Introduction
Arbuscular mycorrhizal fungi (AMF) are a group of fungi that form symbiotic relationships with the roots of most terrestrial plants. These fungi play a crucial role in plant nutrition, soil health, and ecosystem function. This text explores the history, biology, and ecological importance of AMF, along with their applications in sustainable agriculture and environmental restoration.
History and Evolution
Ancient Origins
Arbuscular mycorrhizal fungi belong to the phylum Glomeromycota and have a long evolutionary history dating back to the Devonian period, approximately 400 million years ago. Fossil evidence suggests that early land plants formed symbiotic relationships with AMF, which may have facilitated their colonisation of terrestrial environments (Redecker et al., 2000). This ancient symbiosis was crucial for the transition of plants from aquatic to terrestrial habitats, as the fungi helped the plants to access essential nutrients from the soil.This makes AMF one of the most important symbiotic organisms on Earth, making it possible for ecosystems to thrive. As one of many examples, a large community of researchers agree that Tropical rainforests would not be able to exist without AMF as they grow on very shallow soil where nutrients cycle rapidly and fungus allows plants to take up these nutrients from the soil before they leach out.
Coevolution with Plants
The symbiotic relationship between AMF and plants has coevolved over millions of years. This relationship is highly conserved, indicating its fundamental importance to plant life. AMF colonize the root cortex and form structures known as arbuscules, which facilitate nutrient exchange between the fungus and the host plant (Smith & Read, 2008). This coevolutionary process has led to a highly integrated and mutually beneficial relationship that is essential for the survival and productivity of many plant species.
Biology of Arbuscular Mycorrhizal Fungi
Life Cycle and Morphology
AMF have a unique life cycle that includes both a soil-borne phase and a root-inhabiting phase. The fungi produce spores in the soil, which germinate and produce hyphae. These hyphae seek out plant roots and penetrate the root cortex, where they form arbuscules and vesicles (Smith & Read, 2008).
Arbuscules and Vesicles
Arbuscules are highly branched structures formed within the root cortical cells. They increase the surface area for nutrient exchange between the plant and the fungus. Vesicles are storage structures that can contain lipids and other nutrients. Both structures are essential for the symbiotic relationship (Parniske, 2008). The formation of arbuscules is a key feature of AMF, distinguishing them from other types of mycorrhizal fungi.
Symbiotic Mechanisms
The symbiotic relationship between AMF and plants involves a complex exchange of signals. Plants release strigolactones into the soil, which stimulate spore germination and hyphal branching in AMF. In response, AMF produce signalling molecules called Myc factors, which induce changes in plant root cells to facilitate colonisation (Bonfante & Genre, 2010). This bidirectional molecular dialogue ensures that both partners benefit from the association, with the plant receiving essential nutrients and the fungus obtaining carbohydrates.
Ecological Importance of AMF
Nutrient Uptake and Soil Health
AMF play a vital role in enhancing plant nutrient uptake, particularly phosphorus, which is often limiting in soils. The extensive hyphal networks of AMF explore a larger soil volume than plant roots alone, accessing nutrients that are otherwise unavailable to plants (Smith & Smith, 2011). This nutrient uptake mechanism is particularly important in nutrient-poor soils, where the availability of essential elements such as phosphorus can limit plant growth.
Soil Structure and Fertility
AMF contributes to soil structure and fertility by promoting the aggregation of soil particles. Their hyphae produce glomalin, a glycoprotein that binds soil particles together, improving soil stability and water retention. This process enhances soil health and supports plant growth (Rillig, 2004). The presence of AMF in soil can lead to improved soil structure, increased organic matter content, and enhanced microbial activity.
Plant Health and Stress Tolerance
AMF improves plant health and stress tolerance by enhancing nutrient uptake, improving water relations, and inducing systemic resistance to pathogens. Plants colonised by AMF exhibit increased resistance to drought, salinity, and soil-borne diseases (Pozo & Azcón-Aguilar, 2007). This improved stress tolerance is due to a combination of enhanced nutrient acquisition, increased production of stress-related metabolites, and improved root architecture.
