Written By: Hansini Muppaty
Email: hansini1706@gmail.com
Figure 1: The difference in grass grown with and without the presence of mycorrhizal fungi.
Introduction
Mycorrhizal Fungi are a diverse group of fungi that form symbiotic relationships with the roots of most plant species and play an important role in their nutrition. Due to their small size, their importance is usually overlooked, however, due to their incredible contribution to the environment they are of great importance. Their significant roles and responsibilities include:
Symbiosis with plant roots
Enhanced Nutrient Absorption
Improved soil structure
Increased plant resistance
What is Mycorrhiza?
Mycorrhiza, a symbiotic relationship between a fungus and a root system, plays a crucial role in plant development. The network of fungus filaments surrounds the developing root, providing an extensive surface area for the absorption of water and mineral ions from the soil. In return, the roots receive vital nutrients from the fungus, while providing sugars and nitrogen-containing components to the mycorrhizae.
Certain plants, such as Pinus, rely on this association for their germination and establishment. When two organisms form a mycorrhizal relationship, the fungus colonizes the host plant’s root tissues either intracellularly, as seen in arbuscular mycorrhizal fungi (AMF or AM), or extracellularly, as observed in ectomycorrhizal fungi. Depending on the species or the environment, mycorrhizae may have a parasitic or mutualistic association with host plants, but more often than not, the association is mutualistic.
Historical Background and Discovery Timeline
1840 - German botanist Heinrich Anton de Bary observes fungal structures in plant roots and proposes the term "mycorrhiza" (meaning "fungus root").
1885 - Frank was probably the first to recognize the widespread nature of associations between plant roots and mycorrhizal fungi
1900 - The concept of mycorrhizal associations gains recognition as Frank’s work is published, detailing the benefits of these symbiotic relationships.
1920s - Further research by European scientists establishes the widespread occurrence and ecological importance of mycorrhizal fungi in various plant species.
1950s - Improved microscopy techniques allow for detailed studies of mycorrhizal structures and interactions within plant roots.
1960s - Mycorrhizal research expands to include agricultural applications, particularly in enhancing crop growth and soil health.
1971 - The first International Mycorrhiza Conference is held, bringing together scientists from around the world to share research and advancements.
1974 - Mycologist Tom Harley and botanist James Mosse publish influential work on the role of arbuscular mycorrhizal fungi (AMF) in plant nutrient uptake.
1981 - The development of molecular techniques allows for the identification and classification of mycorrhizal fungi at a genetic level.
1982 - The term "arbuscular mycorrhizal fungi" (AMF) is officially adopted, distinguishing this group from other types of mycorrhizal associations.
1990 - Research on the ecological significance of mycorrhizal networks, also known as "common mycorrhizal networks" (CMNs), begins to emerge, highlighting the role of these fungi in connecting plants within ecosystems.
1995 - Studies demonstrate the potential of mycorrhizal fungi in phytoremediation, the use of plants to remove or neutralize pollutants from the environment.
2000 - The completion of the first genome sequence of a mycorrhizal fungus, Glomus intraradices, provides insights into the genetic basis of mycorrhizal associations.
2004 - The publication of "Mycorrhizal Symbiosis" by Sally E. Smith and David J. Read becomes a seminal reference in the field.
2010 - Advances in next-generation sequencing and bioinformatics accelerate the study of mycorrhizal fungal diversity and their ecological roles.
2013 - Research reveals the complex signaling pathways involved in the establishment and maintenance of mycorrhizal associations.
2017 - The discovery of the extensive role of mycorrhizal fungi in global carbon cycling highlights their significance in climate change research.
2020 - Studies continue to uncover the intricate relationships between mycorrhizal fungi, plant health, and soil ecosystems, with a focus on their applications in sustainable agriculture and environmental conservation.
2023 - Research on mycorrhizal fungi's potential to enhance crop resilience to climate change gains momentum, emphasizing their role in future food security strategies.
Types of Mycorrhiza
The main types of mycorrhizae are
Ectomycorrhiza
The symbiotic relationship known as Ectomycorrhiza (ECM) is a mutually beneficial association between fungi and the feeder roots of higher plants. This partnership is crucial for the existence of both partners, as it provides significant benefits to each other.
Endomycorrhiza
Endomycorrhiza fungi colonize trees, shrubs, and most herbaceous plants and do not form visible structures.
Arbuscular mycorrhiza (AM)
A symbiosis between plants and members of an ancient phylum of fungi, the Glomeromycotan, improves the supply of water and nutrients, such as phosphate and nitrogen, to the host plant. In return, up to 20% of plant-fixed carbon is transferred to the fungus.
Difference between Ectomycorrhiza and Endomycorrhiza
Figure 2: The difference between endomycorrhiza and ectomycorrhiza
The significant distinction between endomycorrhiza and ectomycorrhiza is that in ectomycorrhiza, fungal hyphae do not pierce the cortical cells of the roots. Still, in endomycorrhiza, fungal hyphae pierce the plant roots’ cortical cells. Mycorrhizae is a vital symbiotic interaction between the fungus and higher plant roots. This collaboration helps both fungi and plants. Fungal hyphae enter the earth and offer fiber to the plants, while the plants also benefit from the fungus. As a result, it is an important link. Most significantly, fungal hyphae may expand some meters long and transport nutrients and water to the roots, particularly nitrogen, phosphate, and potassium.
Furthermore, the mycorrhizal connection guards the plant against root infections. As a result, signs of nutrient insufficiency are unlikely to emerge in plants involved in symbiotic relationships. Mycorrhizae are classified into two types: endomycorrhiza and ectomycorrhiza.
