Role of Mycorrhizae in Agriculture

• Among all the mycorrhizal associations, the arbuscular mycorrhizal associations are the most abundant and crucial for agriculture.
• The extended hyphal networks of arbuscular mycorrhizal fungi (AMF) increase water and nutrient availability and translocation for crop plants.
• By changing soil’s chemical and biological properties, the AMF improves nutrient cycling and ensures the biocontrol of soil diseases. AMF also influences the physiological properties of above-ground tissues to increase tolerance to stress and improve plant-water relations.
• The mechanisms through which AMF improves nutrient translocation and disease control are not entirely understood; host-fungus relationships are also unclear for producing biostimulants and biofertilizers, and require more research.

Mycorrhizae are one of the most abundant soil microbial species and form associations with most land plants to improve their establishment, growth, and survival. Knowing the mechanisms by which they operate could help to increase crop productivity, especially in stressful environments. Therefore, learn more about the types of mycorrhizal associations and how they improve crop growth and productivity.

What is Mycorrhizae?

Mycorrhizae means fungus-root and refers to the symbiotic or mutually beneficial association between soil fungi and plant roots.

Mycorrhizal fungi are ubiquitous, and nearly 90% of land plants develop mycorrhizal associations for growth and development. Around 50000 fungal species form mycorrhizal associations with 250 000 plant species. The land plants can be angiosperms, gymnosperms, and pteridophytes. The associations of fungi can be host-specific in some cases.

The fungus sends hyphae into or around plant root tissues to access carbohydrates and other photosynthates. After establishing themselves with the plant’s root system, the fungi grow an extensive network of hyphae in the soil and improve the roots’ absorption of water and nitrogen (N), phosphorus (P), manganese (Mn), zinc (Zn), and copper (Cu), see Figure 1. Nearly 20% of the carbon fixed by plants through photosynthesis is transferred to mycorrhizal fungi in exchange for nutrients and water.

Figure 1: “Close-up of arbuscular mycorrhizal fungi growing from plant roots,” Yoshihiro Kobae. (Image credits: https://www.westernsydney.edu.au/hie/topics/how_plants_benefit_from_partnerships_with_soil_fungi

Types of Mycorrhizae

There are seven types of mycorrhizae, of which endomycorrhizae and ectomycorrhizae are abundant and widespread.

Ectomycorrhizae

Ectomycorrhizal fungi hyphae do not penetrate root cells. The hyphae form a sheath around the root tip called the mantle. The hyphae from the mantle grow between root cells to create a Hartig net of hyphae; in some cases, they can also penetrate root cells. These fungi belong to groups with above-ground fruiting bodies like Basidiomycetes, Ascomycetes, and Pycomycetes.

Endomycorrhizae

Figure 2: “AM fungi in corn root cells magnified 500x,” Mostad, G. 2016. (Image credits: Nichols, K, USDA-ARS, https://masters.agron.iastate.edu/files/mostadgreg-cc.pdf)

Endomycorrhizae are the more common symbiosis associated with around 85% of plants and occur in agricultural land. These are harder to detect on sight as they do not change the morphology of the roots like ectomycorrhiza. There are three types of Endomycorrhizae- Arbuscular, Orchid, and Ericoid Mycorrhizae.

The fungi invade the root cortex and grow between cells but can also penetrate cells to form arbuscules, which are branched finger-like structures that increase the area for nutrient transfer between the root cells and the fungi, see Figure 2. The fungal hyphae also form ballon-shaped vesicles that are used for storage.

Arbuscular Mycorrhizae (AM) are most common in 80% of all plant species and 90% of vascular plants. Earlier, they were called Vesicular-arbuscular mycorrhiza (VAM). However, not all fungi form vesicles, so they are now called Arbuscular mycorrhiza (AM). AM fungi are so abundant and numerous that they form a separate phylum Glomeromycota, with four genera Acaulospora, Gigaspora, Glomus, and Sclerocystis.

AM fungi are obligately biotrophic. That is, they depend entirely on plants for their survival. Unlike ericoid and orchid mycorrhizas, AM fungi have little or no host specificity, so that they can thrive in any soil and with any species.

