Review
Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals

https://doi.org/10.1016/S0169-5347(01)02122-XGet rights and content

Abstract

Plant nutrients, with the exception of nitrogen, are ultimately derived from weathering of primary minerals. Traditional theories about the role of ectomycorrhizal fungi in plant nutrition have emphasized quantitative effects on uptake and transport of dissolved nutrients. Qualitative effects of the symbiosis on the ability of plants to access organic nitrogen and phosphorus sources have also become increasingly apparent. Recent research suggests that ectomycorrhizal fungi mobilize other essential plant nutrients directly from minerals through excretion of organic acids. This enables ectomycorrhizal plants to utilize essential nutrients from insoluble mineral sources and affects nutrient cycling in forest systems.

Section snippets

Organic acids as weathering agents

Soluble organic acids affecting mineral weathering in soils originate from various sources. Medium to high molecular weight organic acids, such as humic substances, are less effective in promoting mineral dissolution than are low molecular weight (LMW) organic acids produced by plant roots and soil microorganisms 5. Although constituting only a minor fraction of the total organic acids in the soil solution, LMW organic acids are generally considered to be the most important biological

Lichenous fungi

In common with free-living fungi, symbiotic fungi also produce and excrete organic anions and protons. Well-known examples include lichens, which are formed by the association of lichenous fungi and photosynthetic algae or cyanobacteria, and which often grow on solid rock. The mycobiont mobilizes elements from the rock by excreting a range of acids, including oxalic acid, derived from carbohydrates provided by the photobiont 9. The hyphae penetrate cracks in the rock surface 13, form

Experimental evidence

Recently, the potential ability of ectomycorrhizal fungi to actively mobilize and translocate essential plant nutrients from minerals has become a focus of attention (Box 3). As in lichenous fungi, ectomycorrhizal fungi receive carbon from their host and solubilize surrounding weatherable minerals through excretion of organic acids. When grown in vitro on agar plates, different ectomycorrhizal fungal species produce oxalic acid, solubilize calcium phosphates deposited on agar 18, 19 and

Prospects

In most boreal forest systems, N is the growth-limiting nutrient for the standing trees 39, 42, 43. Nitrogen limitation impairs the synthesis of amino acids or other N compounds in plant tissues and leads to a relatively increased diversion of assimilates to plant roots 44. An increased below-ground carbon input might also result from elevated levels of atmospheric CO2. With more carbon available, the activity and growth of ectomycorrhizal fungi is stimulated 45, 46, resulting in increased

Acknowledgements

We thank Toine Jongmans (Wageningen University) for providing photographs. Financial support from The Netherlands Organization for Scientific Research (NWO) and the Swedish Council for Forestry & Agriculture Research (SJFR) is gratefully acknowledged.

Glossary

Apatite
group of calcium phosphate minerals.
Axenic
free from living organisms other than those introduced on purpose.
Biotite
mica type of mineral containing potassium (K), iron (Fe) and magnesium (Mg).
Ectomycorrhizal fungus
fungus growing in symbiosis with a host plant, forming ectomycorrhizas.
Ectomycorrhizal plant
plant growing in symbiosis with an ectomycorrhizal fungus.
Etch pit
lens-shaped cavity inside a mineral, caused by the action of water and its solutes.
Extraradical mycelium
network of hyphae

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