Spatial structure within root systems moderates stability of Arbuscular Mycorrhizal mutualism and plant-soil feedbacks

The persistence of mutualisms is paradoxical, as there are fitness
incentives for exploitation.  This is particularly true for
plant-microbe mutualisms like arbuscular mycorrhizae (AM), which are
promiscuously, horizontally-transmitted.  Preferential allocation
by hosts to the best mutualist can stabilize horizontal mutualisms,
however preferential allocation is imperfect, with its fidelity likely
depending upon the spatial structure of symbionts in plant
roots.  In this study, we tested AM mutualisms’ dependence on two
dimensions of spatial structure: the initial dispersion of fungi and the
ease of fungal dispersal, through three complementary experiments. We
found that fitness of the beneficial AM fungus increased when fungi were
initially separate, while initial spatial mixing benefited the fitness of
the non-beneficial fungus. These effects were strongest when dispersal was
limited, and hosts could discriminate.  Additionally, we found
that spatial structure moderated changes in AM fungal composition produced
differential feedbacks on plant growth.  Our results identify
symbiont spatial structure within plant roots as an important modifier of
plant preferential allocation and the dynamics of  mycorrhizal
mutualisms, with cascading effects on plant communities.