Free Market Fungi

For today's post we are going to go below ground and get a little dirty. I read a great paper this week about symbiotic fungi that I really liked because it was a simple, elegant experiment that yielded neat results.

Let's start with defining mycorrhizae (mahy-kuh-rahy-zuh). These are specialized fungi that colonize plant roots, forming a symbiotic relationship, with over 90% of terrestrial plants having these fungi. The plant supplies the fungus with energy substrates, carbon (or sugars), for growth and development. The fungus supplies the plant with water and nutrients from the soil. It grows on the root but also extends an intricate network of hyphae through the soil, greatly increasing the surface absorbing area for the plant, sort of like having extended roots. They can also access hard-to-capture nutrients, such as organic nitrogen and phosphorus, and make them available for the plants to use. The benefits of having mycorrhizae are many. They allow for increased absorption efficiency, increased drought resistance, and increased pathogen resistance. It has also been shown that plants with mycorrhizae are overall healthier and less stressed, grow better as seedlings, and can be transplanted easier. And most plants have several different species of mycorrhizae on them at any one time.

There are three general types of mycorrhizae: (1) ectomycorrhizae, (2) endomycorrhizae, (3) orchid mycorrhizae. The distinction between the types is based on the morphology of the structure formed by the fungi and the plants, if there is penetration of the root cells or not. I'm going to leave out defining the orchid mycorrhizae because they are a pretty specialized group and just stick to the other two main ones. Ectomycorrhizas are mostly Ascomycetes and Basidiomycetes and are often found on woody plants. They are able to penetrate between, but not into, the cortical cells of the plants' root. Because of this, they form a thick hyphal mat that surrounds the root and a network of strands within the cortex called a "Hartig net." This type of fungus also forms a mantle which completely encloses the root tip, this is also where the hyphae extend into the soil. Endomycorrhizae or arbuscular mycorrhizas (alternately or formerly the vesicular-arbuscular mycorrhizas) are far more abundant and classified in the Order Glomales. Arbuscular mycorrhizas (AM) penetrate inside the walls of the cortical cells producing vesicles and highly branched structures called arbuscules.

A new paper in the journal Science takes a closer look at the symbiosis between plants and AM fungi. It is known that plants supply this fungi with carbohydrates and in exchange the AM fungi provide the plants with mineral nutrients (e.g., phosphorus) and the protections I listed above. However, the selective forces that maintain this mutualism are unknown. What keeps the plant from taking nutrients from the fungi and giving nothing back in return? After all, the plant is giving away nutrients that it would otherwise be using, incurring a cost. Or the fungi takes the carbon but gives nothing back to the plant? But if the symbiont interest are tightly aligned then the fungal symbionts should increase their own fitness by helping the plants, and vise versa. Makes sense, right? Now, add multiple fungal species, with each species simultaneously interacting with multiple plants. This is a great opportunity for "cheaters," fungi that exploit the benefits of getting food while avoiding the costs of supplying resources. This paper looks at the question of how these symbiotic partners maintain a fair, two-way transfer of resources. How they keep each other honest.

The researchers used the model plant Medicago truncatula (a small plant that looks kinda like clover) and three arbuscular mycorrhizal (AM) fungal species within the cosmopolitan subgenus Glomus Ab (Glomus intraradices, G. custos, and G. aggregatum). They chose these particular AM fungi because they are closely related and exhibited either high or low levels of cooperation (giving lots or little phosphorus). This cooperation was measured in plant growth responses, costs of carbon per unit phosphorus transferred, and resource hoarding strategies (fungal resource storage). They grew M. truncatula hosts with one, two (G. intraradices versus G. aggregatum), or all three AM fungal species. Then they followed the carbon flux from the plant to the fungi and the incorporation of host carbon into the RNA of the fungal assemblage (because it reflects C allocation patterns).

They found that more carbon was supplied to the more-cooperative fungal species. In both the two-species and three-species experiments the RNA of the cooperative fungus (G. intraradices) was significantly more carbon enriched than the less-cooperative fungus. In fact, the plants showed a host preference in communities where a more cooperative fungus species was available. The extent to which the mutualism can be enforced depends on the scale at which the plants can discriminate in this way. But because we are talking small little fungi, and lots of different kinds, the discrimination scale would need to be fine. This idea lead the researchers to the next part of their experiment, looking at whether fine-scale host discrimination occurs.

To test this they used an in vitro triple split-plate system. It sounds all complex but is actually rather simple. They took a petri dish and divided it into 3 equal sections or compartments.One compartment had mycorrhizal roots and two have fungi composed of the same fungal species but varying in phosphorus supply. Its a good system because it allows the root to "choose" which fungus it wants to partner with based on the amount of nutrients (carbon) transferred from the fungus. They also tested the reverse - if the fungi enforced cooperation by the plant. They used the same split-plate system but used one fungal compartment and two root compartments, making the fungi choose which root it wanted to partner with.

The researchers found that the plant rewarded the fungus that supplied the most phosphorus, more carbon was transferred to the fungus with access to more phosphorus. In the reverse test they found that the cooperative fungus (G. intraradices) transferred more phosphorus to the roots with greater access to carbon resources. So both the plant and the fungi are discriminating. In the less-cooperative species (G. aggregatum) no carbon allocation differences were found, and it transferred more phosphorus to the roots with more carbon access but stored the phosphorus in a host-inaccessible form. It was a little phosphorus hoarder, tsk tsk.

And finally, the researchers wanted to determine whether the fungi are stimulated to provide more phosphorus in direct response to a greater host carbon supply. So they created an experiment to track the simultaneous resource exchange between the plant and fungi by using a two compartment plate and exposing the roots to labeled carbon (U-¹⁴C sucrose) in either high or low concentrations and adding labeled phosphorus (³²P) to the fungal compartment. They found that increasing the carbon supply stimulated the phosphorus transfer by the cooperative fungus but not the less cooperative fungus.

Overall, this paper shows that the mutualism in this system is different than that you find other systems. It isn't controlled by only one partner but, instead, both sides interact with each other to keep the other honest. Each of the partners cooperates with the other, preferentially rewarding them for good service. And this is where the title for this post comes from. The authors equate this system to a market economy, "where there are competitive partners on both sides of the interaction and higher quality services are remunerated in both directions."

Kumbaya.

Read the entire paper here:
Kiers, E. Toby, et al. (2011) Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis. Science: 333(6044), 880-882. (DOI: 10.1126/science.1208473)

Learn more about mycorrhizae here:
The Davies Lab at Texas A&M University
Botany at the University of Hawaii
Foresty 442 Notes from Oregon State University

(image from dirtdoctor.com)