Five Important Arbuscular Mycorrhizal Fungi Research Studies in 2024

• In 2024, many studies will explore frameworks and concepts to fill in gaps in our understanding of the basics of mycorrhizal composition and functions, from microscopic to large scales.
• The Mycorrhizal fungi benefits in agriculture are a key focus, with studies aimed at improving the application of arbuscular mycorrhizal fungi (AMF) to enhance soil health and crop productivity.
• The research covers optimizing mycorrhizal applications to benefit natural and agricultural systems.

Nearly 90% of terrestrial plants have associations with mycorrhizae, a common soil microbe. The two major groups are ectomycorrhizae and endomycorrhizae. The vital contributions that mycorrhizae make to soil fertility and health, plant establishment, growth, and productivity are the focus of a growing body of research. In this article, you can read about insights from five research published in 2024 about mycorrhizae.

1. A Framework of The AM Role in SOM Dynamics

Figure 1. “Diagram showing the updated conceptual framework of arbuscular mycorrhizal (AM) fungi-mediated soil organic matter (SOM) dynamics. Plants fix carbon through photosynthesis, which is then delivered to AM fungi. Arbuscular mycorrhizal fungi influence SOM dynamics through four pathways classified as (1) generating, (2) reprocessing, (3) reorganizing, and (4) stabilizing” Wu et al. 2024. (Image credits: https://doi.org/10.1111/nph.19178)

The arbuscular mycorrhizae (AM), a type of endomycorrhizal fungi, are associated with 80% of terrestrial plants. It is known that AM helps in various soil processes like stabilization, nutrient cycling, microbial diversity, and soil organic matter (SOM) formation.

Wu et al. (2024) proposed a conceptual framework to explain the AM’s role in soil organic matter formation, including factors not considered before, such as the diversity of organic compounds, mineral weathering, chemical interaction, hyphosphere microbial contribution, etc. They propose four pathways that help in SOM generation, reprocessing, reorganization, and stabilization, see Figure 1.

Generating: AM fungi produce exudates, metabolites, mucilage, and necromass. The diversity, composition, and properties of these chemical compounds are called chemodiversity by Wu et al. The scientists consider that this chemodivesity and not individual compounds’ action is crucial in determining SOM composition, biodegradation rate by soil microbes, and persistence.

Reprocessing: AM fungal recruit distinct microbes in their hyphosphere, the soil portion influenced by AM hyphal exudates. These hyphosphere microbes are the main drivers of soil biochemistry as they act on SOM components that AMF cannot decompose. The hyphosphere microbes use internal and extracellular pathways to decompose and assimilate SOM. Their new compounds add to chemodiversity, persistence, and resynthesis SOM. The role of hyphosphere microbial community is called the hyphosphere ‘microbial carbon pump.’

Reorganizing: The fungi’s mycelial growth, expansion, and colonization change the soil’s physical porosity and hydraulic properties. While AM stabilizes macro soil aggregates, the mycelial dynamics increase micro aggregate turnover, water infiltration, soil-water retention capacity, hydraulic conductivity, and redistribution of AM exudates. The changing soil conditions due to mycelial expansion also change nutrient availability, temperature, and oxygen. It results in SOM redistribution and transformation.

Stabilizing: AM fungi cause mineral weathering and alter interactions that influence SOM formation and stabilization. AM rock mineral weathering makes nitrogen, phosphorus, potassium, and magnesium available in soils that form secondary compounds with different sizes, surfaces, and reactivity. That can alter mineral absorption, catalysis, and oxidation of SOM. These processes are called the ‘soil mineral carbon pump.’

Takeaway: The new concept can explain AM’s role in small- to large-scale SOM dynamics, which can help develop mycorrhiza-based technologies to enhance soil health.

2. Spore Dimorphism in AM Fungi

Most AM fungi belong to the Glomeromycotina, one of eight sub-divisions in the Kingdom of fungi. Glomeromycotina AM is associated with 72% of vascular plants and has 350 species. Most species are described based on spore anatomy.

However, there is a similarity in spores of different species due to convergence in evolution, phenotypic plasticity, and dimorphism to a lesser extent. Due to environmental factors and development, morphological plasticity can result in differences in color, size, and shape of even genetically identical spores. Dimorphism causes different ontogeny to produce different spore types. Intragenomic rDNA heterogeneity makes spore molecular identification confusing.

