Mycorrhiza – the roots of the roots

Mycorrhiza is a symbiotic relationship between a fungus and a plant’s roots. Mycorrhiza is highly common in most plant species and is nature’s way of delivering nutrients to plants. The fungus becomes an organ that grows on the roots and acts as an extension and finer branching of these roots.

Rot med hyfer og sporer
Photo: Iver Jakobsen / København Universitet
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The fungus’ function is to scavenge soil for nutrients, which it transports back to the plant.  Therefore, mycorrhiza can be called “the roots of the roots.” In many plants, almost all uptake of nitrogen, phosphorous and some micronutrients is mediated by mycorrhiza fungi.

Mycorrhiza is very common in nature, and approximately 85 percent of all plant species form mycorrhiza. From an evolutionary perspective, mycorrhiza is incredibly old, and fossil data shows that mycorrhiza helped land plants establish on land when they colonized the earth approximately 420 million years ago.

Without mycorrhiza, it is likely that we would not have more than a fraction of land plants that we have today. The fact that some plants do not form mycorrhiza today is a newer evolutionary phenomenon where some plant species have developed new strategies or mechanisms for nutrient uptake.

  

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Root with hyphes. Photo: Iver Jakobsen / Universtity of Copenhagen

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Abstract

Arbuscular mycorrhizal fungi (AMF) colonise roots of most plants; their extra-radical mycelium (ERM) extends into the soil and acquires nutrients for the plant. The ERM coexists with soil microbial communities and it is unresolved whether these communities stimulate or suppress the ERM activity. This work studied the prevalence of suppressed ERM activity and identified main components behind the suppression. ERM activity was determined by quantifying ERM-mediated P uptake from radioisotope-labelled unsterile soil into plants, and compared to soil physicochemical characteristics and soil microbiome composition. ERM activity varied considerably and was greatly suppressed in 4 of 21 soils. Suppression was mitigated by soil pasteurisation and had a dominating biotic component. AMF-suppressive soils had high abundances of Acidobacteria, and other bacterial taxa being putative fungal antagonists. Suppression was also associated with low soil pH, but this effect was likely indirect, as the relative abundance of, e.g., Acidobacteria decreased after liming. Suppression could not be transferred by adding small amounts of suppressive soil to conducive soil, and thus appeared to involve the common action of several taxa. The presence of AMF antagonists resembles the phenomenon of disease-suppressive soils and implies that ecosystem services of AMF will depend strongly on the specific soil microbiome.