The benefits and challenges of Arbuscular Mycorrhizal Fungi (AMF)

August 18,2023

Soil degradation and climate change are affecting agricultural productivity worldwide, posing some of the greatest threats to food security. Yet the news is not all grim – developments in microbial fermentation technology and some very special microbes can mitigate those burning problems, and bring us couple of steps closer to the stable and sustainable food system we are seeking to create.

Who are those special microbes? Well, they do require a bit of a pompous introduction given they come from a very old and respected family. Without further ado…

More than 400 million years ago, ancient plants and very peculiar species of fungi started a molecular dialogue that led to a historic trade deal – the formation of a symbiotic relationship in which fungi deliver mineral macro and micronutrients from the soil to the plant, getting carbohydrates and lipids in return. This trade deal enabled plants to quickly and successfully expand their frontiers, and successfully colonize land. Those fungal species are still with us today, almost in an unchanged form – we call them arbuscular mycorrhizal fungi AMF, and the relationship they form – arbuscular mycorrhiza. Science is currently familiar with more than 240 AMF species that interact with >200.000 plant species.

How do they do it?

Arbuscular mycorrhizal fungi form a close relationship with the plant, growing into its roots, penetrating cortical cells, and forming highly branched structures called arbuscules. These structures are specific to AMF and represent the primary sites of nutrient trade between the plant and the fungus. The rest of the fungal mycelium lies in the soil, forming a broad hyphal network that performs numerous vital functions.

Besides delivering key micro and macronutrients to plants, AMF also aid them indirectly, by improving soil quality and fertility. They do it by aggregating the soil particles physically – with their wide-spreading hyphal network, and chemically – by producing and depositing a sticky glycoprotein called glomalin. Soil aggregates are essential components of soil structure, and their stability has a strong effect on soil’s water-holding capacity, microbial diversity, as well as nutrient cycling and carbon sequestration capacities.[1]

The benefits of AMF

Evolution proves that collaboration is the ultimate tool for success, and arbuscular mycorrhiza is one of the prime examples. Coevolution of plants and AMF helped plants colonize the terrestrial realms and create the world we know and love today. After hundreds of millions of years, arbuscular mycorrhiza didn’t lose its impact on the natural world. Quite the contrary – it has been gaining increasing significance, especially for one species – Homo sapiens.

Almost all plants humans eat form and benefit from arbuscular mycorrhiza, and seizing the advantages of this symbiotic relationship has the perspective to help us address some of the greatest challenges in agriculture today:

  • restoring soil health and fertility
  • increasing crop productivity
  • meeting the growing food demand sustainably
  • mitigating the effects of climate change

Let’s dig a bit deeper and explore the five main benefits of AMF that make this possible:

Nutrient absorption

AMF form mycelial networks that span far and wide, occupying large volumes of soil, giving them access to a vast mineral micro and macronutrient source, unavailable to plants. AMF increases accessible soil volume of the plant’s root system 5–14 times and its nutrient uptake efficiency 175-190%.[2]

Providing a consistent source of necessary food, AMF ensure steady growth and development of plants, increasing their biomass and improving organoleptic qualities of their fruits. Since the fungus enables plants to tap into nutrient sources previously unavailable them, it effectively reduces the need for mineral fertilizers and the negative effects that follow their use (nutrient leaching, contamination of groundwater, unwanted growth of algae and other organisms).

Drought tolerance

Plants that are in symbiotic relationship with AMF display improved water uptake and water economy, which makes them more resilient to drought conditions. The fungus protects the plant from negative effects of drought stress by helping it maintain membrane integrity, stimulating the production of antioxidants, and activating genes that make the plant more tolerant to these stressful conditions.[3]

The production of glomalin also plays an important role in mitigating the effects of drought. By creating more stable soil aggregates, glomalin increases their water-holding capacity and decreases water loss.[3]

Pest & disease resistance

Aside from making plants more tolerant to environmental stress, AMF also increase their resistance to pathogens, especially the ones that lurk in soil, like nematodes, bacteria and fungi (Fusarium, Rhizoctonia, Pythium, Phytophthora). The fungus alters plant root morphology and excretion of exudates, effectively reducing pathogen virulence and supporting beneficial microbes to outcompete the harmful ones.[4]

Soil fertility

Soil depletion limits agricultural production in many areas around the world, and mainly results from poor soil structure, lack of available nutrients, and inadequate agricultural practices. A prime indicator of fertile soil is a rich and active microbial community, and the presence of AMF supports favorable conditions for the growth of beneficial microbes.