Figure 2: “Two spore morphotypes observed by light microscopy in PVLG and PVLG + Melzer buffer,” Kokkoris et al. 2024. (Image credits: https://doi.org/10.1111/nph.19121) 

Rhizophagus irregularis is the AM fungi species widely used for research and plant biostimulants. In this study, Kokkoris et al. (2024) tested if R. irregularis spores also suffered dimorphism. They used cultivated isolates or daughter spores from the same spore in symbiotic and asymbiotic systems. Sanger and PacBio sequencing of the glomalin and partial 45S rRNA genes was also conducted. Several advanced microscopy techniques studied the spores’ characteristics.

The scientists found that four R. irregularis strains produce two types of spores, see Figure 2. One of the spore morphotypes is currently described for R. irregularis, but the second one is closer to known R. fasciculatus spores. The color, hypha thickness, and inner and outer wall properties differ. In the two morphotypes, the glomalin gene sequence is the same, but the

45S rRNA genes are different. The 45S rRNA in the second morphotype has 99.8% similarity to R. fasciculatus spores.

Takeaway: The scientists concluded that  R. irregularis shows dimorphism in spores and could explain the confusion in taxonomy in culture production during AMF research.

3. Defining Hyphosphere Core Microbiome and Their Functions

Figure 3: “Schematic presentation of the hyphosphere core microbiome. In natural or agricultural ecosystems, a stable and taxonomically conserved hyphosphere core microbiome exists within the complex interaction network formed among plants, AM fungi, and hyphosphere bacteria” Wang et al. 2024. (Image credits: https://doi.org/10.1111/nph.19396)

The arbuscular mycorrhizal secrete exudates, much of which are photo-assimilates derived from the host plant, into the soil surrounding the hyphae, called the hyphosphere. This is the area where bacteria that are attracted and feed on AM exudates, grow and reproduce, forming a microbial community distinct from other places in the soil or plant rhizosphere.

Not much is known about the microbiome in the hyphosphere compared to the rhizosphere. Therefore, Wang et al. (2024) wanted to review information on the core hyphosphere, how the members coexist, and establish their significance for AM fungi fitness and functions and their host plants, see Figure 3.

Core microbiome

It was known that microbiome composition changes as a response to AM fungal species and the environment. However, some studies now show that regardless of the AM fungal species, climate zones, and crop-soil systems, certain bacteria at the phylum level are core members and are always present in any hyphosphere. The dominant core bacteria members are Myxococcales, Betaproteobacteriales, Chloroflexales, Fibrobacterales, and Cytophagales. Hence, the holobiont concept that consisted of only a host plant and AM fungi must include this standard associated bacterial community.

Potential coexistence mechanisms

The various bacteria in the hyphosphere microbiome interact, and the mechanisms by which they coexist are still being explored. Wang et al. suggest a few means based on coexistence of bacteria in other systems, such as:

• Resource partitioning, where different members use varying sets of nutrients.
• Syntrophy or metabolic interactions between groups where one set uses intermediate products produced by other bacterial groups. Such cross-feeding is common in natural microbial communities.
• Biofilm formation by bacteria that colonize AM hyphae can form sub-communities. 

Potential ecological functions

Wang et al. consider that the core hyphosphere microbiome could have four potential roles in improving AM fitness- increasing organic phosphorus mineralization, enhancing organic nitrogen mobilization, preying on bacteria, and providing carbon.

Mineralize phosphorus: AM does not secrete or mineralize phosphorus. However, host plants have more soil phosphorus. Several studies have shown that bacteria associated with AM increase phosphorus mineralization. Hence, it is from hyphosphere bacteria that AM gets phosphorus, which is passed on to plants.

Nitrogen mobilization: AMs have 4-7 times more nitrogen in their hyphae than plants and require the element for their growth. The bacteria associated with AM help them access required amounts from the soil.

Predation: Some bacterial predator species, like Myxococcales, attack non-core bacteria and mobilize nutrients from their biomass to increase nitrogen and phosphorus in the hyphosphere.

Takeaway: The concept provides insights into the hyphosphere core microbiome, which can improve understanding of AM symbiosis mechanisms to maximize their beneficial functions.

4. Tree Factors Influencing Root Fungal Communities

Figure 4: “Ordination plots of mycorrhizal type and tree diversity show that monoculture and two species tree stands have distinct fungal communities, but multispecies tree has a fungal converged community. Blue, ectomycorrhiza (EcM) samples; Mono, monospecific stands; Multi, multi-species mixtures; Red, arbuscular mycorrhiza (AM) samples; Two, two-species mixtures. Different plotting symbols represent tree diversity levels” Singavarapu et al. 2024. (Image credits: https://doi.org/10.1111/nph.19722)

Tree species and the environment are essential factors determining the composition of soil fungal communities. Plants are selectively associated with specific fungi through co-evolutions. AM plants are associated with AM fungi, and ectomycorrhizal (EcM) plants select EcM fungi. The dominance of any of these two tree mycorrhizal types will shift the fungal community to AM or EcM fungi. However, as tree diversity increases in an area, the differences in fungi associated with the AM and EcM trees become non-significant.