Endomycorrihzae in Agricultural Soils

Several crops form associations with endomycorrhizal fungi, for example, like

corn, wheat, rice, soybeans, alfalfa, sorghum, cotton, etc.

However, many species do not associate with mycorrhizal fungi, like sugar beets, mustard, canola, buckwheat, cabbage, cauliflower, Brussels sprouts, and broccoli.

Endomycorrhizae help agriculture production by influencing soil and plant characteristics to improve yield, disease resistance, and stress tolerance.

Mycorrhizae Improves Soil Fertility and Structure

Figure 3: “Effects of arbuscular mycorrhizal fungi on improving soil fertility,” Fall et al. 2022. (Image credits: https://doi.org/10.3389/ffunb.2022.723892)

AM Fungi (AMF) improves soil conditions by enhancing their physical, chemical, and biological properties, see Figure 3. AM fungi enriches the soil by:

• Improving soil structure that enhances soil porosity, permeability, and water-holding capacity.
• Promoting microbial diversity in the soil.
• Aiding nutrient cycling.

Improve Soil Structure

AM fungi produce hyphae over 100 meters long, forming networks that bind the soil together to form stable aggregates that prevent soil erosion. The hyphae hold soil aggregates together even after death until they decompose, adding to the soil’s carbon content. The dead hyphae are replaced, so the aggregates remain stable.

The hyphae also produce a protein called glomalin that is insoluble in water and acts as a glue to combine these soil aggregates to form macro aggregates. These soil macro-aggregations stabilize the soil structure further and ensure improved water infiltration and drainage, reduced surface runoff and soil erosion, reduced organic matter and nutrients losses, increased soil aeration, and more retention of water and minerals, especially phosphates. Glomalin can last 7-42 years in the soil, so its influence is long-lasting.

Increase Soil Microbial Biodiversity

Soil nutrient cycling and fertility depend to a large extent on microbial diversity and their activities. Secretions from AMF change the physio-chemical environment in the rhizosphere by increasing root exudates that influence microbial community composition and their actions to improve soil fertility. The positive reactions improve soil fertility, nutrient acquisition, biological control of pathogens, and tolerance of plants to stress.

Aid Nutrient Cycling

AMF plays a significant role in the nutrient cycling of phosphorus, nitrogen, carbon, and micronutrients to enhance soil fertility. The mechanisms are not yet clearly understood, but the results are easier to prove.

Plants take phosphorus as inorganic P, which is limited in soils and quickly depleted as P mobility in soil is slow. AMF improves P availability through chemical and biological actions. AMF recruits phosphate-solubilizing bacteria that produce phosphatase that can mineralize organic P into inorganic P that plants can use. AMF produces organic acids that can help release P in rocks and fertilizers.

Plants use nitrogen as nitrates and ammonium ions. AMF plays a significant role in decomposing and mineralizing dead plant matter to forms usable by plants.

AMFs are essential for carbon cycling as they are a significant source of soil carbon. These fungi account for 9-55% of the microbial biomass and 5-36% of the total soil biomass and are a crucial soil carbon flux.

Moreover, as C in the air increases, plants fix more of it and allocate it to AMF to produce glomalin.

Mycorrhizal Influence on Plants

As a result of the AMF-induced improvement in soil’s physical, chemical, and biological properties, the growing conditions for crop plants are improved.

AMF action helps crop plants in the following ways:

• The hyphal network extends the plant’s access to nutrients and improves nutrient absorption ability.
• AMF increases plants’ resistance to soil diseases, pests, and viruses and tolerance to abiotic stress (drought & salinity).
• AMF ensures plant growth and yield.

Improves Nutrient Access

AMF helps nutrient cycling by mineralizing unavailable P and N and increasing nutrient availability.

AMF also aids in the translocation of nutrients. The fungi help plants increase P absorption through their extended network of hyphae and by exploiting a larger volume of soil. Since P mobility is slow, that increases P availability for plants.

When plants face nitrogen deficiency, they prefer to absorb it as ammonium. The mycorrhizal network can absorb N in the form of ammonium nitrate and amino acids, even though ammonium concentrations are 10-1000 times lower than that of nitrates. So the plant gets N in the form it needs through AMF properties.

