Arbuscular mycorrhiza – plant root endosymbiosis
Mycorrhiza (from Greek μύκης mýkēs, "mushroom", and ῥίζα rhiza, "root") which is a mutual symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, such as its root system. Mycorrhizae play an important role in plant nutrition, biology and soil chemistry.
Biodiversity and the Importance of Mycorrhizae
Our planet is currently experiencing significant biodiversity loss, impacting both natural ecosystems and human food systems. Key drivers of this loss include habitat changes and the heavy reliance on non-ecological agricultural inputs. To address biodiversity and related aspects like organic fertilizers, protein gels, and green crops, it’s crucial to understand the beneficial plant-fungus relationship offered by mycorrhizae, specifically arbuscular mycorrhizal fungi (AMF).
What are Mycorrhizae?
Derived from the Greek words "mýkēs" (mushroom) and "rhiza" (root), mycorrhiza refers to a mutually beneficial association between a fungus and a plant. In this partnership, fungi colonize the plant's root system, aiding nutrient and water absorption while receiving sugars and carbon from the plant. Mycorrhizal fungi are essential for ecosystems, as they support the growth and survival of around 80% of all terrestrial plant species.
Mycorrhizae play a fundamental role in plant nutrition, soil chemistry, and overall ecosystem productivity. There are two main types of associations:
- Arbuscular Mycorrhizae (AMF): Here, fungal hyphae penetrate the root cells of the host plant. They form specialized structures, known as arbuscules (branched) or vesicles (spherical), which enable efficient nutrient exchange. While the fungal structures enter the root cells, they do not invade the cell’s interior (protoplast), allowing close contact with the cell membrane for optimal nutrient transfer.
- Ectomycorrhizae: These fungi remain on the outside of plant root cells, forming a network around the root.
Why Mycorrhizae Matter
Through this symbiosis, plants gain enhanced access to water and nutrients such as nitrogen and phosphorus, while the fungi benefit from the plant's photosynthetic outputs. With evidence of mycorrhizal relationships dating back to early land plants, these partnerships have been vital to plant survival and are a current focus of scientific research for their role in sustainable agriculture and ecosystem health.
Effects on Plants
The movement of carbohydrates produced during photosynthesis flows from the host plant (the autotrophic partner) to the symbiotic fungus (the heterotrophic partner). Vesicular-arbuscular fungi play a significant role in nutrient assimilation, particularly phosphorus, by absorbing nutrients from the soil via their extraradicular hyphae. Nitrogen, essential for the formation of amino acids, purines, pyrimidines, and enzymes, indirectly supports protein formation and nucleic acid synthesis. A lack of nitrogen can negatively impact plant growth and chlorophyll production in leaves.
The mutualistic association between the plant and mycorrhizal fungus provides the fungus with a steady supply of carbohydrates such as glucose and sucrose, which are transported from the plant’s leaves to the root tissue and fungal partner. In exchange, the plant benefits from the fungus’s extensive mycelium network, which increases its ability to absorb water and minerals. The fine, long fungal hyphae have a larger surface area compared to plant root hairs and can access nutrients that are otherwise unavailable to plant roots.
Plants with mycorrhizal associations are generally more resistant to diseases, including those caused by soil-borne pathogens. Mycorrhizae help protect plants both above and below ground by excreting enzymes that are toxic to harmful soil organisms like nematodes. Additionally, mycorrhizal associations stimulate a priming effect in plants, which activates a defense response similar to an immune reaction. As a result, mycorrhizal plants exhibit stronger resistance to future attacks.
When a plant infested with aphids is under attack, it signals neighboring plants by releasing volatile organic compounds that attract the aphids' natural predators. Mycorrhizal fungi also offer protection to plants growing in soils with high metal concentrations, such as acidic or contaminated soils.
Benefits of Using Mycorrhizae
- Enhanced absorption of nutrients, especially nitrogen in organic compounds
- Increased nutrient accumulation in plants
- Production of phytohormones that support plant growth
- Neutralization of phenolic compounds and toxic metals in the soil
- Improved soil structure
- Positive influence on plant species succession at the ecosystem level
- Protection against fungi and nematodes
- Suppression of pathogenic microorganisms, including Fusarium, Alternaria, and bacterial diseases
- Increased resistance to water stress
- Nutrient transfer from dead plants to living plants
