Study Explores the Roots of Cooperation Between Plants and Fungi

Science is commonly regarded as one of the top science journals in the world.

The groundbreaking study helps explain how plants were able to colonize the land in Earth’s distant past, and what mechanisms stabilize cooperation between plants and fungi in the arbuscular mycorrhizal symbiosis.

The work shows that plants can detect and reward fungi that provide more benefits to the plant by reciprocating with more carbohydrates. In turn the fungal partners enforce cooperation by increasing the transfer of mineral nutrients such as phosphorus to roots that provide more carbohydrates.

“Cooperation is only stable because both partners are able to preferentially reward the other,” said associate professor Heike Bücking of South Dakota State University’s Department of Biology and Microbiology, one of the co-authors of the study. “This provides a clear example of how cooperation can be stabilized in a form analogous to a market economy, where there are competitive partners on both sides of the interaction and higher benefits are remunerated in both directions.”

South Dakota State University Ph.D. students Jerry A. Mensah and Carl R. Fellbaum, both in SDSU’s Department of Biology and Microbiology, and Bücking´s former M.S. student Yugandhar Beesetty, are also among the co-authors of the study, “Reciprocal Rewards Stabilize Cooperation in the Mycorrhizal Symbiosis.”

The project was developed in close cooperation with Toby Kiers from the Free University in Amsterdam, Netherlands. Other co-authors from other research institutions include Marie Duhamel, Oscar Franken, Erik Verbruggen, George A. Kowalchuk, Miranda M. Hart, Alberto Bago, Todd M. Palmer, Stuart A. West, Philippe Vandenkoornhuyse, and Jan Jansa.

Bücking said cooperation between plants and arbuscular mycorrhizal fungi, is ancient and is thought to have made it possible for plants to colonize land some 470 million years ago. The partnership is thought to have developed long before mutualisms among insects or vertebrates. But researchers didn´t know what selective forces plants and AM fungi use to maintain cooperation and to prevent “cheating”.

Plants are typically colonized by multiple fungal species and fungal “individuals” can simultaneously interact with multiple host plants, and this would allow “cheaters” to exploit the benefits provided by others while avoiding the costs of supplying resources.

Bücking said a general feature of many mutualisms in nature is that one partner appears to be “in control” and has either domesticated the other partner or enforces cooperation through punishment or sanction mechanisms. In contrast, in the plant-fungal interactions the researchers studied, both sides interact with multiple partners, so neither partner can be “enslaved.”

An explanation of how plants and AM fungi maintain stable, mutually beneficial relationships is that both plants and fungi are able to detect variations in the resources supplied by their partners, allowing them to adjust their resource allocation accordingly — a kind of “biological market” for the trading of commodities that confer benefits.

The study used fungal genotypes that differ in their cooperative behavior to compete directly on a single root system. The researchers used a technique called stable isotope probing, or SIP, to track and quantify the plant resources allocated to individual fungal species. These studies showed that host plants preferentially allocate more resources to fungi that provide more benefit and that this reward mechanism also works on a very small spatial scale.

Control is bi-directional in this symbiosis, Bücking said. In economic terms, partners offering the best rate of exchange are rewarded.

The scientists used a legume called barrel medic or barrel clover, Medicago truncatula, to botanists, for the research. The study used three arbuscular mycorrhizal fungal species within a single subgenus, Glomus intraradices, Glomus custos, and Glomus aggregatum. Researchers also used transformed carrot root tissue cultures grown on Petri dishes.

The studies were supported by the National Science Foundation. Heike Bücking and her collaborators, Toby Kiers and Miranda Hart, recently received an additional award from the National Science Foundation of $450,000 for the project, “Cooperation and punishment in the arbuscular mycorrhizal symbiosis,” that will allow the researchers to continue these groundbreaking studies.