Some scientists decided to use the biodiversity-ecosystem functioning model to find the effects of subtropical tree species richness/number on the composition and roles of fungal communities in different environmental conditions. The subtropical tree communities tested were monospecific, to-species, and multispecies stands. The researchers also extracted DNA and sequenced the ITS2 gene for the tree species.

The results revealed more diversity in fungi associated with AM trees than those related to the EcM. This trend was seen in monocultures, two, and multispecies mixtures.

Tree diversity dampens tree mycorrhizal type and tree species effects on fungal communities. In multi-species stand, the fungal communities converge, and the difference in fungal communities associated with AM and EcM trees is reduced, see Figure 4.

Instead, spatial distance and other tree-related variables (phylogeny and functional traits) had more influence on fungal community composition. Soil variables explained differences in EcM more than the diversity of AM fungal communities.

Takeaway: This research highlights the contributions made by tree and soil abiotic factors in shaping soil fungal communities associated with varying subtropical tree diversity.

5. More AM is Better

Climate change-induced drought stress affects crop growth and yield quantity and quality. It is a challenge that must be overcome to ensure food security and agricultural stability. Biological fertilizers containing arbuscular mycorrhizal fungi (AMF) are one of the various options for increasing crop drought tolerance. AMF helps indirectly by improving nutrient availability, which activates multiple processes to increase a plant’s ability to withstand drought.

However, not all AMF species reduce drought stress. Moreover, research on the comparative benefits of using one vs multiple AMF for crops has significant gaps. Hence, Paravar and Wu (2024) conducted a pot experiment with Lallemantia spp that were inoculated with one and combined AMF (Claroideoglomus etunicatum, Rhizophagus intraradices, and Funneliformis mosseae) in different drought conditions. The combined AMF was given in various mixtures.

Lallemantia royleana (Lady’s mantle) and L.iberica (Dragon head) are used as medicinal plants and vegetables, as they are rich in polysaccharides, and seeds are rich in fatty acids like linolenic acid. Drought reduced oil, fatty acid, and carbohydrate contents. Water shortage also decreased photosynthesis, antioxidant activities, water use efficiency, and nutrient uptake.

The plants inoculated with single and combined AMF, especially combined Mix 4, improved fungal colonization of roots. As a result, water use efficiency and seed yield improved in plants. The seeds produced using Mix 4 had the highest content of sugars, oil, fatty acids, and mucilage compared with the single AMF and other AMF mixtures. Mix 4 also reduced hydrogen peroxide and lipid peroxidation, which cause stress.

Takeaway: The experiment showed that using a combination of AMF improves crop growth and nutritional value more than using a single AMF species.

Tools to Study Mycorrhizal Fungi

New tools helping to investigate underground processes have advanced studies of mycorrhizal fungi. Some onsite tools for short and long-term research are minirhizotron systems that use installed transparent root tubes and root imagers, such as those offered by CID BioScience Inc.- the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager. Find out more about our root imagers and how they can help in your research on root-associated fungi.

Sources

1. Wu, S., Fu, W., Rillig, M. C., Chen, B., Zhu, Y. G., & Huang, L. (2024). Soil organic matter dynamics mediated by arbuscular mycorrhizal fungi–an updated conceptual framework. New Phytologist, 242(4), 1417-1425. Cited:25

 

2. Kokkoris, V., Banchini, C., Paré, L., Abdellatif, L., Séguin, S., Hubbard, K., … & Stefani, F. (2024). Rhizophagus irregularis, the model fungus in arbuscular mycorrhiza research, forms dimorphic spores. New Phytologist, 242(4), 1771-1784. Cited by 12

 

3. Wang, L., George, T. S., & Feng, G. (2024). Concepts and consequences of the hyphosphere core microbiome for arbuscular mycorrhizal fungal fitness and function. New Phytologist, 242(4), 1529-1533. Cited:11

 

4. Singavarapu, B., ul Haq, H., Darnstaedt, F., Nawaz, A., Beugnon, R., Cesarz, S., … & Wubet, T. (2024). Influence of tree mycorrhizal type, tree species identity, and diversity on forest root‐associated mycobiomes. New Phytologist, 242(4), 1691-1703. Cited by 2

 

5. Paravar, A., & Wu, Q. S. (2024). Is a combination of arbuscular mycorrhizal fungi more beneficial to enhance drought tolerance than single arbuscular mycorrhizal fungus in Lallemantia species? Environmental and Experimental Botany, 226, 105853.