Carbon (C)–phosphorus (P) exchange dynamics constrain growth strategies

This study investigates how carbon (C)–phosphorus (P) exchange dynamics constrain growth strategies in arbuscular mycorrhizal (AM) fungi. AM fungi form symbioses with most land plants, supplying phosphorus in exchange for plant-derived carbon. While this reciprocal trade is well known, its quantitative dynamics and consequences for fungal growth strategies have remained unclear.

Arbuscular mycorrhizal fungal growth is constrained by a nearly fixed carbon–phosphorus exchange rate, largely determined by the plant host, which forces fungi into a fundamental trade-off between spreading quickly and growing densely. This exchange-rate constraint shapes fungal strategies and helps explain why symbiotic outcomes vary across plant genotypes and nutrient environments.

Arbuscular mycorrhizal (AM) fungi exchange phosphorus (P) for plant-derived carbon (C), but how this reciprocal trade constrains fungal growth strategies remains unclear. Using robotic imaging and machine learning to quantify entire fungal networks over time, Bisot et al. estimate carbon expenditure and phosphorus uptake dynamically from network morphology. They show that C transfer from the plant is proportional to P supplied by the fungus, yielding a nearly constant C/P exchange rate across fungal genotypes. However, this exchange rate shifts with plant host genotype. Fungal phenotypes exhibit a density–speed trade-off: networks can either expand rapidly (exploration) or grow densely (exploitation), but not both. Mathematical modeling demonstrates that a fixed C/P exchange rate constrains the range of achievable growth strategies, forming a Pareto front in density–speed space. Environmental phosphorus availability alters optimal strategies but not the exchange ratio. These results reveal that symbiotic exchange rate, largely plant-determined, fundamentally shapes fungal growth strategies and symbiotic outcomes.
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The authors combine robotic imaging, machine learning, and mathematical modeling to dynamically estimate carbon and phosphorus fluxes during fungal network growth. Their central finding is that a nearly constant C/P exchange rate constrains fungal growth phenotypes, creating a trade-off between exploration (fast spread) and exploitation (dense growth).

Key Findings

Carbon and phosphorus exchange are proportional

The ratio (the exchange rate) remained approximately constant during growth across time and fungal strains.

However:

• It was independent of fungal genotype
• It depended strongly on plant host genotype

This suggests the plant plays a dominant role in setting the exchange rate.

A nearly fixed carbon–phosphorus exchange rate, largely set by the plant host, constrains how arbuscular mycorrhizal fungi can grow. This constraint creates a fundamental trade-off between spreading quickly and extracting resources densely, shaping fungal strategies and symbiotic outcomes across environments.