of easily recognizable fruiting bodies (like mushrooms) above ground, which release spores for dispersal.
H3.2. Host Plant Specificity and Distribution
The range of plants that form these associations differs dramatically, impacting their ecological distribution:
- Arbuscular Mycorrhizae (AM/VAM): This is the most widespread type of mycorrhizal association, colonizing the roots of the majority of plants, including most agricultural crops, grasses, shrubs, and many tree species, especially those in tropical forests. This broad host range makes AM fungi crucial for global plant productivity and ecosystem health.
- Ectomycorrhizae (EM): In contrast, EM fungi are almost exclusively limited to woody species. They are predominantly found in temperate and boreal forests, forming partnerships with iconic trees such as pines, firs, spruces, oaks, birches, and eucalyptus. This specialization contributes to the distinct characteristics of these forest ecosystems.
H3.3. Structural Interaction with Plant Roots
The way the fungi interact with the plant roots at a cellular level is a defining characteristic:
- Arbuscular Mycorrhizae (AM/VAM): These are endomycorrhizal, meaning the fungal hyphae penetrate the cell walls of the root cortical cells. Inside these cells, they form intricate, tree-like structures called arbuscules, which are the primary sites of nutrient exchange. They also form vesicles, which are storage organs. Importantly, AM fungi do not form a dense fungal sheath around the exterior of the root.
- Ectomycorrhizae (EM): These are ectomycorrhizal, meaning the fungal hyphae form a dense sheath, known as a mantle, around the outside of the root tips. From this mantle, hyphae extend into the soil to absorb nutrients. Critically, the fungal hyphae also grow between the root cortical cells, forming a network called the Hartig net, but they do not penetrate the plant cell walls themselves.
H3.4. Visible Characteristics
The presence of these mycorrhizae can sometimes be observed, albeit differently:
- Arbuscular Mycorrhizae (AM/VAM): Without a microscope, AM colonization is not externally visible on the roots. The internal structures like arbuscules and vesicles can only be observed by staining and examining root sections under a microscope.
- Ectomycorrhizae (EM): EM roots often exhibit noticeable changes. Root tips might appear thicker, shorter, and more branched than uncolonized roots, often covered by the distinct fungal mantle. Furthermore, the presence of specific mushrooms or other fungal fruiting bodies in the vicinity of host trees can indicate an active ectomycorrhizal association.
By understanding these fundamental differences, we can appreciate the diverse ways in which fungi contribute to plant health and ecosystem functioning across various biomes.
