Plants and their arbuscular mycorrhizal fungal symbionts interact in complex underground networks involving multiple partners. This increases the potential for exploitation and defection by individuals, raising the question of how partners maintain a fair, two-way transfer of resources. We manipulated cooperation in plants and fungal partners to show that plants can detect, discriminate, and reward the best fungal partners with more carbohydrates. In turn, their fungal partners enforce cooperation by increasing nutrient transfer only to those roots providing more carbohydrates. On the basis of these observations we conclude that, unlike many other mutualisms, the symbiont cannot be "enslaved." Rather, the mutualism is evolutionarily stable because control is bidirectional, and partners offering the best rate of exchange are rewarded.

https://science.sciencemag.org/content/sci/333/6044/880.full.pdf

Reciprocal Rewards Stabilize Cooperation in the Mycorrhizal Symbiosis

Extraradical mycelia of mycorrhizal fungi are normally the �hidden half� of the symbiosis, but they are powerful underground influences upon biogeochemical cycling, the composition of plant communities, and agroecosystem functioning. Mycorrhizal mycelial networks are the most dynamic and functionally diverse components of the symbiosis, and recent estimates suggest they are empowered by receiving as much as 10% or more of the net photosynthate of their host plants. They often constitute 20%�30% of total soil microbial biomass yet are undetected by standard measures of biomass used by soil scientists and agromomists. Mycorrhizal mycelia provide extensive pathways for carbon and nutrient fluxes through soil, often exceeding tens of metres per gram of soil. We consider the amounts of photosynthate �power� allocated to these mycelial networks and how this is used in fungal respiration, biomass, and growth and in influencing soil, plant, and ecosystem processes. The costs and functional �benefits� to plants linking to these networks are fungal specific and, because of variations in physiology and host specificity, are not shared equally; some plants even depend exclusively on these networks for carbon. We briefly assess the potential contribution of extraradical mycorrhizal mycelium to sustainable agriculture and maintenance of biodiversity and highlight technologies that promise new vistas and improved fine-scale resolution of the dynamic spatial and temporal functioning of these networks in soil.Key words: arbuscular mycorrhiza, ectomycorrhiza, extraradical mycelium, hyphal networks.

Networks of power and influence: the role of mycorrhizal mycelium in controlling plant communities and agroecosystem functioning

In response to the colonization by arbuscular mycorrhizal (AM) fungi, plants reprioritize their phosphate (Pi)-uptake strategies to take advantage of nutrient transfer via the fungus. The mechanisms underlying Pi transport are beginning to be understood, and recently, details of the regulation of plant and fungal Pi transporters in the AM symbiosis have been revealed. This review summarizes recent advances in this area and explores current data and hypotheses of how the plant Pi status affects the symbiosis. Finally, suggestions of an interrelationship of Pi and nitrogen (N) in the AM symbiosis are discussed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2006.01617.x

Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles

Mycorrhizas are considered to be classic mutualisms. Here, we define mutualism as a reciprocal increase in fitness of the symbionts, and we review the evidence for mycorrhizal mutualism at the comm...

Exploring functional definitions of mycorrhizas: Are mycorrhizas always mutualisms?

▪ Abstract Analytical methods for probing plant metabolism are taking on new significance in the era of functional genomics and metabolic engineering. Among the available methods, nuclear magnetic resonance (NMR) spectroscopy is a technique that can provide insights into the integration and regulation of plant metabolism through a combination of in vivo and in vitro measurements. Thus NMR can be used to identify, quantify, and localize metabolites, to define the intracellular environment, and to explore pathways and their operation. We review these applications and their significance from a metabolic perspective. Topics of current interest include applications of NMR to metabolic flux analysis, metabolite profiling, and metabolite imaging. These and other areas are discussed in relation to NMR investigations of intermediary carbon and nitrogen metabolism. We conclude that metabolic NMR has a continuing role to play in the development of a quantitative understanding of plant metabolism and in the character...

Probing plant metabolism with nmr

31P nuclear magnetic resonance (NMR) spectroscopy was used to study phosphate (P) metabolism in mycorrhizal and nonmycorrhizal roots of cucumber (Cucumis sativus L) and in external mycelium of the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith. The in vivo NMR method allows biological systems to be studied non-invasively and non-destructively. 31P NMR experiments provide information about cytoplasmic and vacuolar pH, based on the pH-dependent chemical shifts of the signals arising from the inorganic P (Pi) located in the two compartments. Similarly, the resonances arising from α, β and γ phosphates of nucleoside triphosphates (NTP) and nucleoside diphosphates (NDP) supply knowledge about the metabolic activity and the energetic status of the tissue. In addition, the kinetic behaviour of P uptake and storage can be determined with this method. The 31P NMR spectra of excised AM fungi and mycorrhizal roots contained signals from polyphosphate (PolyP), which were absent in the spectra of nonmycorrhizal roots. This demonstrated that the Pi taken up by the fungus was transformed into PolyP with a short chain length. The spectra of excised AM fungi revealed only a small signal from the cytoplasmic Pi, suggesting a low cytoplasmic volume in this AM fungus.

David C. Lloyd

Papers

31P NMR for the study of P metabolism and translocation in arbuscular mycorrhizal fungi