Mycorrhizal Fungi as Plant Biostimulants

By: Youssef Rouphael, Department of Agriculture, University of Naples Federico II, Italy
email: youssef.rouphael@unina.it

Arbuscular mycorrhizal fungi (AMF) are among the more beneficial microorganisms used as biostimulants in agriculture. What makes the use of AMF more widespread than most equally beneficial symbionts is that they are able to establish a symbiosis with almost all higher plants and are able to growth in  wide range of climatic conditions. The mycorrhizal fungi increase the extension of the host’s root system enhancing the plant's nutrient uptake by acting as a link between the plant and the minerals it can't reach in the soil.

AM fungi germinate in the presence of a host plant. The presence of a host plant root breaks their dormancy, and a structure that links the plant to the fungus is formed. This is followed by the formation of hyphal structures and lipid-rich storage organs in the plant cells.

For the best results, the artificial introduction of AM fungi into the soil should incorporate an inoculum with the following features:

  • more than one AMF species
  • high amounts of infective fungi propagules
  • absence of pests and pathogens
  • presence of beneficial bacteria (‘mycorrhiza helper bacteria’) that enhance AMF root colonization and nutrient uptake
  • use of dry solid propagules as they have been shown to have a longer shelf-life

Crop management practices (crop rotation, the use of organic fertilizers and organic production) affect AMF root colonization either directly or indirectly. Direct impact happens when the fungi are damaged. Indirect implications, on the other hand, are a result of practices that create beneficial or unfavorable conditions to AMF. While the effect of different agricultural practices on AMF has not yet been fully explored, researchers have found crop rotation and organic production to promote soil biodiversity and increase spore number.

Practices with a negative impact include soil tillage, increased availability of nutrients and especially soluble phosphorus (reduces the need for symbiosis), and use of fungicides.

Inoculated crops react differently to such abiotic stresses as drought, salinity, nutrient deficiency, presence of heavy metals, and adverse soil pH, but the reaction will be partly dependent on where and how the AMF are propagated. Generally, however, inoculated crops facing the said stresses tend to behave in distinctive ways:

Drought: AMF are known to create hyphae on plant roots, which increases the root surface area. This loosely translates to an increased surface through which the plant can absorb more nutrients and water.

Salinity: Salinity is believed to affect AMF negatively. In return, AMF has been shown to subdue the adverse effects of salinity on crops. While most of the effects cancel out for AMF propagated in saline soils, you are more likely than not to experience an improved crop performance.

Nutrient Deficiency: Several researches in the recent past have found a positive correlation between AMF inoculation and enhanced nutrient uptake in horticultural crops. Partly, as stated above, this has something to do with the increased root surface area. The fact that AM fungi are better adapted to the absorption of certain nutrient types may also be a contributing factor.

Heavy Metals: AMF impairs the typical manifestation of heavy metals on crops by immobilizing the minerals in their biomass and giving the plant an extra surface to distribute the toxicity to.

Adverse Soil pH:  AMF-inoculated crops have been shown to produce higher biomass and marketable yield than uninoculated plants in the same soil pH conditions, and improve the nutritional status of potassium, calcium, and magnesium cations, which acidic soils are typically low in.AMF inoculation can also increase the nutritional status of iron, zinc, potassium, phosphorus, and manganese cations; increases chlorophyll content; and improves CO₂ assimilation within the plant.

AMF can stimulate the primary and secondary metabolism of a plant and, consequently, the biosynthesis and accumulation of phytochemicals also know as secondary metabolites. This happens as a result of the plant's defense being triggered by the AMF root colonization. A research on several aromatic and horticultural plants was conducted to monitor changes in the production of phytochemicals as a result of AMF inoculation, and a net increase in lipophilic and hydrophilic antioxidant accumulation was recorded. Some plants showed an increased concentration of essential oils while others produced more pseudohypericin and hypericin. Overall, different plant species had varying reactions to inoculation with different species of AMF, therefore an accurate selection of species with AMF strains is a must to obtain nutrient dense crops

AMF have a distinction versus other biostimulants in that they are not inhibited by environmental factors and can be formed by almost all plant species. By accompanying them with favorable practices such as the use of organic fertilizers and crop rotation, users can make arbuscular mycorrhizal fungi a potent substitute to a vast range of horticultural inputs. 

 

You are invited to request access to the entire Research Article that this topic is based on here:
Rouphael, Youssef, et al. "Arbuscular Mycorrhizal Fungi Act As Biostimulants In Horticultural Crops." Scientia horticulturae, v. 196, pp. 91-108. doi: 10.1016/j.scienta.2015.09.002