Structural Diversity of Mycorrhizal Interactions
Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations differ in their structural characteristics and in the plant and fungal species that they involve. In AM roots the fungus penetrates intercellularly and intracellularly into the root cortex, whereas in ECM roots the fungus only penetrates intercellularly into the root cortex. Figure 1 illustrates the main structural differences between AM and ECM associations of angiosperms or gymnosperms, which are discussed in greater detail below.
Figure 3: Structural characteristics of arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) roots of gymnosperms or angiosperms.
Mycorrhizal Effects on Soil Structure
Maintenance of soil structure is of critical importance to the preservation of soil functions and fertility. Mycorrhizal fungi play a major role in soil aggregation through hyphae networking and glomalin (biological glue) production.
Therefore, their presence in the soil is essential to maintain physical soil properties. Better soil structure results in:
Greater water infiltration
Greater water-holding capacity
Better root development
Better resistance to compaction
Better resistance to erosion
In organic farming, the use of synthetic products is prevented. Thus the soil conditions are more favorable to mycorrhiza. The easiest and most effective way to apply mycorrhiza is during sowing: To the substrate we use in the seedbed we will add the bacteria so that from the moment the roots emerge from the seeds, they come into contact with fungi and establish the symbiosis quickly.
How do Mycorrhizal Fungi Contribute to Biodiversity and Ecosystem Stability?
According to ‘biodiversity of arbuscular mycorrhizal fungi and ecosystem function’ ( Jeff R.Powell, Matthias C. Rillig ) Arbuscular mycorrhizal (AM) fungi play important functional roles in ecosystems, including the uptake and transfer of nutrients, modification of the physical soil environment, and alteration of plant interactions with other biota.
Several studies have demonstrated the potential for variation in AM fungal diversity to also affect ecosystem functioning, mainly via effects on primary productivity. Diversity in these studies is usually characterized in terms of the number of species, unique evolutionary lineages, or complementary mycorrhizal traits, as well as the ability of plants to discriminate among AM fungi in space and time. However, the emergent outcomes of these relationships are usually indirect, and thus context-dependent, and difficult to predict with certainty.
The arbuscular mycorrhizal (AM) symbiosis is unique among the various interactions in which plants engage because of its ubiquity from perspectives both geographic (its occurrence on all continents and in most biomes) and evolutionary (the ability to engage in the symbiosis is lost, not gained). It is fundamentally a nutritional symbiosis, in which nutrients (phosphorus and others) are traded with plant hosts for carbon (sugar and lipids). This very exchange, with all its physiological, biochemical, molecular and ultrastructural intricacies, has captured the imagination of many researchers and explains the prominence of work at the organismal level, i.e. the examination of individual plant–fungal modules.
Observations that inoculation with AM fungi could increase host biomass and phosphorus content triggered a flurry of early studies aiming to demonstrate the effects of these fungi across a broad range of plant species, including those of economic importance, in a variety of environmental contexts (Koide & Mosse, 2004; Fig. 1a). Detailed examinations of AM fungal diversity came later (Fig. 1b), particularly facilitated by advances in environmental DNA sequencing. For example, in a global assessment of AM fungal diversity, Davison et al. (2015) observed approximately five-fold variation in virtual taxon (VT; a proxy for ‘species’) richness among their sampled plots.
However, when looking at roots associated with individual plants, they observed extensive variation in diversity in each root sample, ranging from a single VT to as many as 66 (median = 14, 25%/75% quantiles = 8/24). Given the extensive variation in VT richness, which represents a reasonable proxy for species richness, these studies provide further evidence for AM fungi being especially relevant with respect to the ecology of their hosts.
Figure 4: Interest in the functional roles of arbuscular mycorrhizal (AM) fungi from an ecosystem perspective has increased rapidly in the last 40 yr (note y-axis scaling). Before this, work on symbiosis focused mainly on host plant-level research. More recently, work on diversity within the symbiosis has exhibited rapid growth, largely as a result of advances in environmental DNA sequencing methods. (b) Interest in the diverse functional roles played by AM fungi in ecosystems has also grown in the last 30 yr. Each search was performed on the Web of Science Core Collection on 10 November 2017.
Interaction with other Soil Organisms
Mycorrhizal fungi interact with a wide range of other soil organisms, in the root, in the rhizosphere, and in the bulk soil. These interactions may be inhibitory or stimulatory; some are clearly competitive, others may be mutualistic. Effects can be seen at all stages of the mycorrhizal fungal life cycle, from spore population dynamics (predation, dispersal, and germination) through root colonization to external hyphal growth. Two areas that seem likely to be of particular importance to the functioning of the symbiosis are the role of bacteria in promoting mycorrhiza formation and of soil animals in grazing the external mycelium. Mycorrhizal fungi also modify the interactions of plants with other soil organisms, both pathogens, such as root-inhabiting nematodes and fungi, and mutualists, notably nitrogen-fixing bacteria. These interactions are probably important both in natural ecosystems, where pathogens are increasingly recognized as playing controlling roles, and in agricultural systems, where mycorrhizas may be valuable in designing integrated systems of pest control and growth stimulation.
Conclusion
Mycorrhizal fungi provide the soil with numerous advantages. Application of mycorrhizal fungi is presently reaching an industrial stage supported by widespread applied research and marketable applications emphasizing eco-friendly and sustainable aspects. Although they are incredibly small in size, they have a remarkable significance in the great plant kingdom.
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