The mutualistic interactions between plants and mycorrhizal fungi and rhizobia (bacteria) provide ideal examples for tests of the biological market paradigm with traders belonging to two clear cut classes: plants and either fungi or bacteria.

The commodities exchanged on this sort of markets are nutrients, which makes it possible to quantify exchanges, at least for those who know how to do this. These systems are also ideally suited for experiments: nutrients can be added, the same individual can interact with different partners in split-root experiments, nodules formed by root tissue and symbionts can be isolated and so forth.

Reviews contemplating the possibilities of applying biological market theory (BMT) to 'underground markets' can be found here:

• Werner GDA, Strassmann JE, Ivens ABF, Engelmoer DJP, Verbruggen E, Queller DC, Noë R, Johnson NC, Hammerstein P, Kiers ET (2014) Evolution of microbial markets. Proceedings of the National Academy of Sciences 111:1237-1244. pdf

• Werner, G. D. A., & Kiers, E. T. (2015). Partner selection in the mycorrhizal mutualism. New Phytologist 205: 1437-1442 open access ( This paper appeared in a special issue of New Phytologist in which you can find more comments on the application of BMT to mycorrhiza)

• Bragazzi, N., Woldegerima, W. A. & Siri, A. (2024). Economic microbiology: Exploring microbes as agents in economic systems. Frontiers in Microbiology, 15

• Magkourilou, E., Bell, C. A., Daniell, T. J. & Field, K. J. (2025). The functionality of arbuscular mycorrhizal networks across scales of experimental complexity and ecological relevance. Functional Ecology 39 (6): 1384–1399

• Duan, S., Jin, Z., Zhang, L. & Declerck, S. (2025). Mechanisms of cooperation in the plants-arbuscular mycorrhizal fungi-bacteria continuum. The ISME Journal (the references to BMT are somewhat odd, so I hope they are not copied this way in future reviews)

• Frew, A., Varga, S. & Klein, T. (2025). Mycorrhizal networks: Understanding hidden complexity. Functional Ecology, 39(6), 1322-1327 (this is an Introduction to a 'special focus' in Functional Ecology rather than a review. Several papers from that issue are listed elsewhere on this page) 

... and a more critical review:

• Walder, F., & van der Heijden, M. G. A. (2015). Regulation of resource exchange in the arbuscular mycorrhizal symbiosis. Nature Plants, 1, 15159

Toby Kiers and colleagues came to (verbal) blows with these authors:

• Kiers, E. T., West, S. A., Wyatt, G. A. K., Gardner, A., Bücking, H., & Werner, G. D. A. (2016). Misconceptions on the application of biological market theory to the mycorrhizal symbiosis. Nature Plants, 2, 16063

• van der Heijden, M. G. A., & Walder, F. (2016). Reply to ‘Misconceptions on the application of biological market theory to the mycorrhizal symbiosis’. Nature Plants, 2, 16062

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In 2018, I published a review together with Toby Kiers in which we tried to tackle the question of what exactly is a 'trader' on an underground market (Noë, R. & Kiers, E. T. (2018). Mycorrhizal markets, firms, and co-ops. Trends in Ecology & Evolution, 33(10), 777-789. pdf). This is a hairy question  (see also 'the agency problem' on the page 'microbial markets'), notably in the case of arbuscular mycorrhizal fungi (AMFs): which entity is the trading agent that decides about how much to trade and with whom and at what price? And, if plants indeed exert partner choice among their AMF-partners, what is the entity they choose: the whole fungal network (the mycelium, or fungal individual, if one can speak of individuals in this case)? Or, do they choose at the level of single arbuscules, the trading sites inside the root cells that are formed by both fungal and plant tissues?  The answer is probably a bit of both: small groups of fungal nuclei, which we called 'co-ops', perhaps determine what happens at the level of single arbuscules, but the whole mycelium also directs the nutrients it trades over long distances to patches where it fetches a better price, as is described in several other papers from the Kiers-lab. Here a few examples:

• Whiteside, M.D., Werner, G.D.A., Caldas, V.E.A., van’t Padje, A., Dupin, S.E., Elbers, B.,Bakker, M., Wyatt, G.A.K., Klein, M., Hink, M.A., Postma, M., Vaitla, B., Noë, R., Shimizu, T.S., West, S.A.,  Kiers, E.T. (2019). Mycorrhizal fungi respond to resource inequality by moving phosphorus from rich to poor patches across networks. Current Biology, 29(12), 2043-2050.e2048 open access

The economist Loch-Temzelides used the results of this study in his paper in PNAS (2021): "Walrasian equilibrium behavior in nature"  (see the page Biological Market models)

• van’t Padje, A., Oyarte Galvez, L., Klein, M., Hink, M. A., Postma, M., Shimizu, T. & Kiers, E. T. (2020). Temporal tracking of quantum-dot apatite across in vitro mycorrhizal networks shows how host demand can influence fungal nutrient transfer strategies. The ISME Journal open access

• van ’t Padje, A., Werner, G. D. A. & Kiers, E. T. (2020). Mycorrhizal fungi control value of phosphorus in trade symbiosis with host roots when exposed to abrupt ‘crashes’ and ‘booms’ of resource availability. New Phytologist (this is an exceptionally interesting paper from a BMT viewpoint)

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A debate: do plants passively sink or actively trade their carbon?

Many papers on this debate have been published, most of which are rather technical and out of my depth.  I got involved (a bit)  in the fall of 2020 after  Prescott and colleagues published a paper in which they claimed that an organism does not (or cannot?) use a substance it produces as a commodity that can be traded against other commodities with mutualistic partners when this substance is a waste product - in this case surplus C produced by plants which is either stored in, or exudated by, their roots. I reacted to this paper with a short 'Letter' in the same journal, because I have the impression that this is a wide-spread fallacy in this particular sub-field of mutualism research.

• Prescott CE, Grayston SJ, Helmisaari H-S, Kaštovská E, Körner C, Lambers H, Meier IC, Millard P, Ostonen I (2020) Surplus carbon drives allocation and plant–soil interactions. Trends in Ecology and Evolution 35 1110-1118

• Noë R (2021) Waste can be traded with mutualistic partners. Trends in Ecology & Evolution,  36 (3) 175-176 pdf

An important contribution to this debate was published in the fall of 2024 with this rather thorough review 'Tansley Review' by Bunn and colleagues:

• Bunn, R. A., Corrêa, A., Joshi, J., Kaiser, C., Lekberg, Y., Prescott, C. E., Sala, A., Karst, J. (2024) What determines transfer of carbon from plants to mycorrhizal fungi? New Phytologist open access

I am not in a position to judge the details, but I wonder whether natural selection would indeed have skipped all opportunities to install partner discriminating mechanisms in both plants and fungi  - as the sink story suggests - with so many generations that have been in contact over such a long time. Will be continued, I hope ……

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Brian Steidinger published an clever extension to BMT in which  he develops a model that would  explain how mycoheterotrophic plants (plants that are assumed to take up both carbon and other nutrients from fungi) could take part on markets by buying a nutrient from one partner and selling that same nutrient for a better price to another, which in economic jargon is called 'arbitrage'.  This also contradicts the paper by Walder and Van der Heijden (2015) mentioned above, by showing that 'arbitrage' is an alternative to the source-sink story, which is more a description than an explanation anyway.

• Steidinger, B. S. (2025). Mycorrhizal arbitrage, a hypothesis: How mycoheterotrophs could profit from inefficiencies in the biological marketplace. Functional Ecology 39(6): 1431-1440

A lot more information on mycorrhizal fungi, and the networks they form, can be found here: SPUN