Preferential allocation to beneficial symbiont with spatial structure maintains mycorrhizal mutualism

TLDR

Mutualisms are expected to be unstable because benefits entail costs, yet they are common, especially between plants and arbuscular mycorrhizal fungi, and preferential allocation of resources could help maintain them. The study tests whether host plants preferentially allocate photosynthate to the more beneficial of two AM fungal symbionts. Using two distinct systems, the authors measured photosynthate distribution to assess allocation patterns. When fungi are spatially separated, the beneficial symbiont increases biomass, whereas in well‑mixed communities non‑beneficial fungi dominate, showing that spatially structured preferential allocation can stabilize plant‑fungus mutualisms.

Abstract

Mutualisms, beneficial interactions between species, are expected to be unstable because delivery of benefit likely involves fitness costs and selection should favour partners that deliver less benefit. Yet, mutualisms are common and persistent, even in the largely promiscuous associations between plants and soil microorganisms such as arbuscular mycorrhizal fungi. In two different systems, we demonstrate preferential allocation of photosynthate by host plants to the more beneficial of two AM fungal symbionts. This preferential allocation could allow the persistence of the mutualism if it confers sufficient advantage to the beneficial symbiont that it overcomes the cost of mutualism. We find that the beneficial fungus does increase in biomass when the fungi are spatially separated within the root system. However, in well-mixed fungal communities, non-beneficial fungi proliferate as expected from their reduced cost of mutualism. Our findings suggest that preferential allocation within spatially structured microbial communities can stabilize mutualisms between plants and root symbionts.