On the picture below that was captured with a powerful microscope, we can observe how the hyphae of AMF is eliminating a harmful parasitic nematode in the root zone of a plant:
Biodiversity and Ecosystem Function
AMF contribute to biodiversity and ecosystem function by forming symbiotic relationships with a wide variety of plants. This diversity supports the stability and resilience of ecosystems. AMF also play a role in carbon cycling by transferring carbon from plants to the soil (van der Heijden et al., 2015). The presence of AMF in ecosystems can enhance plant community diversity, increase primary productivity, and improve nutrient cycling.
Applications in Sustainable Agriculture
Enhancing Crop Production
The use of AMF in agriculture can enhance crop production by improving nutrient uptake and plant health. AMF inoculation has been shown to increase the yield and quality of various crops, including cereals, legumes, and vegetables. This is particularly beneficial in low-fertility soils where conventional fertilizers are less effective (Jeffries et al., 2003).
Reducing Chemical Inputs
AMF can reduce the need for chemical fertilisers and pesticides in agriculture. By enhancing nutrient uptake and promoting plant health, AMF reduces the dependency on synthetic inputs, leading to more sustainable farming practices. This also helps mitigate the environmental impact of agricultural chemicals (Gianinazzi et al., 2010). The use of AMF can lead to more efficient use of fertilisers, reduced leaching of nutrients, and decreased reliance on chemical pest control methods.
Soil Remediation and Restoration
AMF are valuable tools in soil remediation and restoration efforts. They can help reclaim degraded lands by improving soil structure, fertility, and plant establishment. AMF inoculation is particularly effective in restoring soils contaminated with heavy metals and other pollutants (Gaur & Adholeya, 2004). By enhancing plant growth and promoting soil health, AMF can play a key role in the restoration of degraded ecosystems and the rehabilitation of contaminated sites.
Case Study: AMF in Maize Cultivation
A study by Subramanian and Charest (1998) investigated the role of AMF in maize cultivation under drought conditions. The study found that AMF colonisation significantly improved the nitrogen assimilation and overall nutritional status of maize plants, enhancing their resilience to drought stress.
Key Findings
AMF-colonised maize plants exhibited higher activities of nitrogen-assimilating enzymes compared to non-colonized plants (Subramanian & Charest, 1998).
Total amino acid concentrations were significantly higher in AMF-colonised plants, indicating enhanced nitrogen uptake and assimilation (Subramanian & Charest, 1998).
AMF improved the drought tolerance and recovery of maize plants, demonstrating the potential benefits of AMF inoculation in agriculture (Subramanian & Charest, 1998).
These findings highlight the importance of AMF in improving crop resilience to environmental stressors and their potential to enhance agricultural productivity.
Strock CF, Schneider HM, Galindo-Castaneda T, Hall BT, Van Gansbeke B, Mather DE, Roth MG, Chilvers MI, Guo X, Brown KB, Lynch JP (2019) Laser ablation tomography for visualization of root colonization by edaphic organisms. Journal of Experimental Botany, 70: 5327-5342 https://doi.org/10.1093/jxb/erz271.)
Structural Relationship with Plants
Penetration and Colonisation
The structural relationship between AMF and plants is intricate. Mycorrhizal fungi penetrate the cortical cells of plant roots, forming arbuscules within the cells. This is why they are called "arbuscular mycorrhiza" (from the Greek "mykes" meaning fungus and "rhiza" meaning root). The fungi form a mantle around the roots and extend their hyphae into the intercellular spaces of the roots. This symbiosis is most common in agricultural plants, with over 80% forming these associations (Smith & Read, 2008).