Table 1: “Nutrients are taken in by plants that are infected and those that are not infected with AM fungi, when no P is added to corn,” Mostad. G, 2016. (Credits: https://masters.agron.iastate.edu/files/mostadgreg-cc.pdf

The absorption of micronutrients was also higher in soils with AMF, as shown in the results from an experiment in Table 1 that compares plants with AMF and those without AMF.

By mineralizing nutrients and improving access to nutrients, AMF can help reduce the amount of fertilizers added to the soil and reduce the negative environmental impact of agriculture. Moreover, reducing fertilizer costs can improve profits from food production for growers.

Bio-Control of Disease

AMF changes microbial community composition to increase species or abundance of beneficial microbes for the biocontrol of pests and disease-causing fungi, bacteria, and nematodes.

AMF and the increased number of beneficial microbes can control harmful pathogens through competition for resources and colonization sites on the roots. Also, the microbes can inhibit soil-borne pathogens and reduce their number through antagonistic effects, like nematode parasitism. AMF also strengthens plant defense by increasing phytohormone and protein concentrations, gene expression regulation, and secondary metabolite formation by changing the morphology or anatomy of roots, see Figure 4.

Around 30 species of AMF have been shown to be effective in the biocontrol of diseases and in reducing the severity of infections. However, the mechanism involved and the influence of interaction with environmental factors must be worked out.

Figure 4: “Schematic diagram of the mechanism of joint control of plant diseases by AMF and beneficial microorganisms,” Weng et al. 2022, (Image credits: https://doi.org/10.3390/microorganisms10071266)

AMF can be the sustainable alternative to costly pesticides that destroy soil biodiversity and create public health hazards. Since AMF action is localized, it will not impact nearby flora and fauna outside the field.

Increasing Stress Tolerance

AMF has been known to increase plant tolerance to stress caused by drought and salinity.

AMF improves soil structure by adding glomalin and increasing water-holding capacity to help plants during drought. AMF will also increase the hyphal length to enhance water absorption. As a result, above-ground functions like leaf water potential, stomatal conductance, photosynthesis, and dry matter accumulation are improved. AMF also physiologically alters above-ground tissue, like increasing chlorophyll levels, fluorescence, antioxidant activities, etc., to enhance water stress and salinity tolerance.

As a result, crops in drought and salinity have higher grain biomass and nutritional value than fields without AMF.

Improving Plant Growth and Yield

AMF improves soil conditions and water holding capacity and helps crop plants establish even in stressful situations. Improving water relations can help crops cope with drought, which is increasing due to climate change. Moreover, AMF ensures plants’ proper growth and health by increasing nutrient availability and translocation. The additional benefit of biocontrol of soil diseases and increasing tolerance to stress make crop plants healthier. As a result, the plant productivity increases. Moreover. There is also evidence that AMF can improve biomass and dry matter accumulation, directly enhancing the quantity and quality of food.

Impact of Conventional Farming Methods

A single AMF application artificially can last for a crop cycle to reduce the use of fertilizers, irrigation, and pesticides. Therefore, in open fields and greenhouses, AMF is used as biofertilizers, biostimulants, and bio-protectants.

However, farm management practices can negatively affect the colonization of AMF and must be considered when using AMF.

• Excessive tillage can break up hyphal networks and change fungal species composition in soil.
• Applying too many fertilizers reduces AMF buildup as nutrients are readily available, and plants do not need to depend on associations with AMF for nutrient sufficiency. Over time, this trend will reduce AMF species and abundance. Some AMF species are more sensitive than others to fertilization.
• Crop rotation, especially those involving monocultures of crops that do not form mycorrhizal associations like canola, will reduce AMF diversity and abundance for succeeding crops.

Measuring Mycorrhizae

Though mycorrhizae are ubiquitous, there are still many lacunae in understanding the mechanism involved in its role in agricultural soils and plants. Observing and measuring their growth and functioning will be essential to study them. Minirhizotrons like the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager that allow high-resolution, non-destructive observation of mycorrhizal growth, turnover, and behavior over one or more crop periods will be crucial to increase the use AMF as natural and eco-friendly alternatives to agrochemicals, and reduce water use, to make food production more sustainable.

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