Additionally, AMF produce organic acids and glomalin, improving the soil on both structural and chemical levels. The broad hyphal network and the stable soil aggregates make soil less prone to erosion, as well as to nutrient and water loss. Organic acids and glomalin also trap toxic chemicals like heavy metals, preventing their entry into the food chain.[5]

Carbon sequestration

The broader impact of AMF is seen in their carbon-sequestering abilities. In exchange for their valuable services, AMF get a hefty amount of carbohydrates and lipids from plants – enough to satisfy their needs, but also some to put aside. This essentially means that AMF remove a considerable portion of atmospheric carbon dioxide by storing products of plant photosynthesis in their mycelium – 3.94 Gt CO2e per year to be specific, which equals to about 11% of global CO2 emissions.[6]

The benefits of carbon sequestration in agriculture are numerous – from increasing organic matter content of the soil, to aiding carbon footprint reduction in an already problematic industry.

Challenges

Given this vast evidence of AMF performance, a question that emerges is – why AMF isn’t “a thing” in a market starving for effective, sustainable products? The low adoption rate in agriculture today can be associated with a systematic lack of high-quality AMF products offered at a reasonable price. Producing contaminant-free, effective and convenient AMF products poses a significant scientific and engineering challenge that can be summed up in three key points:

1. Reproducibility

Nature is a complex system, where 1 plus 1 doesn’t always equal 2, as synergies and antagonisms between certain elements can yield quite different outcomes. This also applies to arbuscular mycorrhiza and the specific fungal and plant species that create this relationship. AMF species are generally not host-specific, which means they can interact with many different plant species. However, this doesn’t mean that the level of compatibility and benefit yielded is the same for all plant species. Certain AMF-plant combinations cannot deliver even a fraction of desired benefits, so it is crucial to match the adequate AMF organism with the targeted crop.

In-vivo vs in-vitro

Current state-of-the-art AMF production methods include in-vivo greenhouse production and solid-state microbial fermentation. Both methods are failing to deliver products of consistent quality and efficacy due to contaminations (in-vivo) and lack of control during production process (in-vitro).

Formulation & application

Bringing a convenient, inexpensive and effective biological product to a wide market adoption is not an easy feat. Living organisms behave differently than the agricultural chemicals we are used to; they have their needs and wants, and a shorter expiry date. Bridging the gap between current agricultural practices and delivering effective AMF products to the field necessitates formulation development and an end product that is able to provide:

  • Reasonably long shelf-life;
  • Stability;
  • Consistent quality between batches;
  • Ease of adoption with current ag input machinery.

Overcoming these biological and technical challenges will deliver the full benefits of AMF, bringing an effective, sustainable solution to the most critical problems in modern agriculture – lack of productivity, soil depletion, and climate change.

References

[1] TB Irving. (2021). A critical review of 25 years of glomalin research: a better mechanical understanding and robust quantification techniques are required. New Phytologist.

[2] W Li-ping. (2009). Fertilizing reclamation of arbuscular mycorrhizal fungi on coal mine complex substrate. Procedia Earth and Planetary Science.

[3] H Tang. (2022). The critical role of arbuscular mycorrhizal fungi to improve drought tolerance and nitrogen use efficiency in crops. Frontiers in Plant Science.

[4] W Weng. (2022). Roles of arbuscular mycorrhizal fungi as a biocontrol agent in the control of plant diseases. Microorganisms.

[5] AF Fall. (2022). Roles of arbuscular mycorrhizal fungi on soil fertility: Contribution in the improvement of physical, chemical, and biological properties of the soil. Frontiers in Fungal Biology.

[6] HJ Hawkins. (2023). Mycorrhizal mycelium as a global carbon pool. Current Biology.

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