Different Types of Mycorrhizae
While AMF form arbuscules within root cells, other types of mycorrhizae, such as ectomycorrhizae (ECM), form a sheath around the roots and extend hyphae into the extracellular spaces. ECM live in symbiosis with hardwood trees, conifers, and certain shrubs, but do not form symbiosis with ericaceous plants like blueberries, azaleas, or rhododendrons. ECM are less widespread compared to AMF, colonizing less than 10% of land plant species, primarily conifers and some hardwoods (Smith & Read, 2008).
Microscopic Evaluation
Determining whether a plant is colonized by AMF requires microscopic evaluation. Root samples are stained and examined under a microscope to identify the presence of fungal structures such as arbuscules, vesicles, and hyphae (Brundrett et al., 1996). These structures are responsible for the exchange of nutrients between the fungus and the plant.
Challenges and Future Directions
Challenges in AMF Research and Application
Despite the known benefits of AMF, there are challenges in their research and application. These include the complexity of AMF-plant interactions, the variability of AMF effectiveness in different soils and climates, and the need for efficient inoculum production methods. Addressing these challenges requires ongoing research and collaboration among scientists, farmers, and industry stakeholders (Smith & Read, 2008).
Future Directions in AMF Research
Future research on AMF should focus on understanding the molecular mechanisms underlying AMF-plant interactions, developing efficient and cost-effective inoculum production methods, and exploring the potential of AMF in new agricultural and environmental applications. Advances in genomics, proteomics, and metabolomics will provide new insights into the biology and ecology of AMF, supporting their broader use in sustainable agriculture and environmental restoration (Bonfante & Genre, 2010).
Integration into Farming Systems
Integrating AMF into farming systems involves developing practical guidelines for their use, including inoculation techniques, compatible crop species, and management practices that support AMF colonisation. Extension services and educational programs can help farmers adopt AMF-based practices, promoting sustainable agriculture at a larger scale (Gianinazzi et al., 2010).
Policy and Support
Government policies and support are crucial for promoting the use of AMF in agriculture. This includes funding for research and development, incentives for sustainable farming practices, and regulations that encourage the reduction of chemical inputs. International collaboration and knowledge exchange can also facilitate the global adoption of AMF-based technologies (Gianinazzi et al., 2010).
Conclusion
Arbuscular mycorrhizal fungi are essential symbionts of the plant kingdom, playing a critical role in plant nutrition, soil health, and ecosystem function. Their symbiotic relationships with plants have evolved over millions of years, forming the foundation of natural ecosystems. AMF improve nutrient uptake, enhance plant health and stress tolerance, and contribute to soil structure and fertility. Their applications in sustainable agriculture and environmental restoration offer promising solutions to the challenges of modern farming and land management. As we continue to explore and understand the complex interactions between AMF and plants, we unlock new opportunities for enhancing agricultural productivity, restoring degraded lands, and promoting a more sustainable future.
References
Arbuscular mycorrhiza development and function, Caroline Gutjahr (Technical University of Munich (TUM), Germany)
Bonfante, P., & Genre, A. (2010). Mechanisms underlying beneficial plant-fungus interactions in mycorrhizal symbiosis. Nature Communications, 1(1), 48.
Brundrett, M., Bougher, N., Dell, B., Grove, T., & Malajczuk, N. (1996). Working with mycorrhizas in forestry and agriculture. Canberra: Australian Centre for International Agricultural Research.
Gaur, A., & Adholeya, A. (2004). Prospects of arbuscular mycorrhizal fungi in phytoremediation of heavy metal contaminated soils. Current Science, 86(4), 528-534.
Gianinazzi, S., Gollotte, A., Binet, M. N., van Tuinen, D., Redecker, D., & Wipf, D. (2010). Agroecology: The key role of arbuscular mycorrhizas in ecosystem services. Mycorrhiza, 20(8), 519-530.
Jeffries, P., Gianinazzi, S., Perotto, S., Turnau, K., & Barea, J. M. (2003). The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility. Biology and Fertility of Soils, 37(1), 1-16.
