Reciprocal Rewards Stabilize Cooperation in the Mycorrhizal Symbiosis

TLDR

Plants and arbuscular mycorrhizal fungi form complex underground networks, creating opportunities for exploitation and raising how partners maintain fair, reciprocal resource exchange. The authors manipulated plant–fungus cooperation, demonstrating that plants detect, discriminate, and reward the most cooperative fungal partners with additional carbohydrates. Fungal partners enforce cooperation by increasing nutrient transfer only to roots that supply more carbohydrates, showing that the mutualism is evolutionarily stable with bidirectional control and that partners offering the best exchange are rewarded.

Abstract

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.