Ectomycorrhiza, Friend or Foe?

Abstract

Many ecology textbooks present the interaction between mycorrhizal fungi and their host plants as the archetype of symbiosis or mutualism. However, mycorrhiza drains carbon directly from the plant and also competes with the plant for soil inorganic nitrogen. We developed hypotheses based on a simple model to qualitatively investigate when, in a nitrogen-limited system, the fungal partner returns sufficient extra nitrogen to compensate for the amount of carbon allocated to it by the plant. We showed when the mycorrhizal association can be beneficial to the plant, but also when mycorrhizal immobilization of soil inorganic nitrogen can be a limitation. The amount of carbon and nitrogen that the mycorrhizal fungus can obtain from soil organic matter, by producing extracellular enzymes, is also important. Saprotrophic capability decreases the value of the fungus to the plant, as fungal uptake of soil carbon augments the use of the plant-supplied carbon and increases the fungal requirement for N. The stoichiometric mismatch between low-N soil organic matter and high-N fungal biochemistry turned out to be a bottleneck in making the fungus a net provider of additional N to the plant. The most important properties determining the usefulness to a plant of a mycorrhizal symbiont are plant nitrogen use efficiency and the amount of inorganic N taken up per unit extra fungal growth. The fraction of carbon the fungus allocates to its own growth, relative to its investment in exocellular enzymes, is also a critical property. Our results show that plants could benefit from the association with the fungus, which could explain the ubiquitous nature of this association between fungi and plants.

Use of commercial mycorrhizal fungi in stress-free growing conditions of potted olive cuttings

“…Ectomycorrhizal fungi may also have beneficial interactions with plants. Many tree species form symbiotic associations with ectomycorrhizal fungi which are known to enhance the uptake of nutrients by plants ( Kieliszewska-Rokicka, 1999;Ågren et al, 2019) or which provide protection by alleviating them of heavy metal toxicity (Luo et al, 2014;Egerton-Warburton, 2015).…”

Mycorrhizal Fungi were More Effective than Zeolites in Increasing the Growth of Non-Irrigated Young Olive Trees

“…In the case of leaf P levels, a growing trend was observed over time, but without significant differences between treatments, which may reflect the establishment of symbiotic associations between plants and native microorganisms. It should be noted that the olive tree is the main crop in the region and beneficial microorganisms, particularly ectomycorrhizal and AM fungi, tend to be ubiquitous in nature and establish symbiotic associations with most plant species [13,17,50]. P is the most common nutrient from which plants benefit when these associations are established [20,23,51], particularly in acidic soils, where it is expected to be poorly available to plants [47,52].…”

Digging Deeper: In Search of the Mechanisms of Carbon and Nitrogen Exchange in Ectomycorrhizal Symbioses

“…Fungal growth type and foraging strategy may, for example, be simple parameters for ECM fungal classification, as both relate to the C demand of the fungus and its ability to find and source soil nutrients (Agerer, 2001;Clemmensen et al, 2014). Others have suggested that fungal C cost per unit N provided may be useful in characterizing the association (Corrêa et al, 2008;Agren et al, 2019); however, this is a complicated measure that may change dramatically based on external situations, or even over the lifetime of the mycorrhizal association (Nehls et al, 2016).…”