Parniske, M. (2008). Arbuscular mycorrhiza: The mother of plant root endosymbioses. Nature Reviews Microbiology, 6(10), 763-775.
Pozo, M. J., & Azcón-Aguilar, C. (2007). Unraveling mycorrhiza-induced resistance. Current Opinion in Plant Biology, 10(4), 393-398.
Redecker, D., Kodner, R., & Graham, L. E. (2000). Glomalean fungi from the Ordovician. Science, 289(5486), 1920-1921.
Smith, S. E., & Read, D. J. (2008). Mycorrhizal symbiosis (3rd ed.). Academic Press.
Smith, F. A., & Smith, S. E. (2011). Roles of arbuscular mycorrhizas in plant nutrition and growth: New paradigms from cellular to ecosystem scales. Annual Review of Plant Biology, 62, 227-250.
Subramanian, K. S., & Charest, C. (1998). Arbuscular mycorrhizae and nitrogen assimilation in maize after drought and recovery. Physiologia Plantarum, 104(1), 104-113.
van der Heijden, M. G. A., Martin, F. M., Selosse, M. A., & Sanders, I. R. (2015). Mycorrhizal ecology and evolution: The past, the present, and the future. New Phytologist, 205(4), 1406-1423.
Strock CF, Schneider HM, Galindo-Castaneda T, Hall BT, Van Gansbeke B, Mather DE, Roth MG, Chilvers MI, Guo X, Brown KB, Lynch JP (2019) Laser ablation tomography for visualization of root colonization by edaphic organisms. Journal of Experimental Botany, 70: 5327-5342 https://doi.org/10.1093/jxb/erz271.)
References
Bonfante, P., & Genre, A. (2010). Mechanisms underlying beneficial plant-fungus interactions in mycorrhizal symbiosis. Nature Communications, 1(1), 48.
Brundrett, M., Bougher, N., Dell, B., Grove, T., & Malajczuk, N. (1996). Working with mycorrhizas in forestry and agriculture. Canberra: Australian Centre for International Agricultural Research.
Gaur, A., & Adholeya, A. (2004). Prospects of arbuscular mycorrhizal fungi in phytoremediation of heavy metal contaminated soils. Current Science, 86(4), 528-534.
Gianinazzi, S., Gollotte, A., Binet, M. N., van Tuinen, D., Redecker, D., & Wipf, D. (2010). Agroecology: The key role of arbuscular mycorrhizas in ecosystem services. Mycorrhiza, 20(8), 519-530.
Jeffries, P., Gianinazzi, S., Perotto, S., Turnau, K., & Barea, J. M. (2003). The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility. Biology and Fertility of Soils, 37(1), 1-16.
Parniske, M. (2008). Arbuscular mycorrhiza: The mother of plant root endosymbioses. Nature Reviews Microbiology, 6(10), 763-775.
Pozo, M. J., & Azcón-Aguilar, C. (2007). Unraveling mycorrhiza-induced resistance. Current Opinion in Plant Biology, 10(4), 393-398.
Redecker, D., Kodner, R., & Graham, L. E. (2000). Glomalean fungi from the Ordovician. Science, 289(5486), 1920-1921.
Smith, S. E., & Read, D. J. (2008). Mycorrhizal symbiosis (3rd ed.). Academic Press.
Smith, F. A., & Smith, S. E. (2011). Roles of arbuscular mycorrhizas in plant nutrition and growth: New paradigms from cellular to ecosystem scales. Annual Review of Plant Biology, 62, 227-250.
Subramanian, K. S., & Charest, C. (1998). Arbuscular mycorrhizae and nitrogen assimilation in maize after drought and recovery. Physiologia Plantarum, 104(1), 104-113.
van der Heijden, M. G. A., Martin, F. M., Selosse, M. A., & Sanders, I. R. (2015). Mycorrhizal ecology and evolution: The past, the present, and the future. New Phytologist, 205(4), 1406-1423.
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